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AQA A Level Physics Paper 2 > Thermal Physics > Flashcards

Thermal Physics Flashcards

1
Q

Define Internal energy

A

The internal energy of a body is the sum of the randomly distributed kinetic and potential energies of all its particles

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2
Q

Do all particles in a body travel at the same speed?

A
  • The particles in a body don’t all travel at the same speed
  • Some particles will be moving fast but others much more slowly. The speeds of all the particles are randomly distributed so kinetic energy is randomly distributed too. The largest proportion will travel at about the average speed
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3
Q

What does the distribution of the speeds of particles in a body depend on?

A

The distribution of particle speeds depends on the temperature of the body. The higher the temperature, the higher the average kinetic energy of the particles

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4
Q

What do the potential energies of particles in a body depend on?

A

The particles in a body have randomly distributed potential energies that depend on their relative positions

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5
Q

What is a system?

A

A system is just a group of bodies considered as a whole

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6
Q

Define a closed system

A

A closed system is one which doesn’t allow any transfer of matter in or out. For a closed system, the total internal energy is constant as long it’s not heated or cooled and no work is done

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7
Q

How is energy transferred within a system?

A

Energy is constantly transferred between particles within a system through collisions between particles but the total combined energy of all the particles doesn’t change during these collisions

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8
Q

How can the internal energy of a system be increased/decreased?

A
  • The internal energy of a system can be increased by heating it or by doing work to transfer energy to the system (eg. by changing its shape)
  • The internal energy can be reduced by cooling the system or by doing work to remove energy from the system
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9
Q

What is the effect of a change in internal energy on the average kinetic and potential energies of the particles in a system?

A

Changes in the internal energy will cause the average kinetic and/or potential energy of the particles to change as a result of energy being transferred to or from the system

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10
Q

What is the relationship between internal energy and changes of state?

A

A change of state means a change of internal energy

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11
Q

Describe how the internal energy changes when a substance changes state

A

When a substance changes state its internal energy changes but its kinetic energy stays the same. This is because the potential energy of the particles is altered, not their kinetic energy

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12
Q

Explain what happens to the kinetic energy of the particles in a substance and its temperature when you heat it

A

When you heat something, its particles get more kinetic energy and its temperature rises

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13
Q

Define Specific heat capacity

A

The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1kg of the substance by 1K

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14
Q

State the specific heat capacity equation and its variables

A
  • Q=mcΔθ
  • Q is the energy change
  • m is the mass
  • c is the specific heat capacity
  • Δθ is the change in temperature
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15
Q

What are the units of specific heat capcaity?

A

J/kg/K or J/kg/Celsius

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16
Q

What are the two ways the internal energy of a substance can be increased?

A
  • By heating it
  • By doing work on the system, e.g. by compressing it
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17
Q

What are the two ways the internal energy of a substance can be decreased?

A
  • By removing heat from it (cooling it)
  • By work being done by the system, e.g. by expanding it if is a gas
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18
Q

State the equation given by the first law of thermodynamics

A

Increase in internal energy = Heat supplied to a system + Work done on the system

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19
Q

State the formula used to calculate the Heat gain/loss per second for a heater heating a substance

A

Heat loss/gain per second = massSHCTemperature rise/gain per second

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20
Q

State the formula used to calculate the Heat gain/loss per second for a flowing liquid

A

Heat gain/loss per second = mass flow per secondSHCTemperature rise/fall

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21
Q

Explain the method of finding the specific heat capacity of water using a continuous-flow calorimeter

A

1- Set up the apparatus and let water flow at a steady rate until the water out is a constant temperature
2- Record the flow rate of the water and the duration of the experiment, t, to find the mass of the water. You also need to measure the temperature difference ,Δθ, of the water from the point that it flows in to the point that it flows out between the thermometers. Also record the current and potential difference
3- The energy supplied to the water is Q = mcΔθ + H where H is the heat lost to the surroundings
4- Repeat the experiment changing only the pd of the power supply and the flow rate (mass) so Δθ remains constant. You should now have an equation for each experiment:
Q1 = m1cΔθ + H and Q2 = m2cΔθ + H
5- The values of c, Δθ and H are the same so Q2-Q1 = (m2-m1)cΔθ. Rearranging for c gives: c = Q2-Q1/(m2-m1)Δθ
6- Q is just the electrical energy supplied over time in each case so you can use Q=VIt to find Q1 and Q2 and therefore c, the specific heat capacity of water

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22
Q

Define continuous-flow heating

A

Continuous-flow heating is when a fluid flows continuously over a heating element. As it flows, energy is transferred to the fluid

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23
Q

Define specific latent heat

A

The specific latent heat (l) od fusion or vaporisation is the quantity of thermal energy required to change the state of 1kg of a substance

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24
Q

What is the formula used to calculate specific latent heat and its variables?

A

Q=ml
- Q is the energy change in J
- m is the mass in kg
- l is the specific latent heat

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25
Q

What are the units of specific latent heat?

A

J/kg

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26
Q

Define absolute zero

A
  • Absolute zero is the temperature where the pressure of an ideal gas becomes zero
  • It is given a value of zero kelvins on the absolute temperature scale
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27
Q

What is the kinetic energy of particles at 0K and how does this energy change at higher temperatures?

A

At 0K all particles have the minimum possible kinetic energy, everything pretty much stops. At higher temperatures particles have more energy.

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28
Q

What does a change of 1K equal?

A

A change of 1K equals a change of 1 degrees celsius

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29
Q

What is the formula used to convert between kelvin and degrees celsius?

A

K = C + 273

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30
Q

What unit of temperature do all equations in thermal physics use?

A

Kelvin

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31
Q

What are the three gas laws?

A
  • Boyle’s law
  • Charles’s law
  • The Pressure law
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32
Q

What quantity of mass does each of the gas laws apply to?

A

Each of the gas laws applies to a fixed mass of gas

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33
Q

State Boyle’s law

A
  • At a constant temperature the pressure p and volume V of a gas are inversely proportional
  • P∝1/V
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34
Q

Using Boyle’s law, explain what happens if you reduce the volume of a gas

A

If you reduce the volume of a gas, its particles will be closer together and will collide with each other and the container more often so the pressure increases

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35
Q

What is an ideal gas in terms of Boyle’s law?

A

A gas that obeys Boyle’s law at all temperatures is call an ideal gas

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36
Q

On a graph of pressure against volume explain the relationship between the distance of curves from the origin and the temperature of the gas

A

The higher the temperature of the gas the further the curve is from the origin

37
Q

State Charles’s law

A
  • At constant pressure, the volume V of a gas is directly proportional to its absolute temperature T
  • V∝T
38
Q

Using Charles’s law, explain what happens when you heat a gas?

A

When you heat a gas the particles gain kinetic energy. At a constant pressure, this means they move more quickly and further apart, and so the volume of the gas increases

39
Q

Which gas laws do ideal gases obey?

A

Ideal gases obey all three gas laws

40
Q

For an ideal gas, where does the line meet the temperature axis on a graph of volume against temperature?

A
  • For any ideal gas the line meets the temperature axis at -273.15 degrees Celsius which is absolute zero.
  • If the temperature was in kelvins instead the line would go through the origin
41
Q

Draw a graph of volume against temperature for an ideal gas

A

See page 110 in the revision guide

42
Q

State the pressure law

A
  • At constant volume, the pressure p of a gas is directly proportional to its absolute temperature T
  • p∝T
43
Q

Using the pressure law, explain what happens if you heat a gas

A

If you heat a gas, the particles gain kinetic energy. This means they move faster. If the volume doesn’t change the particles will collide with each other and their container more often and at higher speed, increasing the pressure inside the container

44
Q

What is the relationship between the graphs of volume against temperature and pressure against temperature for ideal gases?

A

They are both the same

45
Q

Explain the method for the experiment to investigate Boyle’s law (the effect of pressure on volume)

A

1- To investigate the effect of pressure on volume set up the apparatus
2- The oil traps a pocket of air in a sealed tube with fixed dimensions. A tyre pump is used to increase the pressure on the oil and the Bourdon gauge records the pressure
3- As the pressure increases, more oil will be pushed into the tube, the oil level will rise and the air will compress. The volume occupied by the air in the tube will reduce
4- Measure the volume of air when the system is at atmospheric pressure, then gradually increase the pressure keeping the temperature constant. Note down both the pressure and the volume of air as it changes. Multiplying these together at any point should give the same value.
5- If you plot a graph of p against 1/V you should get a straight line

46
Q

Explain the method for the experiment to investigate Charles’s law ( effect of temperature on volume)

A

1- To investigate the the effect of temperature on volume set up the apparatus
2- A capillary tube is sealed at the bottom and contains a drop of concentrated sulfuric acid halfway up the tube, this traps a column of air between the bottom of the tube and the acid drop. The beaker is filled with near-boiling water and the length of the trapped column of air increases. As the water cools, the length of the air column decreases
3- Regularly record the temperature of the water and the air column length as the water cools. Repeat with fresh near-boiling water twice more letting the tube adjust to the new temperature between each repeat. Record the length at the same temperatures each time and take an average of the three results
4- If you plot your average results on a graph of length against temperature and draw a line of best fit, you will get a straight line
5- This shows that the length of the air column is proportional to the temperature. The volume of the column of air is equal to the volume of a cylinder which is proportional to its length so the volume is also proportional to the temperature. This agrees with Charles’s law

47
Q

Define the molecular mass of a gas

A

Molecular mass is the sum of the masses of all the atoms that make up a molecule

48
Q

What is molecular mass usually given relative to?

A

Molecular mass is usually given relative to the mass of a carbon-12 atom. This is known as relative molecular mass. Carbon-12 has a relative mass of 12. Hydrogen atoms have a relative mass of 1 but hydrogen molecules are made up of two hydrogen atoms so the relative molecular mass of hydrogen is 2

49
Q

How do you calculate the relative molecular mass of a gas?

A

Break the gas down into each of the elements that make up the gas and add their respective mass numbers

50
Q

Define the term molar mass

A

The molar mass is the mass of one mole of a gas

51
Q

Where does the mole as a unit come from?

A

At a fixed pressure and temperature, a fixed volume of gas will contain the same amount of gas molecules no matter what the gas is. This leads to the unit of a mole

52
Q

What is a mole in relation to a gas?

A

One mole of any gas contains the same number of particles. This number is called Avogadro’s constant and it has the symbol Na

53
Q

What is the molar mass of a substance?

A
  • The molar mass of a substance is the mass that 1 mole of that substance would have (usually in grams).
  • It is equal to the relative molecular mass of that substance.
54
Q

What is the formula used to calculate the number of molecules in a substance and what are the variables?

A
  • N=nNa
  • n is the number of moles in a substance
  • Na is Avogadro’s constant
55
Q

What is the formula used to calculate the number of molecules in a certain mass of a substance?

A
  • N = NaMs/M
  • N is the number of molecules
  • Na is Avogadro’s constant
  • Ms is the mass of the substance
  • M is the molar mass of the substance
56
Q

How do you get the ideal gas equation?

A

You get the ideal gas equation by combining all three gas laws. By doing so you get the equation - pV/T = constant

57
Q

What does the constant in the ideal gas equation depend on and what is the amount of gas measured in?

A

The constant depends on the amount of gas used and the amount of gas can be measured in moles

58
Q

What is the ideal gas equation and each of its variables?

A

-pV = nRT
- P is pressure
- V is volume
- n is the number of moles
- R is the molar gas constant
- T is the temperature

59
Q

Define an ideal gas

A

An ideal gas is one which obeys the ideal gas equation at all temperatures and pressures

60
Q

What does the ideal gas equation work really well for?

A

It works well for gases at low pressures and fairly high temperatures

61
Q

What formula can be used to find out unknown pressures, temperatures and volumes of gases?

A

p1V1/T1=p2V2/T2

62
Q

What is Boltzmann’s constant?

A
  • Boltzmann’s constant ,k, is equivalent to R/Na (molar gas constant/Avogadro’s constant)
  • You can think of Boltzmann’s constant as the gas constant for one particle of gas while R is the gas constant for one mole of gas
63
Q

State the equation of state for an ideal gas

A
  • pV=NkT
  • k is Boltzmann’s constant
64
Q

Explain how work is done to change the volume of a gas at constant pressure

A
  • For a gas to expand or contract at constant pressure, work must be done, there must be a transfer of energy
  • This normally involves the transfer of heat energy
65
Q

What equation can be used to calculate the work done in changing the volume of a gas at a constant pressure?

A
  • Work done = pΔV
  • p is the pressure
  • ΔV is the change in volume
66
Q

How can you work out the energy transferred to change the volume of a gas from a graph of pressure against volume?

A

Find the area under the graph

67
Q

Derive the kinetic theory equation (pV=1/3Nm(Crms)^2) for the x direction (part1)

A

Imagine a cubic box with sides of length l containing N particles each of mass m
1- Say particle Q moves directly towards wall A with velocity u. Its momentum approaching the wall is mu. It strikes wall A. Assuming the collisions are perfectly elastic, it rebounds and heads back in the opposite direction with momentum -mu. So the change in momentum is -mu-mu = -2mu
2- Assuming Q suffers no collisions with other particles the time between collisions of Q and wall A is 2l/u. The number of collisions per second is therefore u/2l
3- This gives the rate of change of momentum as -2mu*u/2l
4- As force equals the rate of change of momentum (Newton’s second law) the force exerted by the wall on this one particle = -2mu^2/2l = -mu^2/l
5- As particle Q is only one of many in the cube. Each particle will have a different velocity u1, u2 etc towards A. The total force F, of all these particles on wall A is F=m(u1^2+u2^2+…)/l
6- You can define a quantity called the mean square speed , ubarsquared as: ubarsquared = u1^2+u2^2+…/N
7- If you put that into the equation above you get: F=Nmubarsquared/l
8- So the pressure of the gas on end A is given by: pressure = force/area = Nmubarsquared/l/l^2 = Nmubarsquared/l^3 = Nmubarsquared/V
9- This is the pressure in the x direction so for general equation we need to think about all 3 directions (x, y and z)

68
Q

Derive the kinetic theory equation (pV=1/3Nm(Crms)^2) for all 3 directions (part2)

A

A gas particle can move in three dimensions
1- You can calculate its speed, c, from Pythagoras’ theorem in 3 dimensions: c^2 = u^2 +v^2+w^2 where u, v and w are the components of the particle’s velocity in the x, y and z directions.
2- If you treat all N particles in the same way this gives an overall mean square speed of cbarsquared = ubarsquared + vbarsquared + wbarsquared
3- Since the particles move randomly: ubarsquared = vbarsquared = wbarsquared so cbarsquared = 3ubarsquared and so ubarsquared = cbarsquared / 3
4- You can substitute this into the equation for pressure that you derived above to give: pV= 1/3Nmcbarsquared

69
Q

Derive the final part of the kinetic theory equation (pV=1/3Nm(Crms)^2) (part3)

A

1- cbarsquared is the mean square speed and has units m^2s^-2
2- cbarsquared is the average of the square speeds of all the particles so the square root of it gives you the typical speed
3- This is called the root mean square speed or usually the rms speed. It is often written as Crms and has units ms^-1
4- √cbarsquared = Crms
5- Therefore we can rewrite the equation as pV=1/3Nm(Crms)^2)

70
Q

What are the simplifying assumptions used Kinetic theory?

A

1- The molecules continually move about randomly
2- The motion of the molecules follow Newton’s laws
3- Collisions between molecules themselves or at the walls of a container are perfectly elastic
4- Except for during collisions the molecules always move in straight lines
5- Any forces that act during collisions last for much less time than the time between collisions

71
Q

Explain the existence of energy of an ideal gas

A
  • Ideal gases have an internal energy that is dependent only on the kinetic energy of their particles
  • The potential energy = 0J as there are no forces between them except when they are colliding.
  • Real gases behave like ideal gases as long as the pressure isn’t too big and the temperature is reasonably high
72
Q

Why does a gas exert a force on a wall?

A
  • The gas molecules collide elastically with the walls of the container
  • Their velocity will change direction but keep the same magnitude
  • This causes the component of their momentum perpendicular to the wall to change
  • By Newton’s second law the molecules must have a resultant force on them to cause this change in momentum
  • The resultant force is exerted by the wall on the molecules
  • By Newton’s third law if the wall puts a force on the molecules the molecules must put an equal but opposite force on the wall
  • The forces from all the molecules cause a pressure on the wall, which can be calculated by pressure = force/area
73
Q

Why does increasing the temperature of a gas increase the pressure?

A
  • If the temperature increases, the average kinetic energy of the molecules increases so their average speed increases
  • Therefore on average there will be a greater change in momentum when a molecule rebounds from a wall
  • Therefore the rate of change of momentum of molecules hitting the walls increases so the force on the walls increases
  • As pressure=Force/area the pressure increases
74
Q

Why does decreasing the volume of a gas increase the pressure?

A
  • As the volume of the gas decreases there will be more collisions of molecules with the container walls each second
  • Therefore the rate of change of momentum of molecules hitting the walls will increase, so the force on the wall will increase as well as the pressure as pressure = Force / area
75
Q

Calculate the mean squared speed and the mean speed squared and show the difference between them for the numbers 1, 2, 3, 4, 5, 6

A

See page 13 in the gases and kinetic theory pack

76
Q

What is the relationship between the average kinetic energy of a gas and its absolute temperature?

A

Average kinetic energy is proportional to absolute temperature

77
Q

Show that the average kinetic energy of a particle is directly proportional to absolute temperature through derivation

A

1- The ideal gas equation is pV=nRT
2- The pressure of an ideal gas given by kinetic theory is pV=1/3Nm(Crms)^2
3- Equating these: 1/3Nm(Crms)^2 = nRT
4- Multiply both sides by 3/2 and dividing by N gives 1/2m(Crms)^2 = 3/2nRT/N
5- 1/2m(Crms)^2 is the average kinetic energy of a particle
6- Substitute Nk for nR where k is the boltzmann constant to give 1/2m(Crms)^2 = 3/2kT. This shows the proportionality relationship
7- The Boltzmann constant is equal to R/Na so you can substitute this for k to give 1/2m(Crms)^2=3/2RT/Na

78
Q

What type of laws are the gas laws and what does this mean?

A
  • The gas laws are empirical
  • Empirical laws are based on observations and evidence. This means that they can predict what will happen but they don’t explain why
79
Q

What type of law is kinetic theory and what does this mean?

A
  • Kinetic theory is a theoretical law
  • This means its based on assumptions and derivations from knowledge and theories we already had
80
Q

Explain the relationship between Ancient Greek and Roman Philosophers and what we now know about gases

A

Ancient Greek and Roman Philosophers including Democritus had ideas about gases 2000 years ago, some of which were quite close to what we now know to be true

81
Q

Describe the order in which the 3 gas laws were discovered

A

1- Robert Boyle discovered the relationship between pressure and volume at a constant temperature in 1662 - Boyle’s law
2- This was followed by Charles’s law in 1787 when Jacques Charles discovered that the volume of a gas is proportional to temperature at a constant pressure
3- The pressure law was discovered by Guillaume Amontons in 1699 who noticed that at a constant volume, temperature is proportional to pressure. It was then re-discovered much later by Lussac in 1809

82
Q

Explain how kinetic theory was discovered

A

In the 18th century a physicist called Daniel Bernoulli explained Boyle’s law by assuming that gases were made up of tiny particles, the beginnings of kinetic theory. It took another couple of hundred years before kinetic theory became widely accepted

83
Q

Who discovered Brownian motion?

A

Robert Brown discovered Brownian motion in 1827 which helped support kinetic theory

84
Q

Explain in terms of kinetic theory why scientific ideas aren’t accepted straight away

A
  • The scientific community only accepts new ideas when they can be independently validated - when other people can reach the same conclusions
  • In the case of kinetic theory, most physicists thought it was just a useful hypothetical model and atoms didn’t really exist. It wasn’t until the 1900s when Einstein was able to use kinetic theory to make predictions for Brownian motion that atomic and kinetic theory became widely accepted
85
Q

What is Brownian motion and how was it discovered?

A
  • In 1827 botanist Robert Brown noticed that pollen grains in water moved with a zigzag, random motion
  • This type of movement of any particles suspended in a fluid is known as Brownian motion. It supports the kinetic particle theory of the different states of matter. It says that the random motion is a result of collisions with fast, randomly-moving particles in the fluid
  • You can see this when large, heavy particles are moved with Brownian motion by smaller, lighter particles travelling at high speeds
  • This is evidence that the air is made up of tiny atoms or molecules moving really quickly
  • So Brownian motion really helped the idea that everything is made from atoms gain acceptance from the scientific community
86
Q

What is meant by Brownian motion?

A

particles suspended in a medium follow a random walk

87
Q

What is meant by a random walk?

A

particles move in straight lines, of random length and in random directions

88
Q

What is Brownian motion of (for example smoke) particles due to?

A

due to collision with very small air molecules

AQA A Level Physics Paper 2 (6 decks)

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