HUGE PACK: IGCSE Flashcards
speed:
-> the distance travelled per unit time
-if the speed of something is changing, it is accelerating
-acceleration of free fall near to the earth is constant
distance-time graphs:
-the gradient is the velocity
-negative gradient = object is returning back to the starting point
-horizontal line = means object is stationary
-if distance is 0 = object is at the starting point
-curved line = means that the velocity is chaniging, and it is accelerating
formula linking average speed, time and distance:
average speed = distance moved/ time taken
practical: investigate the motion of everday objects such as toy cars or tennis balls
- Set up aparatus
- Mark a line on the ramp-this is going to make sure the car starts at the same point each time
- Measure the distance between each light gate-you’ll need this to find the car’s average speed
- Let go of the car just before the light gate so that it start to roll down the slope
- The light gates should be connected to a computer. When the car passes through each gate, a beam of light is broken and a time is recorded by data-logging software
- repeat this experiment several times and get an average time it takes for the car to reach each light gate
- using these times and the distances between gates you can find the average speed of the car on the ramp and the average speed of the car on the runway-just divide the distance between the light gate by the average time taken for the car to travel between gates
velocity and acceleration:
-velocity: is the speed in a given direction
-acceleration: is the change in velocity per unit time
formula linking acceleration, change in velocity and time taken
acceleration = change in velocity/time taken
a = (v-u)/t
velocity-time graphs:
-the gradient is acceleration
-negative gradient (i.e. negative acceleration) = deceleration
-if speed is zero = it is at rest
-horizontal line = means constant speed
-the area under the line = distance travelled
-curved line = means that the acceleration is changing
formula linking final speed, initial speed, acceleration and distance moved:
(final speed)² = (initial speed)² + (2 x acceleration x distance moved)
v² = u² + (2 x a x s)
effects of forces:
-forces can change the speed, shape or direction of a body and they are measure in Newtons (N)
-there are various types of forces (e.g. gravitational, electrostatic)
vectors and scalars:
-vector: have magnitude and direction
examples: displacement, velocity, acceleration & force
-scalar: has just magnitude
examples: distance, speed, time & energy
finding the resultant force:
-to find the resultant of two or more forces acting along the same line, they should be added together if in the same direction and subtracted if in the opposite direction
friction:
-> is a force between two surfaces which impedes motion and results in heating
-air resistance is a form of friction
Newton’s first law:
-Newton’s first law states that an object has a constant velocity unless acted on by a resultant force
Newton’s second law:
-states that:
force = mass x acceleration
f = ma
mass and weight:
-mass: is a measure of how much matter is in an object, measured in kilograms (kg)
-weight: is a gravitational force (the effect of a gravitational field on a mass)
weight = mass x gravitational field strength
w =mg
-the gravitational field strength on earth is 10N/kg
motion of a body falling in a uniform gravitational field: terminal velocity
-initially, there is no air resistance and the only force acting on it is weight
-as it falls, it accelerates which increases its speed and hence air resistance
-this causes the resultant force downwards to decrease
-therefore, the acceleration decreases, so it is not speeding up as quickly
-eventually they are equal and opposite and balance so there is no resultant force
-so, there is no acceleration and the terminal velocity is reached
factors affecting vehicle stopping:
-the distance travelled in the time between the driver realising he needs to brake and actually pressing the brakes is called the thinking distance
-factors which increase the thinking distance include:
-greater speed
-slower reaction time due to alcohol, tiredness or distractions. Reaction time can also be increased by caffeine, which reduced the thinking distance
-the distance travelled in the time between pressing the brakes and the vehicle coming to a stop is called the braking distance
-factors which increase the the braking distance include:
-greater speed or mass
-poor road conditions (icy, wet) or car conditions (worn tires, worn brake pads)
-the stopping distance is the sum of the thinking distance and braking distance
practical: investigate how extension varies with applied force for helical springs, metal wires and rubber bands
- using the ruler, measure the initial length of the first spring when no force is applied
- set up the spring so it is hanging securely from the clamp stand
-you can also secure the ruler to the clamp to ensure it does not move at all during the experiment - add one of the masses and record the extension of the spring
-the extension is the difference between the new length and the initial length - continue adding masses and recording the extension each time
- plot a graph of force applied against the extension of the spring
-force can be calculated from mass x gfs (i.e. 10 x the mass hanging on the spring)
-the gradient of the line of best fit will be the spring constant as k = F/x
Hooke’s Law:
-states that for a spring, F = kx where F is the force applied to the spring, k is the spring constant, and x is the extension
Hooke’s Law: graphs
Linear (straight line) force (y-axis) -extension (x-axis) graph:
-elastic deformation following Hooke’s law
-the point it stops being linear is called the limit of proportionality, from the on, it does not obey Hooke’s law
-gradient is the spring constant, k
Non-linear (curved line) force-extension graph:
-deformation not following Hooke’s Law
-after this region, it will fracture
the properties of a circuit:
-current is the flow of charge round the circuit, electrons carry the charge (-) and will only flow through a component if there is voltage across that component (unit: ampere- A)
-voltage (potential difference) is what drives the current round the circuit (unit: volts- V)
-resistance is anything in the circuit which slows the flow. If you add more components to the circuit there will be a higher overall resistance (unit: ohm)
-if you increase the voltage-then more current will flow
-if you increase the resistance-then less current will flow
the ammeter + voltmeter:
the ammeter:
-measures the current flowing through the component
-must be placed in series anywhere in the main circuit, but never in parallel like the voltmeter
the voltmeter:
-measures the voltage across the component
-must be placed in parallel around the component under test-NOT around the variable resistor or the battery!
ac & dc:
-the Uk mains electricity supply is approximately 230 volts
-it is an a.c supply (alternating current), which means the current is constantly changing direction
-cells and batteries supply d.c (direct current), which means that the current keeps flowing in the same direction
Ohm’s Law: formula linking voltage, current and resistance
voltage = current x resistance
V = IR
- steeper line = lower the resistance
-a straight-line = constant + resistance
-graph curves = resistance is changing
current-voltage graphs:
1) Wire: the current through a wire (at constant temperature) is proportional to voltage
2) Different (fixed) Resistors: the current through a resistor (at constant temperature) is proportional to voltage. Different resistors have different resistances, hence the different slopes
3) Metal Filament Lamp: as the temperature of the metal filament increases, the resistance increases, hence the curve
4) Diode: current will only flow through a diode in one direction
Light-Emitting Diodes: LEDs
-LEDs emit light when a current flows through them in the forward direction. They have lots of practical applications
-they are used for the numbers on digital clocks, in traffic lights and in remote controls
-unlike a light bulb, they don’t have a filament that can burn out
-LEDs, like lamps, indicate the presence of current in circuit. They are often used in appliances to show that they are switched on
Light-Dependent Resistors: LDRs
-LDRs are a special type of resistor that changes its resistance depending on how much light falls on it
-in bright lights, the resistance falls and in darkness, the resistance is highest
-this makes it a useful device for various electronic circuits, e.g: burglar detectors
-in graphs the same + a non-ohmic resistor
thermistors:
-a thermistor is a temperature-dependent resistor
-in hot conditions, the resistance drops and in cool conditions, the resistance goes up
-thermistors make useful temperature detectors, e.g: car engine temperature sensors, thermostats and fire alarms
fixed resistors + wires: graph
-current through a fixed resistor or a wire increases as the p.d across it increases
-current is directly proportional to p.d for a fixed resistor (or a wire)
-this is because the resistance of the fixed resistor (or wire) stays constant
-current on y-axis and p.d on x-axis
-ohmic resistor
filament lamps: graph
-current + voltage is not directly proportional because the resistance of the filament lamp increases as the temperature of the filament increases
-the higher temperature causes the atoms in the metal lattice of the filament to vibrate more
-this causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
-resistance opposes the current, causing the current to increase at a slower rate
-non-ohmic resistor
diodes:
-allows current in one direction only called: forward bias
-in the reverse direction, the diode has a very high resistance, and therefore no current flows called: reverse bias
diodes: IV graphs
-when the current is in the direction of the arrowhead current symbol, this is forward bias
-shown by a sharp increase in p.d and current on the right side of the graph
-when the diode is switched around, this is reverse bias
-shown by a zero reading of current or p.d on the left side of the graph
-non-ohmic resistor
resistors:
-two types: fixed resistors, variable resistors
-fixed resistors have a resistance that remains constant
-variable resistors can change the resistance by changing the length of wire that makes up the circuit
-a longer length of wire has more resistance than a shorter length of wire
thermistors: graph
-the resistance changes a lot for small changes in temperature
-resistance decreases with increasing temperature
-non-ohmic resistor
series circuits: properties
-in series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply (except for voltmeters, which are always connected in parallel, but they don’t count as part of the circuit)
-if you remove or disconnect one component, the circuit is broken and they all stop working. This is generally not very handy, and in practice only a few things are connected in series, e.g: fairy lights
series circuits: actual circuit
-the current is the same everywhere. I1 = I2 = I3 = … The size of the current depends on the total potential difference and the total resistance of the circuit (I = Vtotal divided by Rtotal)
-the total resistance is the sum of the resistance of each component in the circuit- Rtotal = R1 + R2 + R3 + ….
parallel circuits: properties
-in parallel circuits, each component is seperately connected to the +ve and -ve of the supply (except ammeters, which are always in series)
-if you remove or disconnect one component, it will hardly affect the others at all
-this is obviously how most things must be connected, for example in cars and in household electrics
parallel circuits: actual circuit
-the potential difference is the same across all branches. V1 = V2 = V3 = etc
-current is shared between branches. Itotal = I1 + I2 + I3 + etc.
-there are junctions where the current either splits or rejoins. The total current going into a junction equal the total current leaving it, as charge can’t just dissapear or appear
-if two identical components are connected in parallel then the same current will flow through each component
formula linking charge, current and time
charge = current x time
Q = It
formula linking energy transferred, charge & voltage:
energy transferred = charge x voltage
E (in joules) = Q x V
formula linking energy transferred, charge, current & resistance:
energy transferred = charge x current x resistance
E = Q x I x R
wires in a plug:
-there are three wires in a plug-live,neutral and earth
-only the live and neutral wires are usually needed, but if something goes wrong, the earth wire stops you getting hurt
-live wire (brown): provides a path along ehich the electrical energy from the power station travels
-neutral wire (blue): completes the circuit by carrying the current back to the original power source
-earth wire (green + yellow): to protect the user by providing a path for the current to escape without passing you/user
appliances being earthed and insulated:
-all apliances with metal cases must be “earthed” to reduce the danger of electric shock, “earthing” just means that the case must be attached to an earth wire, an earthed conductor can never become live
-if the appliance has a plastic casing and no metal parts showing then it’s said to be double insulated
-the plastic is an insulator, so it stops a current flowing-which means you can’t get a shock, anything with double insulation doesn’t need an earth wire-just a live and neutral
circuit breakers:
-circuit breakers are an electrical safety device used in some circuits, like fuses, they protect the circuit from damage if too much current flows
-when circuit breakers detect a surge in current in a circuit, they break the circuit by opening a switch
-a circuit breaker can easily be reset by flicking a switch on the device, this make them more convenient than fuese-which have to be replaced once they’ve melted
electrical power
-> electrical power is the rate at which an appliance transfers energy
-an appliance with a high power rating transfers a lot of energy in a short time
-this energy comes from the current flowing through it. This means that an appliance with a high power rating will draw a large current from the supply
electrical power (measured in Watts) = current x voltage
P=IV
formula linking energy transferred, current, voltage and time
Energy transferred = Current x voltage x time
E = I x V x t
waves:
-gaves transfer energy and information without transferring matter; the particles oscillate about a fixed point
-transverse waves
-longitudinal waves
transverse waves:
-have peaks and troughs
-vibrations are at right angles (perpendicular) to the direction of travel
-e.g. light
longitudinal waves:
-consist of compressions (particles pushed together) and rarefractions (particles moved apart)
-vibrations are in the same direction (parallel) as the direction of travel
-e.g. sound
important definitions: amplitude, frequency, wavelength and time period
-amplitude: the distance from the equilibrium position to the maximum displacement
-frequency: the number of waves that pass a single point per second
-wavelength: the distance between a point on one wave and the same point on the next wave
-time period: the time taken for one complete wave to pass a fixed point
formula linking speed of a wave, frequency and wavelength:
-the speed of a wave is equal to the product of the frequency and wavelength:
speed = frequency x wavelength
v = f λ
formula linking frequency and time period:
-the frequency of a wave is equal to the recirprocal of the time period, measure in Hertz (Hz):
frequency = 1/time period
f = 1/T
the Doppler Effect:
-if a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength due to the Doppler Effect
-this is because the wavefronts either get bunched together or space apart
-an example of this is when the siren of an ambulance is high-pitched as it approaches you, and low-pitched as it goes away
reflection:
-all waves can be reflected when they travel from a medium of low optical density (such as air) to one of much higher optical density (such as glass)
-the law of reflection states that:
angle of incidence = angle of reflection
-frequency, wavelength and speed are all unchanged
refraction:
-all waves can be refracted, which is when the speed of a wave changes when it enters a denser new medium
-if the wave enters a denser medium, its speed decreases and it bends towards the normal
-if the wave enters a less dense medium, its speed increases and it bends away from the normal
-in all cases, the frequency stays the same but the wavelength changes and as a result, the velocity (speed) must change
electromagnetic spectrum:
order: radio waves, microwaves, infrared radiation, visible light, ultraviolet, x-ray and gamma ray
-colours: roygbiv
-they increase in frequency
-they decrease in wavelength
-all em waves travel with the same high speed in vacuum and approximately the same speed in air
3 x 10^8 m/s
electromagnetic waves: uses
-radio waves: (red) broadcasting and communications
-microwaves: (orange) cooking and satellite transmissions
-infrared: (yellow) heaters and night vision equipment
-visible light: (green) optical fibres and photography
-ultraviolet: (blue) fluorescent lamps
-x-rays: (indigo) observing the internal structures of objects and materials, including for medical applications
-gamma rays: (violet) sterilising food and medical equipment
electromagnetic waves: dangers + protective measures
-there are detrimental effects of excessive exposure of the human body to em waves:
-microwaves: internal heating of body tissue (shielding to prevent them from reaching the user)
-infrared: skin burns (using insulating materials to reduce the amount of IR radiation reaching your skin)
-ultraviolet: damage to surface cells and blindness (sun cream and sun glasses prevent over-exposure)
-gamma rays: cancer, cell mutation (using protective shielding made of very dense materials such as lead)
light:
-ligh waves are transverse waves and can be reflected and refracted
-reflection of light can be shown when light reflects at a plane mirror and forms an image -> this can be represented by a ray diagram
practical: investigate the refraction of light, using rectangular blocks, semi-circular blocks and triangular prisms
- connect the ray box to the power supply and insert the single slit slide so that it produces a clear and thin beam of light
- place one of the blocks onto the sheet of paper and draw around it
- remove the block and then mark the position on the outline that you are going to shine the light ray at with a cross
- using a protractor, draw a normal to that point (perpendicular line)
- mark on a selection of different angles of incidence by measuring angles from the normal line
- replace the block on top of the outline, and then shine the ray of light along each incident line. For each angle, mark the position on the other side of the block where the light exits
- turn off the ray box and remove the block
- using a ruler, connect up the entry position and the exit position for each angle of incidence
- using a protactor, measure the angles of refraction for the different angles of incidence
- repeat for the other two shaped blocks and compare the results
Snell’s Law:
-relates the angle of incidence and the angle of refraction to the refractive index of a medium by n1sin(i) = n2sin(r) where n is the optical density and i is the angle of incidence and r is the angle of refraction
formula linking refractive index, angle of incidence and angle of refraction:
n = sin(i)/sin(r)
practical: investigate the refractive index of glass, using a glass block
- draw around a rectangular glass block on a piece of paper and direct a ray of light through it in an angle. Trace the incident and emergent rays, remove the block, then draw in the refracted ray between them
- you then need to draw in the normal at 90º to the edge of the block, at the point where the ray enters the block
- use a protractor to measure the angle of incidence (i) and the angle of refraction (r), remember- these are the angle made with the normal
- calculate the refractive index (n) using Snell’s Law: n= sin(i)/sin(r)
total internal reflection:
-total internal reflection occurs when the angle of incidence is greater than the critical angle and the light reflects back into the medium
-for total internal reflection to occur, the light must also be travelling from a more optically dense medium into a less optically dense medium (most common examples is glass to air)
-the critical angle c can be related to the refractive index by:
n = 1/sin sin(c) or sin c = 1/n
optical fibres:
-an optical fibre is a long thin rod of glass surrounded by cladding which uses total internal reflection to transfer information by light, even when bent
-they are used extensively in medicine (endoscopes, inside-body flexible cameras) and communications (high speed data transfer)
-sound waves are longitudinal waves and can be reflected and refracted
energy stores and energy transfers:
-energy stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear
-energy transfers: mechanically, electrically, by heating, by radiation (light and sound)
energy transfers: mechanically, electrically, by heating and by radiation
-mechanically e.g. when gravity accelerates an object and gives it kinetic energy
-electrically e.g. when a current passes through a lamp and it emits light and heat
-by heating e.g. when a fire is used to heat up an object
-by radiation e.g. when vibrations cause waves to travel through the air as sound, or an object emits electromagnetic radiation
formula linking efficiency, useful energy output and total energy output
-energy is always conserved, the total energy before is equal to the total energy after
efficiency = useful energy output/total energy output x 100%
-Sankey diagrams can be used to represent the transfer of input energy into useful energy and wasted energy
conduction:
-main method of thermal energy transfer in solids
-metals are extremely good at conducting heat
-non-metals are poor at conducting heat whilst liquids and gases are extremely poor (insulators)
-substance is heated, then atoms start to vibrate more and bump into each other-transferring energy from atom to atom
-delocalised electrons can collide with atoms helping to transfer vibrations through material and heat better
convection:
-main way that heat travels through liquids and gases (can’t in solids)
-when a fluid (a liquid or a gas) is heated:
-molecules push eachother apart- making fluid expand
-this makes the hot fluid less dense than the surroundings
-hot fluid rises, and the cooler fluid replaces it
-eventually, hot fluid cools, contracts and sinks back down again
-resulting motion = convection current
radiation:
-heat transferred by infrared
-the hotter the object, the more infrared radiation it radiates
-colour of object affects how well it emits and aborbs radiation
-black objects-best at emitting + absorbing radiation
-shiny objects-worst at emtting + absorbing radiation
practical: investigate thermal energy transfer by conduction, convection and radiation
- set up the equipment
- using a small amount of petroleum jelly, attach a drawing pin to the end of each of the rods
-try to make this the same amount of petroleum jelly for each rod - bring together the other ends of the rods (without the pins) so that they can each be heated the same amount
- using a bunsen burner, begin heating the ends of the rods without the pins and start the stopwatch
- record the time taken for the pins to fall off the end of each rod and use this to determine the order of conductivity of the metal
-the first pin to fall will be from the rod that is the best conductor
work done:
-work is done when a force moves something through a distance (whenever energy changes forms), the work done is equal to the energy transferred
work done = force x distance
W = Fd
gravitational potential energy:
-the conservation of energy produces a link between gravitational potential energy, kinetic energy and work. For example, when a ball is dropped, gravity does work on it and its gravitational potential energy becomes kinetic energy as it accelerates downwards:
kinetic energy = 1/2 x mass x speed²
Ek = 1/2mv²
gravitational potential energy = mass x gravitational field strength x height
Ep = mgh
power:
-power is the rate at which energy is transferred or the rate at which work is done. For example, a lamp with a greater power will be brighter because it transfers more energy from electrical energy to light and heat energy in a given time
power = work done/time taken
P = W/t
formula linking density, mass and volume
-the density of a substance is defined as the mass per unit volume and is measured in kilograms per metre cubed (kg/m^3)
density = mass/volume
p = m/V
practical: investigate density using direct measurements of mass and volume
- to measure the density of a substance, use a balance to measure its mass
- if it’s a box shape, start by measuring its length, width and height with an appropiate piece of equipment (e.g. ruler). Then calculate its volume by multiplying the length, width and height together
- for an irregular solid, you can find its volume by submerging it in a eureka can filled with water. The water displaced by the object will be transferred to the measuring cyclinder
- record the volume of water in the measuring cylinder, this is also the volume of the object
- plug the object’s mass and volume into the density fomula
formula linking pressure, force and area + knowledge:
-pressure is defindes as the force per unit area and is measured in Pascals (Pa)
pressure = force/area
p = F/A
for example, lying down on a bed of nails compared to a single nail:
-the force applied is the weight of you body
-the total area is either a single pin point or many points spread out over a larger area
-therefore, on a bed of nails, the pressure is lower as the area is greater
pressure at a point in a gas or liquid:
-the pressure at a point in a gas or liquid at rest acts equally in all directions and causes a force at right angles to any surface
-pressure in a fluid (gas or liquid) is created from the movement of particles (as they collide with a surface)
formula linking pressure difference, height density and gravitational field strength + knowledge:
-the pressure beneath a liquid surfaces increases with depth, the density of the liquid and the gravitational field strength
-it is given by:
pressure difference = height x density x gfs
p = pgh
-deeper in the fluid the more particles above the point, hence the greater their weight
-fluids with higher density have more particles per unit of volume, hence greater weight
-weight depends upon gravitational field strength
ideal gas molecules: molecules
-gas molecules move rapidly and randomly due to collisions with other gas molecules
-gases exert pressure on a container due to collisions between gas molecules and the wall
-when the molecules rebound off the walls, they change direction so their velocity and therefore momentum changes
-this means they exert a force because force is equal to the change in momentum over time
ideal gas molecules: pressure and temperature
-at a constant volume, if the temperature increases, the pressure increases because the molecules move faster so they collide harder and more frequently with the walls
-the temperature at which the pressure is zero is called absolute zero (-273ºC), the Kelvin scale of temperature defines absolute zero to be 0K with an increment of one Kelvin equal to an increment of one degree Celsius, this means that:
-temperature in kelvin = temperature in degrees celcius + 273
-for a gas at fixed mass and volume, where the temperature is measure in Kelvin:
P1/T1 = P2/T2 pr P/T = constant
-at a constant temperature, if the volume increases, the pressure decreases because the molecues collide less frequently with the walls and over a greater area
-for a gas at fixed mass and temperature:
P1V1 = P2V2 or pV = constant
Boyle’s Law:
-the temperature in Kelvin of a gas is proportional to the average kinetic energy of the molecules
-the higher the temperature, the greater the average kinetic energy and so the faster average speed of the molecules
magnetism:
-magnets repel and attract other magnets and attract magnetic materials, like poles of magnets repel and opposite poles attract
-non-magnetic materials are material that are not attracted to magnets and cannot be magnetised (e.g. glass, plastic)
-magnetic materials are materials that are attracted to magnets and can be magnetised (e.g. iron, steel, cobal, nickel)
-magnetism can be induced in magnetic materials by placing them in a magnetic field
-magnetic materials that can be permanently magnetised are described as magnetically hard (e.g. steel)
-magnetic materials that are only temporarily magnetised are described as magnetically soft (e.g. soft iron)
magnetic field lines:
-magnetic field lines represent the magnetic force on a north pole at a given point
-the direction of a magnetic field line shows the direction of the force
-how close together the magnetic field lines are shows the magntidue of the force
-field lines from magnets point from north to south
-there is a uniform magnetic fielf between the opposite poles of two magnets placed close together, as the field lines move from the north pole of one straight towards the south pole of the other, the field lines are parallel and evenly spaced
practical: investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets
-method 1: iron fillings
1) sprinkle some iron fillings onto a sheet of paper
2) place the permanent bar magnet onto the paper and the fillings shouls move into the shape of the magnetic field
3) repeat this with two permanent bar magnets placed a short distance apart
-method 2: plotting compass
1) place the permanent bar magnet onto a plain piece of paper
2) place the plotting compass somewhere around the field and then draw the direction of the needle at that point
3) continue placing the needle in various positions on the paper and drawing arrows
-the arrows should show that the field lines go from the north pole to the south
5) you can also try this with two permanent bar magnets placed a short distance apart
-experiment with placing like poles and unlike poles facing eachother
electromagnetism + Fleming’s left hand rule:
-an electric current passing through a conductor produces a magnetic field around it
-a force acts on a current-carrying conductor in a magnetic field
-Fleming’s left-hand rule shows the relative directions of the force, field and current…
-thumb = direction of force
-index finger = direction of field
-middle finger = direction of current
The motor effect:
-if the current is reversed or the magnetic field is reversed, the force will be reversed
-if the magnitude of the current or the magnetic field is increased, the magnitude of the force will increase
DC motors:
-DC motors consist of a coil of wire in between two permanent magnets
-direct current flows through the wire and it expericiences a turning effect due to the forces exerted on it in the magnetic field
-as the current flows in opposite directions on each side of the coil, the forces on each side are in opposite directions-making it turn
-the turning effect can be increased by:
-increasing the current
-using a stronger magnetic field
-increasing the number of turns on the coil
a commutator:
-a split ring commutator is used to ensure that the direction that the current flows in the coil reverses every half turn
loudspeakers:
-loud speakers consist of a coil attached to a cone in a magnetic field
-when alternating current flows through the coil, the cone is continuously pushed away and pulled back, making a sound
-the frequency (and therefore pitch) of the sound can be altered by changing the frequency of the alternating current used
electromagnetic induction: the generator effect
-when there is relative movement between a conducting wire & a magnetic field, a voltage will be induced
-for example, if conducting wire moves across a magnetic field, a voltage is induced in it, if it is part of a complete circuit, this causes the a current to flow
-this is called the generator effect
electromagnetic induction: voltage
-the induced voltage can be increase by:
-moving the wire more quickly
-using a stronger magnetic field
-increasing the length of the wire inside the magnetic field (e.g. by making it more coiled)
-a voltage is also induced in a coil with a changing magnetic field through it. For example, when a magnet is moved into a coil, a voltage is induced in it
-the more quickly the magnetic field changes, the greater the voltage
electromagnetic induction: electricity
-electricity can be generated by rotating a magnet within a coil or by rotating a coil in a magnetic field
-as they rotate, the magnetic field through the coil changes, which induces a voltage and therefire a current in the coil
an atom:
an atom consists of:
-a positive charged nucleus made of:
-positive protons
-neutral neutrons
-surrounded by negatively charged electrons which orbit the nucleus
-the radius of the nucleus is a lot smaller than the radius of the entire atoms, almost all the mass of the atoms lies in the nucleus
proton: relative mass 1, relative charge +1
neutron: relative mass 1, relative charge 0
electron: relative mass 0.0005, relative charge -1
isotopes:
-atoms of the same element have the same number of protons and electrons
-isotopes are forms of an element’s atom with the same number of protons and electrons but a different number of neutrons
-for a given nuclide (distinct nucleus):
-X is the symbol of the element
-A is the mass (nucleon) number -> number of neutrons and protons
-Z is the atomic (proton) number -> number of protons
radioactive decay:
-radioactive decay is the spontaneous transformation of an unstable nucleus into a more stable one by the release of radiation
-it is a random process which means one cannot know what nucleus will decay or when it will decay because it is down to chance
decay processes: alpha
-aplha (α) particles are made up of 2 protons and 2 neutrons- they are big, heavy and slow-moving
-they therefore don’t penetrate far into materials but are stopped quickly
-because of their size they’re strongly ionising, which means they bash into a lot of atoms and knock electrons off them before they slow down, which creates a lot of ions
-because they’re electrically charged (positive charge), aplha particles are deflected (direction changes) by electric and magnetic fields
-emitting an alpha particle decreases the atomic number of the nucleus by 2 and the mass number by 4
decay processes: beta
-(electrons) 0,-1 e-(on top)
-a beta (β) particle is an electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron
-when a beta particle is emitted, the number of protons in the nucleus increases y 1, so the atomic number increases by 1 but the mass number stays the same
-they move quite fast and they are quite small
-they penetrate moderately before colliding and are moderately ionising too
-because they’re charged (negative), beta particles are deflected by an electric and magnetic fields
decay processes: gamma
-gamma (γ) rays are the opposite of alpha particles, they have no mass-they’re just energy
-they can penetrate a long way into materials without veing stopped
-this means they are weakly ionising because they tend to pass through rather than collide, but eventually they hit something and do damage
-gamma rays have no charge, so they’re not deflected by electric or magnetic fields
-gamma emission always happend after beta or alpha decay, you never get just gamma rays emitted
-gamma ray emission has no effect on he atomic or mass numbers of he isotope, if a nucleus has excess energy, it loses this energy by emitting a gamma ray
uses of radioactivity: alpha
industry:
-smoke detectors:
-long half-life alpha emitters are used in smoke detectors
-alpha particles cause a current in the alarm, if smoke enters the detector, some of the alpha particles are absorbed and the current drops, triggering the alarm
uses of radioactivity: beta
industry:
-thickness monitoring:
-long half-life beta emitters can be used for thickness monitoring of metal sheets
-a source and receiver are placed on either side of the sheet during its production
-if there is a drop or rise in the number of beta particles detected, then the thickness of the sheet has changed and needs to be adjusted
uses of radioactivity: gamma
medicine:
-sterilisation of equipement:
-gamma emitters are used to kill bacteria or parasites on equipment so it is safe for operations (this means they can be sterilised through their protective packaging to eliminate the risk of contamination)
-diagnosis and treatment:
-short half-life gamma emitters such as technetium-99m are used as tracers in medicine as they concentrate in certain parts of the body, the half-life must be long enough for diagnostic procedures to be performed, but short enough to not remain radioactive for too long
-other gamma emitters such as cobalt-60 can be used to destroy tumours with a high dose of radiation
decay processes: how they are blocked
-alpha particles: are blocked by paper, skin or a few cm of air
-beta particles: are blocked by thin metal
-gamma rays: are blocked by thick lead or very thick concrete
practical: investigate the penetration powers of different types of radiation using either radioactive source or simulations
1) set up a Geiger counter without any of the radioactive sources nearby and records the background activity over a period of about 15 minutes and calculate the count rate in counts per minute (divide the total counts by the number of mins)
2) set up a clamp stand directly infront of where the source will be, pointing towards the clamp stand
3) place the geiger counter around 5 cm from where the source will be, pointing towards the clamp stand
4) move the first radioactive source into position and with no absorbers in place, record the number counts over a 5 minute period and calculate the count rate
5) attach different absorberd to the clamp stand, one at a time, and repeat
6) correct all count-rate readings for background radiations by subtracting the background radiation measure in step 1
7) repeat for the other two source and then compare the count rates for each source with each different absorber
8) a higher count rate for a given material means that more radiartion has passed through the absorbed and so the radiation type is more ionising
balancing nuclear equations:
-alpha emission: mass number decreases by 4, atomic number decreases by 2
-beta emission: mass number stays the same, atomic number increases by 1
-gamma emission: mass number stays the same, atomic number stays the same
-neutron emission: mass number decreases by 1, atomic number stays the same
dangers of ionising radiation:
-radiation can cause mutations in living organisms
-radiation can damage cells and tissue
-the problems arising from the disposal of radioactive waste and how the associated risk can be reduced
detecting ionising radiation:
photographic film:
-the more radiation absorbed by the film, the darker it gets (the film is initially white)
-they are worn as badges by people who work with radiation, to check how much exposure they have had
Geiger-Müller tube:
-a gm tube is a tube which can detect radiation
-each time it absorbs radiation, it transmits an electrical pulse to the machine, which produces a clicking sound
-the greater the frequency of clicks, the more radiation present
safety measure of ionising radiation:
-minimising the time of exposure to radiation, keeping as big a distance from the radioactive source as possible, and using shielding against radiation (such as protective clothing made from dense materials such as lead)
-radioactive waster from nuclear reactors must be disposed of carefully, usually by burying it in sealed drums deep underground and remotely handling it after it has been thoroughly cooled
background radiation:
-weak radiation can be detected from external sources called background radiation
sources of background radiation include:
-from space:
-cosmic rays include high-energy charged particles penetrating the atmosphere
-from earth:
-radioactive rocks which give off radioactive radon gas
-food and drink which contains radioactive isotopes (such as Carbon 14)
-fallout from nuclear weapons testing
-medical source such as x-rays from MRI scanners
-nuclear power plants which produce radioactive waste
activity of radioactive sources:
-the activity of radioactive source is the number of decays which occur per unit time and is measured in becquerels (Bq where 1Bq = 1 decay per second)
-the activity of a radioactive source decreases over a period of time
half-life:
-the half-life of an isotope is the time taken for half the nucleis to decay, or the time taken for the activity to halve
-a short half-life means the activity falls quickly, because of lots of the nuclei decay quickly
-a long half-life means the activity falls more slowly because most of the nuclei don’t decay for a long time-they just sit there, basically unstable, but kind of biding their time
contamination and irradiation:
-contamination: occurs when a radioactive source has been introduced into or onto an object, the contimanated object will be radioactive for as long as the source is in or on it
-irradiation: occurs when an object is exposed to a radioactive source which is outside the object, the irradiated object does not become radioactive
nuclear fission:
-the process of splitting a nucleus is called nuclear fission
-when uranium-235 nucleus absorbs a thermal (slow-moving) neutron, it splits into two daughter nuclei and 2 or 3 neutrons, releasing energy in the process
-the neutrons then can induce furhter fission events in a chain reaction by striking other uranium-235 nuclei
-when uranium-235 splits into two it will form two daughter nuclei, both lighter elements than uranium
-these nuclei are usually radioacive, this is a big problem with nuclear power-radioactive waste
-each nucleus splitting gives out a lot of energy-this energy is in he kinetic energy stores of the fission products
-in a reactor, this energy is transferred to thermal energy stores to produce steam to drive a turbine
nuclear reactor:
-control rods: (usually made of boron) are used to absorb excess neutrons and keep the number of neutrons such that only one fission neutron per event goes on to induce further fission
-the moderator: (usually water) slows down neutrons by collisions so that they are moving slow enough to be absorbed by another uranium-235 nucleus
-a coolant: (also water) is used to prevent the system from overheating
-the reactor core: is a thick steel vessel which withstands the high pressures and temperature and absorbed some of the radiation
-the whole core is kept in a building with thick reinforced concrete walls that act as radiation shields to absorb all the radiation that escaped the reactor core
nuclear fusion:
-the process of fusing two nuclei to form a larger nucleus is called nuclear fusion
-there is a very small loss of mass in the process, accompanied by a release of energy
-nuclear fusion is how the sun and other stars release energy
-nuclear fusion dows not happen at low temperatures and pressure because the electrostatic repulsion of the protons is too great
-which is because the positively charged nuclei have to get very close to fuse, so they need to be moving very fast to overcome the strong force due to electrostatic repulsion
important terms: universe, galaxy and solar system
-the universe: is a large collection of billions of galaxies
-a galaxy: is a large collection of billions of stars
-a solar system: is a collection of plantes orbiting a common star, our solar system is in the Milky Way galaxy
gravitational field strength:
-the gravitational field strength is the force per unit mass on a body in a gravitational field and is measure in Newtons per kilogram (N/kg)
-it varies with the mass and size of the body and is therefore different on other planets and the moon compared to the earth
weight = mass x gravitational field strength
W = mg
gravitational force:
-gravitational forces enables the various bodies to orbit around other, for example:
-moons orbit plants -> the orbits are slightly elliptical with near constant orbital speed
-planets and comets orbit the sun -> the orbits of planets are slightly elliptical with near constant orbital speed, the orbits of comets are highly elliptical
-artificial satellites orbit the earth
formula linking orbital speed, orbital radius and time period:
-the greater the orbital radius or the smaller the time period, the greater the orbital speed:
orbital speed = 2 x pi x orbital radius/time period
v = 2πr/T
-comets have a greater speed nearer to the star (when r is smaller) because the ice inside them melts as they get closer (and warmer)-causing their mass to decrease
stellar evolution:
-a star begins as a cloud of dust and gas called a nebula, the particles experience a weak attraction towards each other due to gravity and begin to clump together
-they continue to clump together until the pressure and temperature is great enough for nuclear fusion to occur
-hydrogen nuclei fuse together to form helium nuclei which releases a large amount of energy and causes a great outwards pressure
-this outwards pressure balances with the inwards pressure due to gravity and the star is now stable and called a main sequence star
-eventually the hydrogen star is used up, there is no longer enough outwards pressure from nuclear fusion and it collapses under its own gravitational attraction, becoming unstable
-if the star has a similar mass to the sun, it expands massively and becomes a red giant, it then becomes a white dwarf (and finally cools into a black dwarf)
-if the star has a mass larger than the sun, it exapnds and becomes a red super giant, before exploding into a supernova, what remains is either a neutron star, or if it was expecionally massive, a black hole
classification of stars:
-stars can be classified according to their colour, the colour of a star is related to its surface temperature, with hotter stars being bluer and cooler stars being redder
