Physics Flashcards
Define current
The rate of flow of charge, measured in amperes (A) and assigned I as its symbol
Define potential difference
The driving force that move charge around a circuit, measured in Volts (V) and assigned symbol V
Define resistance
Anything that sows the flow of charge, measured in Ohms ( Ω ), assigned the symbol R
Current through a component depends on 2 things
- The components resistance (greater resistance = smaller current)
- The potential difference across the component (greater pd = larger current given a fixed resistance)
Size of current is the same as…
Rate of flow of charge
Charge (Coulombs, C) =
Current (A) x Time (s)
Potential difference (V) =
Current (A) x Resistance ( Ω )
Ohmic conductor (resistor at a constant temperature) I/V graph
Linear - as current will be directly proportional to charge - which means resistance doesn’t change
Filament lamp I/V graph
Current increases so temperature of filament increases which means resistance increases - S shaped graph
Diode I/V graph
High resistance in one direction, so current only flows in a single direction - gives a grapy of y = 2^x shape
LDR resistance depends on
Light intensity
LDRs have a lower resistance in
brighter light
Where are LDRs used?
Automatic night lights
Thermistor resistance depends on
Temperature
Thermistors have a lower resistance in
Hotter temperatures
Thermistors are used in
Thermostats
Current in series
The same EVERYWHERE so I1 = I2
Voltage in series circuits
Is shared between each component Vtot = V1 + V2
Resistance in series circuits
Total resistance of components is the sum of their resistances Rtot = R1 + R2
Current in parallel circuits
Sum of currents through each branch Itot = I1 + I2
Adding a resistor in series…
Increases the total resistance of the circuit
Voltage in parallel circuits
Is the same as the source of voltage V1 = V2 = Vtot
Resistance in parallel
Total parallel resistance is less than the resistance of the smallest resistor
Adding a resistor in parallel…
Decreases the total resistance of the circuit
Alternating current
Current that constantly changes direction and is produced by an alternating voltage - mains supply
Direct current
Current that always flows in the same direction - supplied by batteries
UK mains systems
AC supply, frequency of 50Hz and voltage around 230V
Electric shocks
A large potential difference is produced across the body which causes current to flow across the body
Energy transferred (J)
Charge flow (Q(Coulombs)) x Voltage (V)
Amount of energy an appliance transfers depends on
An appliance’s power and how long an appliance is on for
Energy transferred (J)
Power (W) x time (s)
Power is the…
Energy transferred per second
Power rating is the…
Maximum safe power an appliance can operate at
Power (W)
Current (A) x Voltage (V)
Power (W) =
Current ^2 (A) x Resistance ( Ω )
Transformers
Step up - increase resistance
Step down - decrease resistance
Why is transferring at a high current not efficient?
There would be a lot of energy to the thermal energy stores of the surroundings
How does something become charged?
Rubbing two insulating materials together to move electrons from one to the other which means both materials become electrically charged - charge will be equal and opposite. Only electrons move, positive charges don’t
When do electric sparks occur?
Electric charge builds on an object which builds a p.d. between the object and the earth (which is at 0V) increases, when p.d. is big enough there is a spark
Electric fields around charged spheres
Carries from positive to negative
When field lines are close together…
The field is stronger
How do charged objects exert forces on each other in a electric field
They either attract or repel each other (non-contact force)
How is an alternating current generated?
Rotating generators to constantly change current direction
Current = (given charge and time)
Charge (C)/ Time (s)
NTC =
Negative temperature coefficient thermistor - non-linear resistors that alter resistance with temperature
Ideal diode
Allows current to flow in one way but not the other
Energy transfer (J, symbol Q) =
Current (I) x Time (s) x Voltage (V)
Magnetic field lines
Flow from north to south
What is a soft magnetic material?
Iron/ iron-nickel alloy - are easily magnetised/ demagnetised
Hard magnetic materials
Tungsten steel/ chromium steel - retain magnetism for long periods of time
What is induced magnetism?
Magnetic material placed in the field of a magnet - does not retain magnetism out of field
Define the electromagnetic effect?
A current carrying wire produces a magnetic field, noticed by the points of a compass (Right hand thumb rule)
What is an electromagnet?
A soft metal core made into a magnet by the passage of electric current through a coil surrounding it
What 3 things affect the strength of an electromagnet?
Strength of the core material (iron core is best), strength of the current and number of turns of the wire
The motor effect
A wire carrying a current in a magnetic field experiences a force
What is the left hand rule?
Force (thumb) Magnetic field (index) and current (middle finger)
What does the direction of a force depend on?
Direction of magnetic field and direction of the current
What factors can increase the magnitude of a force felt by a wire carrying a current?
Increasing the magnetic field strength, increasing wire length and increasing the current in a wire
Force (N) =
B (magnetic field strength in N/Am or T) x I (current) x Length (m)
How do electric D.C. motors work?
A force on a conductor in a magnetic field causes rotation in motors
How do electric D.C. motors work? (dynamo)
A force on a conductor in a magnetic field causes rotation in motors
How is the force on a conductor in a motor increased?
More coils on the wire, increased current, stronger magnet and reduced friction
What are electromagnets used in?
Motors, generators, electric bells, loudspeakers and headphones
How can you induce a voltage in a wire?
Using a changing magnetic field or by moving the wire in a magnetic field will induce a current to flow
How does an A.C. generator work? (alternator)
As the coil rotates, the p.d. is induced and the alternating current is produced
What is a split ring commutator?
Ring that constantly alters the direction of the flow of current - ensures spin on a dynamo
What does a current/ time graph look like for an alternator?
A transverse wave
What does a current/ time graph look like for a dynamo?
Camel humps
Applications of electromagnetic induction
Microphones - diagram over a permanent coil that moves from air pressure and in loudspeakers - similar to microphones
Vp / Vs =
Np / Ns
How does a transformer work?
A changing magnetic field induces a changing potential difference in the 2nd coil
If a transformer results in 100% total transfer of electrical power, what equation does it result in?
As P = IV, the following is formed: Vp x Ip = Vs x Is
What does voltage measure?
Measures the energy transferred by each Coulomb of charge that flows through a wire
Even though some power is lost to heat in transformers, the power to the primary coil must…
The power supplied to the second (Law of the conservation of energy)
A higher voltage on a transformer means
More efficient transfer
Scalar and vector
Single and double magnitude measurements respectively
Displacement
Distance travelled as the crow flies
Velocity
Speed in a given direction
Speed =
Distance (m) / Time (s)
Velocity
Change in displacement (m) / Time (s)
Acceleration =
Change in velocity (m/s) / Time (s)
In a distance time graph what does the gradient of the line equal?
Speed
In a distance time graph what does a horizontal line mean?
Stationary
In a distance time graph what does a straight slope equal?
Constant speed
In a displacement time graph what does the gradient equal?
Velocity
In a displacement time graph what does a horizontal line mean?
Stationary
In a velocity time graph how do you calculate the displacement?
Area under the graph
In a velocity time graph what does a horizontal line mean?
Constant velocity
In a velocity time graph what does a straight slope mean (upwards or downwards)?
Constant acceleration/ deceleration
V^2 - U^2 =
2as V = final velocity (m/s) U = initial velocity (m/s) a = acceleration (m/s^2) s = displacement (m)
Weight is the
Force of gravity of an object
Normal contact force =
Force exerted upon an object when in contact with another stable object (perpendicular to the surface)
Drag force
Resistance caused by motion of a body through a fluid that opposes the direction of oncoming flow (air resistance)
Friction
Force between 2 sliding surfaces
Magnetic force
Attraction/ repulsion of electrically charged molecules
Electrostatic force
Attraction/ repulsion based on electric charge
Thrust
Push force that causes acceleration in one direction
Upthrust
Resultant upwards force from complete/ partial submersion where the base has a greater pressure
Lift
Component force in a liquid that is perpendicular to the oncoming direction of flow
Tension
Pulling force that is transmitted axially by chain/ rope
How to calculate the magnitude and direction of a force?
Net force (opposites minus) and the strength of any force in an alternate direction
What do unbalanced arrows on a force diagram mean?
Unbalanced forces
Net force =
All forces acting on an object
Force / extension for a rubber band =
Non-linear as shape alters but not constantly
Force / extension graph for a rubber band
Non linear as stretch isn’t constant
Force / extension graph for a spring
Linear initially but once force is too great the shape is changed permanently so becomes non-linear
Elastic extension
Changes shape under force but returns to original when the force is removed
Inelastic extension
Permanently changed in shape under movement of a force
Hooke’s Law equation
Force = K (x)
K = spring constant (N/m) x = extension (m)
Hooke’s Law written
Strain in a solid is directly proportional to the applied stress within the elastic limit of the solid
Energy transferred in stretching (J - same as work done) =
1/2 K x^2
K = Spring constant (N/m) x = Extension (m)
Energy transferred in stretching given only force and extension (J) =
1/2 F x
when F is the force applied in N, and x is the extension in m
Newton’s 1st Law =
A body will remain at rest of in a state of uniform motion in a straight line unless acted upon by a resultant external force
Main property of mass
Resists change in motion (inertia)
Newton’s 2nd Law
Force (N) = Mass (Kg) x Acceleration (m/s^2)
Newton’s 3rd Law
If Body A exerts a force on Body B, then Body B exerts an equal and opposite force of the same type on Body A
Gravitational field strength on Earth?
10 N/ Kg
Weight =
Mass (Kg) x Gravitational field strength (N/ Kg)
What factors affect air resistance?
Mass (higher with a higher mass), Surface area (more room for the working of air resistance) and acceleration due to gravity (works to balance to frictional force over time)
Momentum (Kg m/s) =
Mass (Kg) x Velocity (m/s)
Law of the conservation of momentum
Momentum of an isolated incident must remain constant, so momentum is conserved over time and not created or destroyed
Force is also equal to the rate of change of momentum according to Newton’s 2nd Law - this gives what equation
Force (N) = (Mass (Kg) x Change in velocity (m/s)) / Change in time (s)
Work done =
Force (N) x distance moved in direction of force (m)
1J of work is done when…
1N of force causes movement by 1m - so work done can be measured in Nm
GPE (J) =
Mass (kg) x Gravitational field strength (N/Kg) x Height (m)
Kinetic energy (J) =
1/2 mass (Kg) x Velocity ^2 (m/s)
Power (W) =
Energy transferred (J) / Time (s)
Law of the conservation of energy
Total energy of an isolated system remains constant, so is conserved over time
Useful vs Wasted energy
Can/ cannot be usefully transformed for a respective use
Percentage efficiency =
(Useful output / total input) x 100 (given the same units)
What 4 factors affect the rate of conduction?
Temperature difference (bigger = higher rate), cross sectional area, length and time
What happens when a liquid is heated?
The molecule movement speeds up, sending them further apart, occupying a higher volume which lowers the density
What causes heated particles in a fluid to flow?
Convection - a result of temperature and density variations
What is thermal radiation?
Electromagnetic radiation generated by the thermal motion of particles in matter
Specific heat capacity definition
The energy (heat) required to raise the temperature of the unit mass of a given substance by a usually one degree Celsius
Specific heat capacity (J /kg C or K) =
Thermal energy (J) / Mass (Kg) x Temperature change (C or K)
This equation is the same as Q = mc∆t - where Q is the symbol for heat transfer, m is mass, c is SHC and t is time
Ideal gas behaviour
Random moving particles not subject to inter-particular interactions
Pressure on a gas volume
Constant for an ideal gas if kept at isothermal conditions
Latent heat of fusion
Enthalpy change resulting from providing energy, typically heat, to a specific quantity of substance to change its state from solid to liquid at a constant pressure
Latent heat of vaporisation
Amount of energy added to a liquid substance to form a gas
Density (g/cm^3) =
Mass (Kg) / Volume (cm^3)
Pressure (Pa or N/m^2) =
Force (N) / Area (m^2)
Hydrostatic pressure (N/m^2) =
Height (m) x Density (Kg/m^3 or g/cm^3) x Gravity (N)
How do waves transfer energy?
Without the net movement of water, disturbance causes the energy transfer
Transverse movement
Movement is perpendicular to the movement of energy
Longitudinal movement
Parallel to the movement of the wave
Compression and rarefaction
Close and loose packing respectively
Frequency
Number of waves per point per unit time
Period definition
Time for 1 particle on a medium to make 1 complete vibration per cycle
Frequency (Hz) =
1/ Period (s)
WaveSpeed =
Distance/ Time
Wavespeed (m/s) =
Frequency (Hz) x Wavelength (m)
Law of reflection
Angle of incidence = Angle of reflection
Specular and diffuse reflection
Flat surfaces and rough respectively
Refraction at a boundary
Change of wave direction the result of changed density
Wave speed from less to more dense
Slows down - so bends towards the normal
What property of waves is constant at a boundary?
FREQUENCY
What value is constant with regards to light?
Speed at 330 m/s
What happens to wavespeed from deep to shallow water?
Slows down towards normal with a shorter wavelength
What is the Doppler effect?
Change in the frequency of a wave in relation to an observer moving relating to the wave source.
Receding - Redshifted so lower frequency but longer wavelength
Approaching - Blueshift so higher frequency and smaller wavelength
Plane mirror ray diagram requirements
Upright and laterally inverted, same height object, as far behind as in front and virtual image
Vibrations release…
A sound wave
Can sound waves travel in a vacuum?
No - no particles to transfer energy
Larger amplitude =
Higher sound
Larger pitch =
Smaller frequency (no affect on volume)
Frequency is the same as
Pitch
Human hearing range
20Hz to 20,000Hz
Ultrasound definition
Sound waves released and a picture built up from the time taken to travel - used in sonar and medical imaging
Are EM waves transverse?
Yes - and travel at the CONSTANT speed of light in a vacuum
Order of EM waves from lowest frequency to highest
Radio, micro, IR, visible, UV, X-Rays and Gamma
Radio wave properties
Used in broadcasting and communications, can travel very long distances and are reflected off the upper atmosphere - Lowest energies, lowest frequencies with the longest wavelength
Microwave uses
Cooking food, can penetrate the atmosphere
Infrared uses
Heater and night vision equipment and television remote controls
Visible light uses
Human vision, photography and optical fibres
Ultraviolet use
Fluorescent lamps
X-Ray uses
Medical equipment, view internal structures (absorbed by more dense tissues)
Gamma ray uses
Sterilising food and medical equipment - Highly penetrative - Highest energy, highest frequency and shortest wavelength
Mnemonic for EM waves
Roman Men Invented Very Unusual X-Ray Guns
Hazards of EM waves
Microwaves - Internal heating of body tissues Infrared - Heat/ burning UV - Mutations causing cancer/ blindness X-Rays - Mutations and cell death Gamma rays - Mutations and cell death
Atomic number =
Proton number
Mass number =
Proton + neutron number
2 main features of radioactive emission
Emissions are from an unstable nucleus and random
Alpha emission
Emits a helium nucleus (^4/2 He)
Beta emission
Emits one electron
Gamma emission
No change - just a ray
What does Beta emission do to mass number?
+1
Alpha decay penetrating power and ionisation?
Low penetrating (stopped by paper) and highly ionising
Beta decay penetrating power and ionisation?
Medium penetrating (stopped by a layer of aluminium) and medium ionising
Gamma decay penetrating power and ionisation?
Very penetrating (stopped by a block of lead only) and weakly ionising
What decay can be reflected by electric fields?
Alpha and Beta because they are both charged (Gamma isn’t)
How do Alpha and Beta behave in a magnetic/ electric field?
They experience a deflecting force - if not, they move in parallel motion
Sources of background radiation
Cosmic rays, rocks, radon gas, animals, medical uses and buildings
Radioactivity hazards inside the body
Alpha very dangerous as absorbed by cells, beta and gamma low risk as the pass right through
Radioactivity hazards outside the body
Alpha doesn’t reach cells, beta and gamma penetrate and mutate cells
Half life graph
Activity/ time and demonstrate the RANDOM decay of a material - show how long taken for % of atomic nuclei in a substance to emit spontaneously
How to calculate number of half lives?
(1/2)^n
What do all conductors have?
At least some resistance
What does a neutron change into in Beta emission?
Proton
When is there not always an induced current in a conductor which is experiencing a change in
magnetic field?
When the conductor is not part of a complete circuit
What is red shift?
Further away = longer wavelength and faster moving = redshifted
Do sounds travel faster or slower in a solid?
Faster - closeness of particles
What doesn’t change when medium changes?
Frequency - waves cannot be created or destroyed
Regarding a gear what is radius directly proportional to
Turning force transmitted
