Physics R#3 Flashcards

Question

What are the SI units of volume?

Answer 1

Cubic centimeters (cm³), cubic millimeters (mm³), **cubic meters (m³).**

Answer 2

The same as length but raised to the power of 3 | cm³ to m³ (x 100^3)

Answer 3

L × W × H.

Answer 4

Using the displacement method.

Answer 5

Measuring cylinder, water, object, sinker (if needed).

Answer 6

* Add sufficient water to the measuring cylinder and record volume as V1. * Submerge the object and record new volume as V2. * Subtract V1 from V2 (V2 - V1) to get the volume of the object.

Answer 7

* Avoid parallax error. * Ensure the object is fully submerged. * Use a sinker if the object is less dense than water.

Answer 8

The mass per unit volume of a substance.

Answer 9

Density = Mass / Volume (D = m/v).

Answer 10

**Kilograms per cubic meter (Kg/m³)** or grams per cubic centimeter (g/cm³).

Answer 11

* Measure the mass using a digital balance. * Measure the volume using the displacement method. * Divide mass by volume (D = m/v).

Answer 12

A quantity that only has magnitude (size) only.

Answer 13

Mass, time, distance, volume, density, speed, energy and temperature.

Answer 14

A quantity that has both magnitude and direction.

Answer 15

Displacement, force, weight, velocity, acceleration, momentum, electric field strength and gravitational field strength.

Answer 16

* Distance: The total path traveled between two points, with no direction * Displacement: the straight-line change in position from start to end, including direction

Answer 17

Average speed = distance / time

Answer 18

Use the gradient: (𝑦2− 𝑦1) / (𝑥2 − 𝑥1) (rise over run)

Answer 19

The highest speed recorded, found by identifying the steepest straight line on a graph.

Answer 20

Speed at a specific moment in time.

Answer 21

Instantaneous velocity is found by drawing a tangent at the point of interest on a velocity–time graph; the gradient of this tangent line represents the instantaneous velocity.

Answer 22

Draw a tangent to the curve and measure its gradient.

Answer 23

Acceleration is change in velocity per unit time

Answer 24

**a = (v−u) / t** where: * a = acceleration * 𝑣 = final velocity * 𝑢 = initial velocity * 𝑡 = time

Answer 25

Acceleration that increases at an increasing rate. * Example: 1𝑚/𝑠, 3𝑚/𝑠, 7𝑚/𝑠 (+2,+4)

Answer 26

Acceleration that slows down over time. * Example: 1𝑚/𝑠, 5 𝑚/𝑠, 7 𝑚/𝑠 (+4, +2) * Occurs in cars reaching terminal velocity.

Answer 27

1. The gradient (slope) of a velocity–time graph gives the acceleration 2. The area under the curve represents the displacement of the object.

Answer 28

Acceleration that stays the same over time. * Example: 1𝑚/𝑠, 3 𝑚/𝑠, 5 𝑚/𝑠 (+2, +2).

Answer 29

Velocity decreases at a constant rate. * Example: 10 𝑚/𝑠, 7 𝑚/𝑠, 4 𝑚/𝑠 (−3, −3)

Answer 30

A deceleration is a negative acceleration. Negative change in velocity per unit time.

Answer 31

Constant speed.

Answer 32

Changing speed (acceleration or deceleration).

Answer 33

Constant velocity (speed stays the same over time).

Answer 34

Find the area under the graph.

Answer 35

1/2 × Base × Height

Answer 36

Length × Width

Answer 37

1/2 × (Base 1+ Base 2) × Height

Answer 38

* Find the midpoint of the curve. * Divide the area into two shapes. * Approximate each area and sum them.

Answer 39

Deceleration (slowing down).

Answer 40

Constant acceleration.

Answer 41

Motion under the effect of gravity only.

Answer 42

9.8𝑚/𝑠^2

Answer 43

* Speed increases as the object falls. * Acceleration remains constant at 9.8 𝑚/𝑠^2

Answer 44

No, all objects fall at the same rate when air resistance is ignored.

Answer 45

A force is any push or pull on an object. It is a vector quantity.

Answer 46

The total force acting on an object in a certain direction.

Answer 47

Add them and keep the same direction.

Answer 48

Subtract the smaller force from the larger one and keep the direction of the larger force.

Answer 49

Use the Pythagorean theorem: 𝐹 = Sqrt(𝐹(1)^2 +𝐹(2)^2)

Answer 50

* If they have the same starting point → Use the Parallelogram Method. * If they follow each other → Use the Triangle Method.

Answer 51

w = m × g where: * 𝑤 = weight * 𝑚 = mass (kg) * 𝑔 = acceleration due to gravity (9.8 𝑚/𝑠², use 10 𝑚/𝑠² in exams).

Answer 52

The force that pulls objects toward Earth.

Answer 53

A force that opposes motion between solid surfaces.

Answer 54

A force that opposes motion in air. It increases with speed or larger surface area.

Answer 55

An upward force that occurs in liquids and gases.

Answer 56

A force that acts in strings, wires, and cables (e.g., a ceiling fan hanging from a wire).

Answer 57

A contact force exerted by a solid surface perpendicular to an object.

Answer 58

If the resultant force is 0, **an object remains at rest or moves at constant velocity.**

Answer 59

The resultant force acting on an object is equal to mass × acceleration: * 𝐹 = 𝑚 x 𝑎

Answer 60

* Force (F) is directly proportional to mass (m). * Force (F) is directly proportional to acceleration (a). * Mass (m) is inversely proportional to acceleration (a).

Answer 61

For every action, there is an equal and opposite reaction.

Answer 62

He falls under the effect of his weight only → Acceleration is constant, speed increases.

Answer 63

His speed increases, so air resistance increases → Acceleration and resultant force decrease. * Resultant force = weight − air resistance

Answer 64

Air resistance = weight → Resultant force = 0 → Speed becomes constant. (Newton’s first Law)

Answer 65

Surface area increases, so air resistance > weight → The skydiver decelerates. * Resultant force = air resistance − weight

Answer 66

His speed decreases, so air resistance decreases until it equals weight → He falls at a safe constant speed.

Answer 67

Motion where an object moves at constant speed but is always accelerating because its direction changes.

Answer 68

Centripetal force.

Answer 69

Toward the center of the circular path.

Answer 70

Friction between the object and the surface (e.g., car tires on a road).

Answer 71

A free‐body diagram is a simplified drawing that shows all the forces acting on an object. It helps visualize forces, resolve them into components, and set up equilibrium equations.

Answer 72

Pressure is the force that acts on an area.

Answer 73

𝑃 = 𝐹/𝐴 (Pressure = Force / Area).

Answer 74

N/cm², N/m², or **Pascals (Pa), where 1 Pa = 1 N/m².**

Answer 75

Pressure in liquids is the force exerted on the surrounding container.

Answer 76

P = ρgh (Pressure = Density × Gravity × Height).

Answer 77

Pascals (Pa), **ensuring all values are in meters.**

Answer 78

The pressure exerted by the air around us.

Answer 79

Total Pressure = Atmospheric Pressure + Pressure due to Liquid.

Answer 80

It means that pressure in a fluid is exerted equally in all directions—a principle known as Pascal's Principle.

Answer 81

The turning effect of a force about a pivot. Formula: * **Moment = Force × Perpendicular Distance from Pivot**

Answer 82

The perpendicular distance d = 0, so the moment = 0.

Answer 83

The object will turn clockwise.

Answer 84

The object will turn anticlockwise.

Answer 85

The object is balanced and in equilibrium.

Answer 86

No resultant force acts on the object: * Force up = Force down * Force right = Force left No resultant moment acts on the object: * Total Moment Clockwise = Total Moment Anticlockwise

Answer 87

1. Calculate the moment caused by all given forces. 2. Determine which forces are clockwise and which are anticlockwise. 3. Use Total Moment Clockwise = Total Moment Anticlockwise to solve for missing forces or distances.

Answer 88

Use the rule Force up = Force down (since the force direction is downward).

Answer 89

The point at which the weight of an object acts.

Answer 90

Move the object over a pivot until it balances. That point is the center of mass.

Answer 91

The center of mass is located at the center of the 2D or 3D shape.

Answer 92

1. Hang the object from a pin at any point. 2. Use a plumbline to draw a line of weight. 3. Repeat from at least three different points. 4. The point of intersection of the lines is the center of mass.

Answer 93

It is shifted towards the heavier parts.

Answer 94

* Lower center of mass (closer to the ground). * Larger base area. * The line of weight must be within the base area of the object. | Explain by: “center of mass doesn’t/does fall outside its base area”

Answer 95

The SI unit for moment is the Newton-meter (N·m). Moment is defined as the force multiplied by the perpendicular distance from the pivot.

Answer 96

Extension is directly proportional to load.

Answer 97

The original unstretched length of an object.

Answer 98

The stretched length of an object when a load is added.

Answer 99

Extension = L₂ - L₁

Answer 100

* Straight line * Passes through the origin * As force increases, extension increases

Answer 101

Force = K × Extension (F = KX)

Answer 102

The spring constant (stiffness of the spring).

Answer 103

K = F/X (Use any values from the straight part of the graph).

Answer 104

The maximum load where a spring still obeys Hooke’s Law.

Answer 105

The graph bends towards the extension axis and the object no longer obeys Hooke’s Law.

Answer 106

Yes, but when calculating, we must account for the difference between L₁ and L₂.

Answer 107

The elastic limit is the maximum load or extension a spring can sustain and still return to its original shape. Hooke’s Law applies only within this elastic (or proportional) region.

Answer 108

The spring constant 𝑘 is measured in Newtons per meter (N/m).

Answer 109

Work = Force × Distance

Answer 110

**Joules (J)** or Newton-meters (Nm)

Answer 111

Energy is the capacity to do work; work and energy are equivalent.

Answer 112

GPE is the energy an object has due to its height. Formula: GPE = mgh

Answer 113

By increasing mass or height.

Answer 114

KE is the energy due to motion. **Formula: KE = ½ m v²**

Answer 115

* **Elastic potential energy** (due to change in shape) * **Chemical energy** (stored in chemical compounds) * **Thermal/Heat energy** (lost due to friction) * **Nuclear energy** (from radioactive elements) * **Solar energy** (from the Sun’s light) * **Electric energy** (due to charges)

Answer 116

Energy cannot be created nor destroyed, only transformed from one form to another

Answer 117

v = √(2gh)

Answer 118

* If it stops immediately, all KE is lost as heat and sound. * If it rebounds, some KE is lost as heat and sound. | If describing energy loss, say thermal energy & specify where it's going

Answer 119

Some energy is lost as heat and sound due to friction with air. | If describing energy loss, say thermal energy & specify where it's going

Answer 120

* At the highest points: All energy = GPE * At the lowest points: Energy = KE + GPE

Answer 121

Some energy is lost as heat due to friction, reducing the KE at the lowest point.

Answer 122

Power is the rate of doing work. * Power = Work / Time * Power = Energy / Time * Power = Force × Velocity

Answer 123

**Watts (W)** or Joules per second (J/s)

Answer 124

Efficiency is the ratio of useful output energy to input energy. * Efficiency = (Useful Output Work / Input Energy) × 100% * Efficiency = (Useful Output Power / Input Power) × 100%

Answer 125

* Increase useful output energy * Reduce wasted energy * Use less input for the same result

Answer 126

Coal, natural gas, oil, and petrol.

Answer 127

They provide a high output of energy and can generate electricity continuously.

Answer 128

They are non-renewable and cause pollution (including acid rain and global warming).

Answer 129

Through nuclear fission, which produces heat to create steam that drives a turbine connected to a generator.

Answer 130

It offers a very high output of **useful** energy and continuous generation of electricity.

Answer 131

It requires strict safety measures and produces dangerous nuclear waste.

Answer 132

Fossil fuels and nuclear fuels.

Answer 133

They burn fuel or use nuclear reactions to boil water, producing steam that turns a turbine connected to a generator.

Answer 134

Water is heated by hot rocks deep underground, creating steam that drives a turbine and generator.

Answer 135

It is renewable and has no direct fuel costs.

Answer 136

Limited suitable locations and the high cost/difficulty of deep drilling.

Answer 137

They use the potential energy of water (falling or tidal) to drive turbines connected to generators.

Answer 138

They are renewable and have no fuel costs.

Answer 139

They can be expensive to build and may cause environmental damage.

Answer 140

The kinetic energy of moving air turns turbine blades, which drive a generator.

Answer 141

It is clean, renewable, and has no fuel cost.

Answer 142

It provides relatively low output and requires a large, consistently windy area.

Answer 143

They convert sunlight (light energy) directly into electrical energy.

Answer 144

It is renewable, clean, and requires no fuel cost.

Answer 145

It depends on weather/sunlight availability and can be expensive

Answer 146

The kinetic energy of ocean waves can drive turbines connected to generators.

Answer 147

It’s often less efficient, location-specific, and can be technologically challenging or expensive.

Answer 148

The chemical energy in organic materials (like wood or waste) is released by burning, producing steam to drive a turbine and generator.

Answer 149

Renewable sources do not run out (or can be used repeatedly), while non-renewable sources are finite and eventually deplete.

Answer 150

Solar panels

Answer 151

Nuclear fission, geothermal, and tidal.

Answer 152

Momentum is mass × velocity. Formula: **P = m × v**

Answer 153

* If momentum is in the same direction: ΔP = m(v - u) * If momentum is in opposite directions or changes direction: ΔP = m(v - (-u)) = m(v + u)

Answer 154

F = ΔP / t or F = m (v - u) / t

Answer 155

Impulse is equal to change in momentum (ΔP) or Force × Time of contact

Answer 156

Total momentum before a collision = Total momentum after a collision In a closed system where no external forces act

Answer 157

* Elastic collision: KE is conserved **(m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂)** * Perfect inelastic collision: Objects stick together; KE is lost **(m₁u₁ + m₂u₂ = v(m₁ + m₂)**)

Answer 158

Before launch, momentum of fuel and rocket is zero. When fuel burns, it gains a downward momentum so the rocket gains an equal and opposite momentum upwards, making the rocket lift off.

Answer 159

At first, the rifle and bullet have a momentum of 0. When a bullet is fired forward it gains forward momentum, so the rifle recoils and gains equal and opposite backward momentum.

Answer 160

Efficiency is the ratio of useful output energy (or work) to the input energy, expressed as a percentage: * Efficiency = (Useful output energy / Input energy)×100%

Answer 161

Impulse, defined as force multiplied by the time of contact (Impulse = F × t), equals the change in momentum of an object (ΔP). Momentum is a vector quantity given by mass times velocity.

Answer 162

Solid, Liquid, Gas

Answer 163

* Solids: Regular arrangement, closely packed * Liquids: Irregular arrangement, close together * Gases: Irregular arrangement, far apart

Answer 164

* Solids: Molecules are closely packed * Liquids: Molecules are close together * Gases: Molecules are far apart

Answer 165

* Solids: Very strong * Liquids: Strong * Gases: Very weak

Answer 166

Only gases are compressible; solids and liquids are incompressible.

Answer 167

* Solids: Vibrate in fixed positions * Liquids: Pass by each other * Gases: Move randomly and freely | Say "liquid molecules move in clusters" when describing their movement

Answer 168

* Solids: Molecules are tightly packed with strong forces. * Liquids: Molecules are close together with strong forces. * Gases: Molecules are far apart with weak forces, allowing compression.

Answer 169

The random movement of microscopic dust/pollen (particles) suspended in a fluid due to collisions with fast-moving molecules in the fluid.

Answer 170

It moves freely and randomly in a zigzag path and is seen as a speck of light.

Answer 171

Fast-moving air molecules hit the dust particle from all directions, causing random motion.

Answer 172

Expansion in solids < Expansion in liquids < Expansion in gases.

Answer 173

As temperature increases, particles gain kinetic energy and move further apart. For gases, the **average** distance between molecules is greater AND they have weaker intermolecular forces than solids, so they can easily move further apart than solids. (2 mark question, describe solids explicitly for more marks)

Answer 174

Gas molecules are far apart with weak forces, allowing more expansion when heated.

Answer 175

It bends towards the metal with lower expansion.

Answer 176

* Expansion in train railways * Expansion in electric cables

Answer 177

At constant temperature, pressure is inversely proportional to volume **P₁V₁ = P₂V₂**

Answer 178

Pressure is inversly proportional to volume. As volume decreases , pressure increases because the average distance between the molecules is lower causing the molecules tocollide more frequently with container walls and rebound causing a change in momentum which exerts more force and pressure. | ALWAYS state the relationship between both factors

Answer 179

Pressure is directly proportional to temperature.

Answer 180

Temperature and pressure are directly proportional. As temperature increases, molecules gain kinetic energy, move faster, and collide more frequently and rebound causing a change in momentum which exerts more force and pressure. | ALWAYS state the relationship between both factors

Answer 181

Volume is directly proportional to temperature.

Answer 182

Temperature and volume are directly proportional. As temperature increases, particles gain kinetic energy and move faster, away from each other, so average distance between molecules increases so volume increases.

Answer 183

Randomly and freely moving air molecules collide with the walls of the cylinder and rebound causing a change in momentum, which exerts force and pressure

Answer 184

Q = mcΔT where: * Q = Energy (Joules) * m = Mass (kg or g) * c = Specific heat capacity (J/kg°C | J/g°C) * ΔT = Temperature change (°C)

Answer 185

The amount of energy needed to raise the temperature of a body by 1°C.

Answer 186

When comparing between the thermal capacities of different substances make sure that the **energy supplied to both substances is equal**

Answer 187

The amount of energy required to raise the temperature of **1 kg/g** of a substance by 1°C.

Answer 188

Calculated specific heat is always higher than actual specific heat, as Q and C are proportional so when there are heat losses to the surroundings Q decreases and thus C decreases as well

Answer 189

They require more energy to change temperature.

Answer 190

* Kinetic energy increases * Potential energy remains constant

Answer 191

* Kinetic energy (temperature) remains constant * Potential energy increases (for melting/boiling) or decreases (for condensation/freezing)

Answer 192

The temperature at which liquid converts to gas/solid converts to a liquid (at a fixed temperature)

Answer 193

The lowest possible temperature (−273 °C / 0°K), where the particles have least kinetic energy | Not "No kinetic energy"

Answer 194

K = C + 273 | 0°K = -273°C

Answer 195

The most energetic surface molecules absorb energy from remaining liquid, leaving the remaining liquid with less average kinetic energy which decreases temperature

Answer 196

* Increase temperature * Increase air flow * Increase surface area * Decrease humidity * Decrease liquid depth

Answer 197

Thermal energy is transferred through metals by conduction. This is due to collisions between vibrating atoms and free electrons through atoms going from the hot to cold part

Answer 198

Metals (e.g., copper, aluminum) because they have many free electrons.

Answer 199

Using insulators like plastic, wood, and rubber.

Answer 200

In liquids and gases.

Answer 201

It has lower density and is replaced by cooler, denser air.

Answer 202

Cool air from the sea replaces warm air rising from land.

Answer 203

Cool air from land replaces warm air rising from the sea.

Answer 204

* Using a vacuum * Using insulators * Trapping air between layers * Using a lid

Answer 205

Through electromagnetic (infrared) waves, even in a vacuum.

Answer 206

Dark, dull, rough, black surfaces.

Answer 207

Shiny, polished, white, and silver surfaces.

Answer 208

Greenhouse gases trap infrared radiation, increasing Earth's temperature.

Answer 209

Incoming radiation from space = Outgoing radiation from Earth.

Answer 210

1. Heat transfers from engine to fluid by conduction. 2. Heat transfers from fluid to radiator by conduction. 3. The radiator is black with a large surface area to increase radiation emission.

Answer 211

* Double walls with vacuum: Reduces conduction & convection. * Silvered walls: Reduces radiation loss. * Lid: Reduces heat loss by convection.

Answer 212

* Specific heat capacity is the energy required to raise the temperature of 1 kg (or 1 g) of a substance by 1°C, * Thermal capacity is the total energy needed to raise the temperature of an object by 1°C (specific heat capacity multiplied by its mass).

Answer 213

* Conduction: Energy transfer via direct collisions between particles (depends on thermal conductivity). * Convection: Energy transfer through the bulk movement of fluids (driven by density differences). * Radiation: Energy transfer via electromagnetic waves (does not require a medium).

Answer 214

* At constant temperature (Boyle’s Law): 𝑃 ∝ 1/𝑉 * At constant volume: 𝑃 ∝ 𝑇 * At constant pressure: 𝑉 ∝ 𝑇

Answer 215

During a phase change, the energy is used to break or form intermolecular bonds rather than increasing the kinetic energy of the particles, so the temperature remains constant until the phase change is complete.

Answer 216

A wave is a disturbance that transfers energy through a medium. * Mechanical waves: Need a medium, cannot travel through a vacuum (e.g., sound waves, water waves, P-waves). * Electromagnetic waves: Do not need a medium, can travel through a vacuum (e.g., light waves, radio waves).

Answer 217

* Transverse waves: Particles vibrate perpendicular to wave propagation, consist of crests and troughs (e.g., all waves except sound and P-waves). * Longitudinal waves: Particles vibrate parallel to wave propagation, consist of compressions and rarefactions (e.g., sound waves, P-waves).

Answer 218

* Compression: Particles move closer, pressure increases. * Rarefaction: Particles move farther apart, pressure decreases.

Answer 219

* Amplitude (A) is the maximum displacement from the rest position. * Measured in mm, cm, or m.

Answer 220

* Wavelength is the distance between two successive crests/troughs (transverse) or two successive compressions/rarefactions (longitudinal). * Formula: λ = total distance / number of waves.

Answer 221

* Frequency is the number of waves that pass in one second. * Formula: f = number of waves / total time. * Unit: Hertz (Hz). * Depends on the source of the waves.

Answer 222

* The time required for one complete wave. * Formula: T = 1 / f or T = total time / number of waves.

Answer 223

* The speed at which a wave travels. * Formula: V = λf or V = λ / T.

Answer 224

* Angle of incidence = Angle of reflection. * Angle of incidence: Between the incident ray and the normal. * Angle of reflection: Between the reflected ray and the normal.

Answer 225

Refraction is the change in direction of a wave due to a change in speed and wavelength when moving between mediums. * Deep to shallow / rarer to denser: Bends toward the normal, speed and wavelength decrease, frequency remains constant. * Shallow to deep / denser to rarer: Bends away from the normal, speed and wavelength increase, frequency remains constant.

Answer 226

* A wavefront is a surface where all points have the same wave properties. * Types: Plane or spherical.

Answer 227

* Mechanical and longitudinal waves. * Need a medium to transfer (cannot travel through a vacuum). * Consist of compressions and rarefactions.

Answer 228

**Solids > Liquids > Gases** * Gases: 100-900 m/s | Air: 320-340 m/s * Liquids: 1000-2000 m/s | Water: 1500 m/s * Solids: 2000+ m/s | Steel: 5000 m/s

Answer 229

1. Fire a pistol and start the stopwatch when smoke is seen. 2. Stop timing when the sound is heard. 3. Measure the distance, repeat for accuracy, and calculate speed using distance/time.

Answer 230

1. Produce a sound and time the delay until it is heard again. 2. Measure the distance to the reflecting surface. 3. Calculate speed using **(2 × distance) / time.**

Answer 231

* Use a larger distance between observers. * Repeat and take an average.

Answer 232

* Amplitude is proportional to loudness: larger amplitude → louder sound. * Loud sounds have narrower compressions and wider rarefactions.

Answer 233

* Frequency is proportional to pitch: higher frequency → higher pitch. * Higher pitch means more compressions and rarefactions in a given time.

Answer 234

* Infrasonic: f < 20 Hz. * Audible for humans: 20 Hz to 20,000 Hz (20 kHz). * Ultrasonic: f > 20,000 Hz.

Answer 235

* The frequency remains constant because it is solely determined by the source * Only the wave’s speed and wavelength change during refraction.

Answer 236

Diffraction is most pronounced when the slit width is significantly smaller to the wavelength of the wave, allowing the wave to spread out.

Answer 237

The speed of sound in air increases with temperature because the air molecules move faster, leading to more rapid energy transfer.

Answer 238

* Light is an electromagnetic wave. * It does not need a medium to transfer (it can travel through a vacuum). * It is a transverse wave (energy transfer is perpendicular to wave direction). * It transfers energy from one point to another. * It can reflect, refract, and diffract. * Its speed in air/vacuum is 3×10^8 m/s. * It consists of 7 colors (ROYGBIV).*

Answer 239

* Monochromatic light has one frequency/wavelength. * Polychromatic (white) light contains multiple frequencies (the full ROYGBIV spectrum).

Answer 240

**Law of reflection: Angle of incidence = Angle of reflection.** Image properties in a plane mirror: * Upright (erect). * Same size as the object. * Virtual image. * Laterally inverted. * Distance from object to mirror = distance from mirror to image.

Answer 241

* Real image: Can be formed on a screen; formed by actual rays coming from the object. * Virtual image: Cannot be formed on a screen; formed by rays that only appear to come from the object.

Answer 242

* Refraction is the change in direction of a wave as it passes from one medium to another. * It happens because the wave’s speed and wavelength change in the new medium, causing the direction to bend.

Answer 243

* Air to medium: **𝑛 = sin(𝑖) / sin (𝑟)** * Medium to air: **𝑛 = sin(𝑟) / sin (𝑖)**

Answer 244

* 𝑖 = angle of incidence. * 𝑟 = angle of refraction. * 𝑛 = refractive index, which is the ratio: speed of light in air / speed of light in medium * A medium with a higher 𝑛 has lower light speed. * In formula form: 𝑛 = (3×10^8) / 𝑣

Answer 245

* 𝑛(1) < 𝑛(2) and 𝑣(1) > 𝑣(2) || (fast to slow). * The ray bends towards the normal (𝑖 > 𝑟) * Use 𝑛 = sin(𝑖) / sin(𝑟) .

Answer 246

* 𝑛(1) > 𝑛(2) and 𝑣(1) < 𝑣(2) || (slow to fast). * The ray bends away from the normal (𝑖 < 𝑟) * Use 𝑛 = sin(𝑟) / sin(𝑖)

Answer 247

* The critical angle 𝑐 is the angle of incidence in a denser medium that produces a 90° angle of refraction in air (𝑟 = 90∘). * 𝑖 = 𝑐: means the refracted ray just grazes the surface. * 𝑛 = 1 / sin(𝑐)

Answer 248

* Total internal reflection (TIR) happens when 𝑖 > 𝑐 (angle of incidence is greater than the critical angle) and light travels from a denser medium to a rarer medium. * All light is reflected inside; no refraction leaves the medium.

Answer 249

The ray travels straight through without bending because there is no angle of incidence.

Answer 250

* Reflecting prism: Can use total internal reflection or angled reflection (e.g., in a periscope) to change the light path. * Parallel glass plate: The angle of incidence on the first surface equals the emerging angle from the second surface, making **the incident ray parallel to the emerging ray.**

Answer 251

Any ray incident on the curved (semicircular) surface at its center enters normally (Case 5), so it passes straight through without deviation.

Answer 252

* Optical fibres have a high 𝑛 interior layer and lower 𝑛 exterior layer. * Light entering at suitable angles reflects internally (TIR) along the fibre without escaping.

Answer 253

* They are used in endoscopes for medical imaging. * A fibre is inserted into a blood vessel or body cavity to reach an organ. * Light travels inside the fibre by TIR to illuminate the organ and returns the image by TIR to a screen.

Answer 254

* They carry telephone, internet, and TV signals at high data rates with more security over longer distances. * TIR ensures minimal signal loss.

Answer 255

* Dispersion is the splitting of white (polychromatic) light into its 7-color spectrum (ROYGBIV). * Different wavelengths refract by different amounts, causing the colors to separate.

Answer 256

* Red has the longest wavelength and thus the highest speed in a medium. * It refracts the least and emerges first in dispersion (ROYGBIV order).

Answer 257

A convex lens bends light rays inward, focusing them. **Ray diagram steps:** 1. A ray parallel to the principal axis refracts through the focus. 2. A ray through the center of the lens continues undeviated. 3. A ray through the focus refracts parallel to the principal axis.

Answer 258

* A near-sighted person can see near objects clearly but not far objects. * The image forms in front of the retina. * It is corrected with a diverging (concave) lens.

Answer 259

* A far-sighted person can see far objects clearly but not near objects. * The image forms behind the retina. * It is corrected with a converging (convex) lens.

Answer 260

* They travel at the speed of light in vacuum (3×10^8 m/s). * They can propagate through a vacuum (no medium required). * They are transverse waves.

Answer 261

Radio waves → Microwaves → Infrared (IR) → Visible light → Ultraviolet (UV) → X-rays → Gamma (γ) rays.

Answer 262

* Radio waves are used in non-satellite communications (TV, radio) and Bluetooth. * Their long wavelength and low frequency let them penetrate walls.

Answer 263

* Uses: Cooking (microwave ovens), satellite communications, mobile phones. * Adverse effect: Heating effect on tissues.

Answer 264

* Uses: Remote controls, security alarm systems, night vision cameras. * Adverse effect: Can cause skin burns.

Answer 265

* Uses: Sterilization of medical equipment, hot missile tracking, security marking, sterilization of water. * Adverse effects: Skin cancer, blindness/damage to retina.

Answer 266

* X-rays: Bone photography, airport security, detecting hidden weapons. * Gamma rays: Security systems, special cameras, sterilization of food. * Both: Can cause cancer, mutations, and kill cells.

Answer 267

* Analogue signals: Continuous variation, prone to noise. * Digital signals: Two-step voltages (0 = low, 1 = high), can be regenerated accurately, reduce noise, allow higher data transfer rates, and can be converted from analogue.

Answer 268

* Diffraction is the spreading of waves when they pass through a gap or around an obstacle. * It is most significant when the gap size is similar to the wavelength. * Longer wavelengths (e.g., radio waves) diffract more than shorter ones (e.g., light).

Answer 269

* White light is a combination of all visible colours (ROYGBIV). * Dispersion occurs because different wavelengths refract by different amounts when passing through a prism.

Answer 270

When light passes from air to a denser medium, its speed decreases, the wavelength shortens, and it bends towards the normal, **while its frequency remains constant.**

Answer 271

A diverging (concave) lens is used to correct near-sightedness **because it moves the image from in front of the retina onto it.**

Answer 272

* Draw one ray parallel to the principal axis (which refracts through the focal point) * Draw one ray through the centre (which is undeviated) * Draw one ray through the focal point (which refracts parallel to the principal axis)

Answer 273

Digital signals are less prone to noise, can be regenerated without quality loss, and support higher data transfer rates, leading to more reliable long-distance communication.

Answer 274

Positive charge (+) and Negative charge (–)

Answer 275

Like charges repel, unlike charges attract.

Answer 276

* Positively charged: more (+) than (–) * Negatively charged: more (–) than (+) * Neutral: equal number of (+) and (–)

Answer 277

Metals, because they contain a large number of freely moving electrons allowing charge to pass.

Answer 278

By the movement of electrons

Answer 279

When the metal can is heated, the metal atoms/ions start to vibrate. These vibrating atoms/ions hit free moving electrons and propel them through the metal towards the colder part of the can. (3 mark question, 4 point answer)

Answer 280

Rubber, wood, plastic, because they contain few freely moving electrons and don’t allow charge to pass.

Answer 281

By rubbing, transferring electrons between the objects.

Answer 282

Water is a conductor and would prevent effective charge transfer.

Answer 283

By bringing a charged rod close, allowing electrons to move, then grounding and removing the charge source.

Answer 284

The region surrounding a charge where an electric force is exerted.

Answer 285

The direction and strength of the electric field; they originate from positive charges and enter negative charges.

Answer 286

Converts chemical energy into electrical energy.

Answer 287

Acts as an indicator for electric current.

Answer 288

Controls the flow of current, typically drawn in an open position.

Answer 289

Measures current (A) and must be connected in series.

Answer 290

The amount of charge passing per second: 𝐼 = 𝑄 / 𝑡

Answer 291

From the positive terminal of the battery to the negative terminal.

Answer 292

Electrons flow from the negative terminal to the positive terminal as they are repelled by the negative charge of the negative terminal AND they flow against the flow of current.

Answer 293

Measures voltage (V) and must be connected in parallel.

Answer 294

It increases: 𝑅(𝑇) = 𝑅(1) + 𝑅(2) + 𝑅(3)

Answer 295

If one component fails, all fail; brightness of bulbs decreases.

Answer 296

𝑅(𝑇) = (𝑅[1] x 𝑅[2]) / (𝑅[1] + 𝑅[2]) **multiply / add**

Answer 297

If one component fails, others work; bulbs maintain same brightness.

Answer 298

Longer wires have higher resistance.

Answer 299

Greater area results in lower resistance.

Answer 300

As temperature increases, metal atoms gain kinetic energy and vibrate more, due to the increased collisions between the freely moving electrons and the vibrating lattice, resistance increases

Answer 301

Higher temperature decreases resistance.

Answer 302

R = (ρxL) / A

Answer 303

* Current: Delivery crew carrying sandwiches * Voltage: Energy sandwiches from battery to components * Resistance: Traffic affecting crew’s movement

Answer 304

A variable resistor used to control current.

Answer 305

A temperature-dependent resistor; lower resistance at high temperatures.

Answer 306

A resistor that decreases in resistance when exposed to light.

Answer 307

A semiconductor that allows current to pass in one direction.

Answer 308

It has low resistance and allows current to pass.

Answer 309

It has high resistance and blocks current.

Answer 310

A diode that emits light when forward-biased.

Answer 311

* Live wire (carries electricity) **[Blue]** * Neutral wire (closes circuit) **[Brown]** * Earth wire (prevents shocks) **[Yellow/Green]**

Answer 312

A safety device that shuts down power when excessive current generates a strong magnetic field.

Answer 313

When high current passes through circuit (greater than normal current) they melt breaking the circuit and protecting any connected components

Answer 314

The fuse must be slightly larger than the normal operating current.

Answer 315

The outer case of a device is made of an insulator to prevent electric shocks.

Answer 316

They provide low resistance path for electrons to flow through to the ground preventing electric shock in case the metal case becomes live or the insulation fails

Answer 317

A circuit that divides voltage based on resistor values **connected in series**.

Answer 318

Moving the slider changes the length, resistance, and voltage.

Answer 319

* DC (Direct Current): Flows in one direction (batteries). * AC (Alternating Current): Changes direction periodically (generators).

Answer 320

To charge negatively by induction, bring a positively charged rod near a neutral conductor so that electrons in the conductor are attracted toward the rod. While the rod is in place, connect the conductor to ground; electrons will flow from the ground into the conductor, giving it a net negative charge. Then remove the ground connection and finally remove the rod.

Answer 321

* As current flows through a filament, it heats up, causing its temperature to increase. * As temperature increases lattice of material vibrates more and collides more with electrons, therefore current decreases and resistance increases. * This rising resistance results in a non-linear (curved or plateauing) V–I graph rather than a straight line as predicted by Ohm’s law for constant resistance.

Answer 322

The resistance of a wire depends on its material’s resistivity (ρ), its length (l), and its cross-sectional area (A). The formula is: * 𝑅 = (𝜌 × 𝑙) / 𝐴

Answer 323

Electric field lines start from positive charges and end on negative charges. Their direction shows the field’s direction, and the closer the lines are to each other, the stronger the field at that region.

Answer 324

In a potential divider, the voltage drop across each resistor is proportional to its resistance. By adjusting the resistor values (or moving a slider on a resistance wire), you obtain a desired fraction of the total voltage.

Answer 325

An ammeter is connected in series to measure the current flowing through the circuit, whereas a voltmeter is connected in parallel across a component to measure the voltage drop.

Answer 326

* DC flows in one constant direction (typical of batteries) * AC periodically reverses direction and its magnitude varies with time (typical of generators)

Answer 327

* In forward bias, a diode has low resistance and allows current to pass * In reverse bias, it exhibits high resistance and blocks current flow. * This property is used, for example, in converting AC to DC.

Answer 328

Like poles repel; unlike poles attract.

Answer 329

Magnets have a north and a south pole (there are no monopole magnets), are made of iron or steel, and do not affect non-magnetic materials.

Answer 330

Permanent magnets are made of steel and keep their magnetism for a long time.

Answer 331

Temporary magnets are made of iron and lose their magnetism after a short time.

Answer 332

By moving a magnet on a steel bar in one direction many times.

Answer 333

By moving a steel bar inside a coil that is carrying a DC current.

Answer 334

* By hammering repeatedly * By heating until red hot * By moving a magnet inside a coil carrying AC

Answer 335

It is the region surrounding a magnet where magnetic force acts.

Answer 336

They come out of the north pole in all directions and go into the south pole from all directions.

Answer 337

As the spacing between lines increases, the magnetic field strength decreases.

Answer 338

It is strongest near the poles and weakest near the center.

Answer 339

Place the magnet on paper sprinkled with iron filings; the pattern shows the field strength.

Answer 340

Place needle compasses at different positions around the magnet to observe the orientation of the needles.

Answer 341

* Increase the current/voltage * Increase the length of the wire * Move closer to the wire

Answer 342

* Increase the current/voltage * Increase the number of turns in the coil * Use an iron core ## Footnote Inside the coil the field lines are straight and parallel.

Answer 343

* Electromagnets are magnets that can be switched on or off. When switch is closed, current passes through the coil and magnetizes an iron core (producing a strong field) * When open, no current flows and no field occurs.

Answer 344

* Increase the current/voltage * Increase the number of turns in the coil * Use an iron core

Answer 345

* A relay uses a small current circuit to operate a circuit that uses a large current * Closing the switch sends current through a coil, creating a magnetic field that attracts a free contact to close the circuit and power a motor.

Answer 346

It uses the magnetic effect to reduce the current to its normal value when excess current flows through a wire in a magnetic field.

Answer 347

* By increasing current/voltage * By increasing the number of turns of the coil * By using a stronger magnet

Answer 348

Two equal and opposite forces (one acting downward on AB and one upward on CD) create a moment that rotates the coil.

Answer 349

* By increasing current/voltage * By increasing the number of turns of the coil * By using a stronger magnet

Answer 350

* Reverse the current * Reverse the poles of the magnet

Answer 351

A DC motor converts electric energy into kinetic energy.

Answer 352

They reverse the direction of current every half cycle, ensuring that the coil rotates continuously in one direction.

Answer 353

Carbon brushes connect the rotating coil with the battery.

Answer 354

When a wire moves inside a magnetic field, the rate at which magnetic lines are cut changes, inducing a current.

Answer 355

* By increasing the length of the wire * By increasing the strength of the magnets * By moving the wire faster

Answer 356

The induced emf is directly proportional to the rate at which magnetic field lines are cut by the wire.

Answer 357

The induced current is opposite to the normal current.

Answer 358

It is alternating current (AC).

Answer 359

* By increasing the number of turns in the coil * By increasing the strength of the magnets * By rotating the coil faster

Answer 360

* An AC generator converts kinetic energy into electrical energy * The current is zero when the coil is vertical and maximum when the coil is horizontal

Answer 361

Slip rings are used in AC generators to provide a continuous transmitting of power (they aren't split in the middle)

Answer 362

It is the process by which a changing magnetic field in one circuit induces a current in a nearby circuit.

Answer 363

* When the magnet moves toward the coil, current is induced * When it moves away, induced current occurs in the opposite direction * If both move together at the same speed in same direction, no current is induced.

Answer 364

* The coil moving toward the magnet induces a current, and moving away induces current in the opposite direction * If both remain at rest, no induced current occurs.

Answer 365

By electromagnetic induction. When alternating current/voltage passes through primary coil it produces an alternating magnetic field. This magnetic field magnetises the iron core. The magnetic field transfers to the secondary coil using iron core. The rate of magnetic field lines cut changes by secondary coil so induced current/e.m.f. occur in secondary coil.

Answer 366

A transformer wouldn't work using DC as rate of magnetic field lines cut wouldn't change

Answer 367

* Power is constant: 𝐼(𝑃) x 𝑉(𝑃) = 𝐼(𝑆) x 𝑉(𝑆) * In step-up transformers: 𝑁(𝑃) < 𝑁(𝑆) so 𝑉(𝑃) < 𝑉(𝑆) and 𝐼(𝑃) > 𝐼(𝑆) * In step-down transformers: 𝑁(𝑃) > 𝑁(𝑆) so 𝑉(𝑃) > 𝑉(𝑆) and 𝐼(𝑃) < 𝐼(𝑆)

Answer 368

1. To reduce current 2. To reduce heating effect 3. To reduce energy lost 4. Use thinner wires 5. Cheaper 6. Less metal used 7. Less insulating material used

Answer 369

Thick wires are used to carry large current because they are more efficient, there is less, risk of fire, have a lower resistance so less energy lost, and because thin wires would melt due to heating effect

Answer 370

* Increase current/voltage * Increase the number of turns in coils * Use stronger or iron cores * Adjust proximity or movement speed * When needed, reverse current or magnetic poles

Answer 371

Extend your **right thumb in the direction of the conventional current** (from positive to negative). **Your curled fingers then show the circular magnetic field lines around the wire**. Use this rule to determine the field direction around any straight current‑carrying conductor.

Answer 372

**Curl the fingers of your right hand in the direction of the conventional current flowing through the coil loops.** **Your thumb will point in the direction of the magnetic field inside the coil**, indicating the north pole of the electromagnet. Use this rule to determine the polarity and field direction inside coils or solenoids.

Answer 373

Hold your left hand so that **the thumb, forefinger, and middle finger are perpendicular. The index finger shows the direction of the magnetic field, the middle finger the direction of the current, and the thumb the direction of the force (motion) on the conductor.** Use this rule to predict the force direction on a current‑carrying conductor in electric motors.

Answer 374

Extend your right hand so that **your thumb points in the direction of the conductor’s motion relative to the magnetic field, your index finger points in the direction of the magnetic field, and your middle finger then indicates the direction of the induced current.** Use this rule when a conductor moves within a magnetic field, as in generators, to find the direction of the induced current.

Answer 375

* The magnetic field inside a solenoid is strong, uniform, and consists of straight, parallel lines. * It resembles the field of a bar magnet, with distinct north and south poles at the ends.

Answer 376

* A beam of α‑particles was directed at a thin gold foil. * Most particles passed straight through, indicating that atoms are mostly empty space. * A few were deflected at small angles (showing a concentrated positive charge) and a very few were deflected back, confirming a tiny, dense, positively charged nucleus.

Answer 377

* Most α‑particles were undeflected, showing that the atom contains empty space * A few were slightly deflected, indicating a positive charge that repels α‑particles * A few were greatly deflected, proving that the atom contains a tiny, heavy particle—the nucleus.

Answer 378

He proposed that an atom consists of a tiny, heavy, positively charged nucleus surrounded by electrons in a largely empty space.

Answer 379

Atomic notation is (_Z^A)X, where A (the mass or nucleon number) is the total number of protons and neutrons, and Z (the atomic or proton number) is the number of protons (with electrons equal to protons in a neutral atom).

Answer 380

* Isotopes are elements with the same number of protons but different numbers of neutrons. * For carbon, all isotopes have 6 protons, C‑12 has 6 neutrons, C‑14 has 8 neutrons, and C‑15 has 9 neutrons.

Answer 381

Stable elements have no excess energy, whereas unstable elements contain excess energy that is released as nuclear radiation.

Answer 382

The three types are: 1. Alpha particles: (2^4)α 2. Beta particles: (-1^0)β 3. Gamma rays: (_0^0)γ

Answer 383

* Alpha particles consist of 2 protons and 2 neutrons (like a helium nucleus) * They can penetrate only a few centimeters in air * They can be stopped by a sheet of paper

Answer 384

* They have the highest ionization * They are heavy (4 amu) * They carry a charge of +2e (3.2x10^-19 C) * They deflect toward the negative plate in an electric field * They deflect anticlockwise in a magnetic field (using Fleming’s left-hand rule).

Answer 385

* Beta particles are fast-moving electrons * They can travel a few meters in air * They are stopped by aluminium foil (>5mm/ 3m concrete)

Answer 386

* Beta particles have intermediate ionization * They are light (mass equal to an electron) They carry a charge of –1e (-1.6x10^-19 C) * Deflect toward the positive plate in an electric field * Deflect clockwise in a magnetic field (using Fleming’s right-hand rule).

Answer 387

* Gamma rays are high-energy electromagnetic waves with high frequency and short wavelength * They have no mass and no charge * They have the lowest ionization * They are nearly unstoppable in air * They are significantly reduced by lead (>2cm/stopped by 30 cm)

Answer 388

Gamma rays are high‑energy electromagnetic waves that have no mass and no charge, so they remain undeflected in both electric and magnetic fields.

Answer 389

Ionization is the ability of radiation to make an atom lose an electron.

Answer 390

* The general alpha decay equation is (_Z^A)X → (_Z–2^(A–4))Y + (_2^4)α. * For example: (_92^236)U → (_90^232)Kr + (_2^4)α

Answer 391

* The beta decay equation is (_Z^A)X → (Z+1^A)Y + (-1^0)β. * For example: (_92^236)U → (93^236)Bl + (-1^0)β.

Answer 392

The gamma decay equation is (_Z^A)X → (_Z^A)X + (_0^0)γ, which indicates that the nucleus remains unchanged.

Answer 393

* In alpha decay, the atomic number decreases by 2 and the mass number by 4 * In beta decay, the atomic number increases by 1 while the mass number remains unchanged * In gamma decay, there is no change in either number

Answer 394

* Activity is the number of decays per second * Half-life (T₁/₂) is the time required for the activity to decrease to half its initial value OR the time for radioactive nuclei to decrease to half its initial value.

Answer 395

* Too long can cause a radioactive substance to be active in the body for a long time * Too short might be insufficient time for investigation

Answer 396

* Too long can cause water to become radioactive and harmful * Too short might be insufficient time to detect leak

Answer 397

* First, take the maximum value (e.g., 600 counts) and divide it by 2 (giving 300). * Draw a horizontal line at 300 counts until it meets the decay curve * Then drop vertically to the time axis; that time is the half-life (e.g., 1.5 hours).

Answer 398

Background radiation is the natural radiation present around us in absence of radioactive material. The main sources are: * Outer space (Sun and stars) * Radon gas (about 60% of BG radiation) * Rocks * Nuclear experiments * Nuclear wastes * Medical imaging

Answer 399

It is measured using a GM (Geiger-Muller) counter and is usually around 10-30 counts, although it can vary significantly.

Answer 400

The total count rate equals the sum of the count rate from the radiation source and the background count rate.

Answer 401

Fleming’s left-hand rule is used when a current or positive particle (such as an α‑particle or proton) moves in a magnetic field.

Answer 402

Fleming’s right-hand rule is used for induced current or β‑particles (electrons) moving in a magnetic field.

Answer 403

* α‑particles have a larger mass * They carry a higher charge * They have more kinetic energy So, they interact more strongly with matter, resulting in greater ionization.

Answer 404

To lessen ionization risks: * Reduce exposure time * Work from a large distance * Use a protective screen

Answer 405

* **Alpha particles** are used in smoke detectors * **Gamma rays** are used for sterilizing food and treating cancer * **Beta particles** are used to measure the thickness of paper sheets.

Answer 406

Nuclear fission: * Splits a large nucleus into smaller nuclei * Releases energy * Ex: (_0^1)n + (_92^235)U → (_56^141)Ba + (_36^92)Kr + 3(_0^1)n * Its a chain reaction Nuclear fusion: * Combines two small nuclei at very high temperatures to form a larger nucleus * Releases more energy, * Ex: (_1^2)H + (_1^3)H → (_2^4)He + (_0^1)n. Contoling chain reactions: * By moderators (to slow the reaction) * By control rods (to absorb neutrons)

Answer 407

Nuclear fission: 1. Produces less energy than fusion 2. Involves heavy nuclei 3. Occurs in a fission reactor 4. Occurs at normal pressure and temperature Nuclear fusion: 1. Produces more energy than fission 2. Involves lighter nuclei 3. Occurs in the Sun 4. Occurs at very high pressure and temperature

Answer 408

The nuclei of the two isotopes are both positively charged so they repel each other. High amount of energy is required to overcome these repulsive forces and combine both nuclei. High temperature produces high kinetic energy allowing the nuclei to fuse together.

Answer 409

* Radiometric dating compares the measured activity of a radioactive isotope (e.g., Carbon‑14) in a sample with its expected decay rate. * Using the known half-life, the age of the sample can be estimated.

Answer 410

**Mass number:** Total number of nucleons remains constant. **Atomic number:** Total positive charge is conserved. **Charge:** Overall electrical charge is conserved. **Energy:** Energy (including the energy carried by radiation) is conserved.

Answer 411

* The Earth is a planet travelling in a nearly circular orbit around the Sun. * The Moon orbits the Earth as a satellite.

Answer 412

Motion of Earth: * The Earth spins about its axis (the line passing through the north and south poles) and makes one complete rotation every 24 hours. This causes day and night. * The Earth’s rotation on its axis causes the Sun to have an apparent daily journey from east to west.

Answer 413

Seasons (Summer, Autumn, Winter, Spring) happen due to two factors: * The rotation of Earth about the Sun once in 365 days (1 Year). * The tilt of the Earth’s axis by 23.5° to the plane of its path around the Sun.

Answer 414

The Earth orbits the Sun in approximately 365 days, and this orbit is nearly circular.

Answer 415

Light from the Sun takes about 500 seconds (around 8 minutes 20 seconds) to reach Earth.

Answer 416

Length of Days: * The longest day for the Northern Hemisphere and the shortest day for the Southern Hemisphere is 21 June. (The Sun is highest above the horizon.) * The longest day for the Southern Hemisphere and the shortest day for the Northern Hemisphere is 21 December. (The Sun is lowest above the horizon.)

Answer 417

Motion of the Moon: * The Moon is a satellite of the Earth. * It rotates around Earth once every month. * It rotates about its axis every month and always has the same side facing the Earth, so we never see the dark side of the Moon. Other properties: * It does not have an atmosphere. * It has a low gravitational field compared with Earth.

Answer 418

Phases of the Moon: * In the new Moon phase, the Moon is between the Sun and the Earth, and the side facing the Earth is unlit, so it is not visible from the Earth. * A thin new crescent appears along one edge as it travels in its orbit. * The size of the crescent increases until it reaches the first quarter phase. In the first quarter phase, half of the Moon can be seen. * At full Moon, the Moon is on the opposite side of Earth from the Sun and appears as a complete circle. * After that, it wanes through the last quarter phase until only the crescent can be seen again. * The Moon appears to have a daily trip across the sky, rising from east to west.

Answer 419

Orbital Speed: The average orbital speed. 𝑣 of the Moon is calculated by: **𝑣 =** * circumference / time of one rotation * **2𝜋𝑟 / 𝑇** where: * 𝑟 is the radius of the orbit, * 𝑇 is the time taken by the Moon to make one complete rotation around the Earth.

Answer 420

The Solar System: * It consists of one star (the Sun) and eight planets moving around it in elliptical orbits. * It includes dwarf planets and asteroids which orbit the Sun. * Moons that orbit many planets. * It contains comets and natural satellites.

Answer 421

The Four Inner Planets: Mercury, Venus, Earth, and Mars. They are: * Small in size * Solid * Rocky * Have high density

Answer 422

The Four Outer Planets: Jupiter, Saturn, Uranus, and Neptune. They are: * Larger in size * Colder * Consist mainly of gases * Have low densities

Answer 423

Dwarf Planets: For example, Pluto. They: * Orbit the Sun at an average distance greater than Neptune.

Answer 424

Asteroids: * Pieces of rock of various sizes * Mostly orbit the Sun between Mars and Jupiter (the asteroid belt) * Have a density similar to the four inner planets * If asteroids enter the Earth’s atmosphere, they will burn up and fall to Earth as meteors. * Larger asteroids are classified as dwarf planets. * Asteroids are classified as minor planets, which are defined as any object that orbits a star that does not have a large enough mass for gravitational attraction. * Famous asteroid: Ceres

Answer 425

Comets: * Consist of dust embedded in ice made from water and methane. * Have a density similar to the four outer planets. * They orbit the Sun in highly elliptical orbits. Their distance from the Sun varies significantly. * When comets approach the Sun, the dust and gas are blown backwards by radiation from the Sun, and the comet shows a bright head and a long tail pointing away from the Sun. * Famous comet: Halley’s Comet

Answer 426

* Planets, dwarf planets, and comets orbit the Sun in ellipses. * The Sun is not at the center of the ellipse. * For approximating the circular paths of planets, the point can be taken as the center of the ellipse.

Answer 427

The same formula 𝑣 = 2𝜋𝑟 / 𝑇 can be used for any planet (including the Earth) to calculate average orbital speed Where: * 𝑟 is the average orbital radius and * 𝑇 is the orbital period

Answer 428

More than 99% of the mass of the Solar System is concentrated in the Sun. Because of this, the Sun has a much stronger surface gravitational field than the planets, and its gravitational attraction keeps all the objects in orbit.

Answer 429

The Sun is thought to have formed when gravitational attraction pulled together swirling clouds of hydrogen and dust called nebulae in a region of space where their density was high.

Answer 430

* The planets are created from discs of matter that remain from the nebula that formed the Sun. * As this material rotated around the Sun, gravitational attraction between small particles caused them to join together and grow in size in an accretion process. * The evidence for this **accretion model of the formation of the Solar System** is the approximate age of the Earth being the same as the age of the Moon and the Solar System. * Also, all planets orbit the Sun in the same plane and rotate in the same direction.

Answer 431

They might have come from an exploding supernova. During the lifetime of a star, atoms of hydrogen and other light elements are fused into atoms of heavier elements.

Answer 432

* As the Sun grew, it became hotter. * In the region of space where inner planets were forming, the temperature would be high for light elements to form heavier elements. * The inner planets are built of materials with high melting temperatures, such as metals and silicates. * Further away from the Sun, in cooler regions, light molecules could exist in a solid icy form. The outer planets could grow large enough to capture the light elements. * The outer planets are large, gaseous, and cold; together their mass is about 99% of the mass orbiting the Sun. * More than 99% of the mass of the Solar System is concentrated in the Sun.

Answer 433

The gravitational field strength of a planet increases when: 1. The mass of the planet increases. 2. The distance between the planet and the object decreases (the attraction force is inversely proportional to the square of the distance).

Answer 434

* The planet’s year (time to orbit the Sun) increases with increasing distance from the Sun. * The orbital speed decreases with increasing distance from the Sun. * The surface temperature of the planet decreases as distance from the Sun increases.

Answer 435

* For planets orbiting the Sun in near-circular paths, the force of gravity between the Sun and the planet provides the necessary centripetal force. * The strength of the Sun’s gravitational field decreases as the distance between the Sun and the planet increases, so the centripetal force will decrease. This results in a lower orbital speed and longer orbital duration.

Answer 436

* For a comet with a large elliptical path, its speed increases as it approaches the Sun and decreases as it moves away from the Sun. * **As the comet approaches the Sun, its kinetic energy increases.** * **When it moves away, some of the kinetic energy is converted into potential energy.**

Answer 437

* The Sun is a medium-sized star which consists mainly of hydrogen and helium. * The radiant energy it emits is mostly in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. * This radiation is emitted from glowing hydrogen, which is heated by energy released in nuclear fusion within the Sun.

Answer 438

Nuclear Reactions in Stars: * Stable stars such as our Sun are hot and dense enough for nuclear fusion of hydrogen into helium to occur, releasing large amounts of energy. * The Sun is powered by a nuclear fusion process in its core. * Some of the energy generated in the core transfers to the surface of the star. These surface layers are cooler and less dense, but they are hot and contain hydrogen gas that glows and emits electromagnetic radiation into space.

Answer 439

Stars vary in size, mass, surface temperature, color, and brightness. Color and brightness both depend on the surface temperature, which increases with the mass of the star. **A white or blue star is hotter than a red or yellow star.**

Answer 440

* Stars are seen at night and appear close, but the distance between stars is very great. * A new unit is used to measure these distances, which is the light-year. * A light-year is the distance traveled in space or vacuum by light in one year. * **1 light-year = 9.5×10^12 km = 9.5×10^15 m.**

Answer 441

* When interstellar clouds of dust and gas containing hydrogen collapse under the force of gravitational attraction, a protostar is formed. * As the mass of the protostar increases, its core temperature increases. When the core becomes hot enough, nuclear fusion can start. * If the young star has a large mass, it forms a blue or white star. If it has a smaller mass such as our Sun, it forms a red or yellow star.

Answer 442

* In a stable star (as in the Sun), the very strong forces of gravity pulling inwards are balanced by the opposing forces trying to make it expand due to extremely high temperatures. * When forces are balanced, the star is in a stable state (for 10,000 million years). **During this time, most of the hydrogen in the core is converted into helium.** | (Refer to the Star Life Cycle diagram for a visual overview.)

Answer 443

* It becomes unstable due to less energy being produced by nuclear fusion in the core. * The core collapses inward under its gravitational force of attraction. * Potential energy is converted into kinetic energy that makes the core hotter. A fast burn of remaining hydrogen takes place, and a large expansion occurs. * The expansion and cooling of the surface gases makes the star turn into a red giant (most stars) or red supergiant if the star is massive. * **As the temperature of the core increases again, fusion of helium into carbon occurs.** | (Refer to the Star Life Cycle diagram for a visual overview.)

Answer 444

* **When all the helium is used up**, the core collapses and becomes a white dwarf at the center of a glowing shell of ionized gas known as a planetary nebula. * The white dwarf cools into a cold black dwarf containing carbon. | (Refer to the Star Life Cycle diagram for a visual overview.)

Answer 445

* Massive stars burn up hydrogen more quickly than low-mass stars, so their stable stage is shorter. * The core collapses into a supergiant, and nuclear fusion of helium to carbon occurs. * When all helium has been used up, the core collapses further due to gravitational forces, and **its temperature increases and becomes hot enough for the nuclear fusion of carbon into oxygen, nitrogen, and finally iron to occur.** * Nuclear fusion stops, and the energy of the star is released in a supernova explosion. * In the explosion, there is a large increase in the star’s brightness, and the temperature becomes high enough for fusion of nuclei **into heavier elements than iron. These heavy elements are thrown into space as nebula and become ready to form new stars and planetary systems.** | (Refer to the Star Life Cycle diagram for a visual overview.)

Answer 446

* The center of the supernova collapses to a very dense neutron star, which spins rapidly and acts as a pulsar, sending out pulses of radio waves. * If the red giant is very massive, a black hole is formed. In a black hole, the matter is packed so densely (the mass of Earth would occupy the volume of 1 cm³) that nothing can escape from its gravity, not even light. Intense X-ray radiation may be emitted, which alerts us to its presence. | (Refer to the Star Life Cycle diagram for a visual overview.)

Answer 447

* A galaxy is a large collection of stars; there are billions of stars in a galaxy. * The Milky Way is a spiral galaxy to which our Solar System belongs. * Galaxies can be seen on dark nights as a narrow band of light spreading across the sky. * Galaxies vary in size and number of stars. They travel in groups. The nearest spiral galaxy in our local cluster is the Andromeda Galaxy.

Answer 448

The Milky Way has a diameter of about 100,000 light-years.

Answer 449

* The Universe is made of millions of galaxies. * **When light emitted from stars in distant galaxies is shifted to the red end of the electromagnetic spectrum, it is called redshift (the wavelength of light increases).** * **The greater the redshift, the farther away the galaxy is from us. This is explained by the Doppler effect.** * If the source of light approaches us, the waves are crowded into a smaller space, and their wavelength decreases (blueshift). If the source moves away, the wavelength increases (redshift). * **From the size of the redshift of starlight, the speed of recession of the galaxy can be calculated, providing evidence that the Universe is expanding.**

Answer 450

* If the galaxies are receding from each other, it follows that **in the past they were closer together**. Therefore, the matter of the Universe was packed together in an extremely dense state. * **The Big Bang theory proposes that this happened from one place with a huge explosion (Big Bang).** * Scientists cannot predict what will happen in the future because the density of the Universe is difficult to calculate. This is partly because 80% of the Universe is made of invisible materials that do not emit radiation. * The gravitational force between masses is used to determine the motion and evolution of planets, stars, and galaxies, and it controls the fate of the Universe.

Answer 451

Microwave Background Radiation: * The Big Bang produced radiation energy in the form of cosmic microwave background radiation (CMBR) of a specific frequency. **It fills the whole Universe with the same intensity in all directions**. * The CMBR was produced shortly after the Universe was formed. * Since the Universe is still expanding, this results in a redshift of the cosmic background radiation into the microwave region of the electromagnetic spectrum.

Answer 452

Age of the Universe: * It is found that the speed of recession (𝑣) of a galaxy is directly proportional to its distance away (𝑑). This is called Hubble’s Law: * **𝑣 = 𝐻0 × 𝑑** * 𝐻0 (Hubble’s Constant) is defined as the ratio of the speed at which the galaxy is moving away from Earth to its distance from Earth. * Hubble’s constant represents the rate at which the Universe is expanding at present. Its value is found by measuring the speed of recession of large galaxies whose distances are known. * Redshifts are used to find the speed of recession, and distance can be calculated from the brightness. The apparent brightness decreases as the inverse square of the distance from the supernova. * The estimated value of 𝐻0 is 2.2×10^−18 per second. * The age of the Universe is equal to: * **1 / 𝐻0 =1 / 2.2×10^−18 = 4.5 × 10^17 s.**