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OCR A level Physics Revision guide/Summary (A level)
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Contents5.1 Thermal physics ..................................................................................................................................................................................
4
Temperature ........................................................................................................................................................................................
4
Energy and the three states of matter ................................................................................................................................................ 4
Properties of substances .....................................................................................................................................................................
6
Ideal gases ...........................................................................................................................................................................................
8
5.2 Circular motion ...................................................................................................................................................................................
11
Introduction to circular motion ...........................................................................................................................................................
11
Centripetal forces ................................................................................................................................................................................
11
5.3 Oscillations .........................................................................................................................................................................................
13
Introduction to oscillations .................................................................................................................................................................
13
5.4 Gravitational fields .............................................................................................................................................................................
17
Introduction to gravitational fields ......................................................................................................................................................
17
Planetary motion .................................................................................................................................................................................
18
Gravitational potential and energy ......................................................................................................................................................
19
5.5 Astrophysics and cosmology .............................................................................................................................................................. 20
Objects in the Universe .......................................................................................................................................................................
20
Stars .....................................................................................................................................................................................................
20
Analysing electromagnetic radiation from stars .................................................................................................................................. 22
Measuring distances in astronomy ..................................................................................................................................................... 24
Cosmology ...........................................................................................................................................................................................
25
Module 5 ticklist .................................................................................................................................................................................
29
6.1 Capacitors and capacitance ................................................................................................................................................................
30
The exponentional nature of charge, current and p.d. ....................................................................................................................... 31
Uses of capacitors ...............................................................................................................................................................................
33
6.2 Electric fields .......................................................................................................................................................................................
35
Introduction to electric fields ..............................................................................................................................................................
35
Uniform electric fields .........................................................................................................................................................................
36
Electric potential and energy ..............................................................................................................................................................
37
6.3 Magnetic fields ...................................................................................................................................................................................
39
Magnetic fields and field lines .............................................................................................................................................................
39
Charged particles in uniform magnetic fields ...................................................................................................................................... 41
Electromagnetic induction ..................................................................................................................................................................
42
6.4 Nuclear and particle physics ...............................................................................................................................................................
46
Alpha particle scattering experiment .................................................................................................................................................. 46
Types of particle ..................................................................................................................................................................................
47
Quarks .................................................................................................................................................................................................
48
Beta decay ...........................................................................................................................................................................................
48
Radioactive decay ................................................................................................................................................................................
49
Nuclear reactions, fission and fusion .................................................................................................................................................. 52
6.5 Medical imaging .................................................................................................................................................................................
56
Producing and using x-rays ..................................................................................................................................................................
56
Diagnostic methods in medicine .........................................................................................................................................................
58
Using ultrasound .................................................................................................................................................................................
60
Module 6 ticklist .................................................................................................................................................................................
63
, 5.1 - Thermal physics
Temperature
- Temperature is a measure of the hotness of an object
- Thermal energy will transfer from a hotter object to a colder one until both objects reach the same temperature
- If there is no net flow of thermal energy between two objects, they must be the same temperature and are in
thermal equilibrium
Temperature scales:
- A temperature scale requires two fixed points with defined temperatures
- Celsius scale:
o Measured in degrees Celsius
o Uses the freezing and boiling points of water (at a certain pressure) as fixed temperatures – 0oC and 100oC
respectively
o The problem with this is that freezing/boiling points of water vary with surrounding atmospheric pressure, so
it’s difficult to use the scale in the real world
- Absolute/Thermodynamic scale:
o Measured in Kelvin
o Uses absolute zero and the triple point of pure water as fixed points
o Absolute zero is the lowest possible temperature (where particles have no kinetic energy)
o The triple point of water is the only temperature at which water can exist in all three states in thermal
equilibrium
o The absolute temperature scale is better suited for scientific use since its fixed points do not depend on the
surrounding conditions (e.g. atmospheric pressure)
o For ease of use, the absolute temperature scale uses the same size increments as the Celsius scale:
Temperature in Kelvin ≈Temperature in degrees Celsius+273
T(K)≈T(℃)+273
Energy and the three states of matter
The kinetic model of solids, liquids and gases:
- Solids:
o Particles (atoms or molecules) are arranged in a regular, 3D structure
o The stable structure means there are strong electrostatic forces of attraction
between the particles, preventing them from moving apart
o The particles have kinetic energy, which causes them to vibrate without
leaving their position in the structure
o Since the particles are usually closest together, solids tend to be most dense
- Liquids:
o The forces of attraction between the particles are much weaker
o This means they can move around without retaining any structure or shape
o The particles have more kinetic energy than in solids of the same type of
atom/molecule, resulting in particles having a fast, random motion
- Gases:
o The forces of attraction between particles are tiny (negligible)
o This means the particles are much further apart and so occupy a larger
volume than if a liquid or a gas, making them the least dense of the three states
o The particles have more kinetic energy than liquids, meaning they move with
random speeds and directions – this is Brownian motion (see next page)
4
, 5.1 - Thermal physics
microscope
Observing Brownian motion:
lamp
- Brownian motion is the erratic, random motion of particles
in a fluid (a liquid or a gas)
- Particles move at fast but random speeds in random
directions as a result of frequent elastic collisions with other
particles in the fluid
- Brownian motion can be observed using a smoke cell, in
lens to focus light glass cell with smoke which smoke particles are suspended in air (opposite)
particles
Internal energy:
- The internal energy of a substance is the sum of the randomly distributed kinetic and potential energies of the
atoms/molecules in the system
o Kinetic energy depends on the speed and mass of the particles
o Potential energy depends on the electrostatic forces of attraction between the particles and is negative –
it’s the work required to completely separate the particles (meaning a gas has greater potential energy
(e.g. -0.5J) than a liquid (e.g. -3.5J))
Changing the internal energy of a body:
- When energy is given to a body, its internal energy will increase
- Whether this becomes kinetic or potential energy depends on the temperature of the body
- Typically, giving energy to a body will increase the kinetic energy of its particles and thus the temperature of the body
- If the temperature of the body is a melting/boiling point, the energy is used to increase the body’s potential energy
– the body changes state
- Increasing the potential energy makes it less negative – less energy is required to separate the atoms/molecules -
this means the potential energy is greatest in a gas (close to 0J)
- Increasing the potential energy means work is done against the attraction between the particles (‘breaking bonds’),
moving them further apart
- This idea is best explained with a temperature-time graph for a substance being supplied with a constant power:
Temp (K)
s
ga
boiling
id
liqu
melting
lid
so
Time (s)
- When an object freezes or condenses, its internal energy decreases and so energy is released to the surroundings
5
, 5.1 - Thermal physics
Properties of substances
Specific heat capacity, C:
- The energy per unit mass required to change the temperature of a substance by 1K (or 1oC)
- where E is the energy supplied to the substance, m is the mass of the substance and ∆θ is the change in
temperature of the substance Heater circuit
- This is commonly written as E=mc∆θ
Determining the specific heat capacity of a substance: Variable
resistor
- You need to know how to determine the specific heat capacity of a (solid) metal or
a liquid
Thermometer
- Equipment for a metal:
1. A metal block is hollowed out in two places to allow a thermometer and electric
heater to be placed inside, close to the middle
2. Measure the mass of the metal block using a digital balance
3. Place the heater and thermometer in the block (as shown), and Metal block
Insulation
surround the block with insulation
4. Connect the heater to a circuit with an ammeter in series and Heater circuit
voltmeter in parallel
- Equipment for a liquid: Variable
1. Measure the mass of the liquid using a digital balance resistor
(by zeroing the balance with a container, then pouring
the liquid into the container)
2. Place the heater and thermometer in the container Insulating lid
3. Surround the container with insulation, including an
Thermometer/stirrer
insulating lid with holes to allow the heater/thermometer
to enter
4. Connect the heater to a circuit with an ammeter in
series and voltmeter in parallel Liquid
- Method: Insulation
1. Record an initial temperature reading from the thermometer. Turn on the heater and start a stopwatch
2. Every 10 seconds, record the temperature on the thermometer and the ammeter and voltmeter readings. If
using a liquid, stir regularly using the thermometer to ensure heat is evenly distributed
3. After 10 or so minutes, turn off the heater and stop the stopwatch Temp (K)
4. From your data, determine the mean current supplied and potential Gradient =
difference across the
heater – the product of these is the mean power of the heater
5. From , we can divide by the time t to have Time (s)
6. From the data, a temperature-time graph can be drawn. Since the gradient is ,
we have , where P is the mean power supplied and m is the measured mass of the metal
- Determining specific heat capacity using method of mixtures:
o Two known masses of different liquids at known temperatures are mixed together and left in an insulated
container until they reach thermal equilibrium
o Once at thermal equilibrium, their final temperature is recorded
o Since the energy gained by one liquid is equal to the energy lost by the other (assuming none was lost
through the insulation):
o Thus, given the specific heat capacity of one of the liquids, we can find the other
6
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