5.1 - Thermal Physics

Question 1

How do you convert from Celsius to Kelvin?

Answer 1

Add 273.
Eg. 10 degrees C = 283 K

Question 2

Why is the absolute scale used?

Answer 2

It doesn’t arbitrarily depend on the properties of a given substance (eg. water’s melting and boiling point for the Celsius scale).
0K (absolute zero) means that the particles have minimum internal energy.

Question 3

Describe the arrangement and energy of particles in a solid, a liquid and a gas.

Answer 3

Solid - regular arrangement, vibrate around fixed positions.
Liquid - close together, constantly moving past each other.
Gas - spaced very far apart, free to move in all directions.

Question 4

How does Brownian motion give evidence for the particle model of matter?

Answer 4

Smoke particles suspended in air can be seen to move randomly in all directions. This must be as a result of random collisions with particles making up the air.

Question 5

What is Internal energy?

Answer 5

The sum of the potential and kinetic energies of a system.

Question 6

True or false: At a given temperature, all particles in a material have the same kinetic energy.

Answer 6

False. The kinetic energies will be randomly distributed around a central ‘most likely’ amount.

Question 7

How can you increase the thermal energy of a system?

Answer 7

We can increase it by heating it up or doing work on the object.

Question 8

Explain the energy changes that occur during a change of state.

Answer 8

During change of state the potential energy of the particles change but the kinetic energies don’t change.

Question 9

What equation can be used to determine the energy required to change the temperature of a substance?

Answer 9

Q = mcΔθ
Where Q = energy, m = mass, c = specific heat capacity, Δ𝜭 = temperature change.

Question 10

What is the specific heat capacity of substance?

Answer 10

The energy required to raise the temperature of 1kg of a substance by 1K.

Question 11

Give the equation to work of the energy for change of state?

Answer 11

Q=ml

Where Q = energy, m = mass, l = specific latent heat (‘of fusion’ if melting/freezing, ‘of vaporisation’ if condensing/evaporating)

Question 12

What is the specific latent heat of a substance?

Answer 12

The energy required to change the state per unit mass of a substance, while keeping the temperature constant.

Question 13

In an experiment to find ‘c’ for water, lots of energy input escapes to the surroundings. Will this lead to an over or underestimate of specific heat capacity?

Answer 13

• An overestimate.
• Specific heat capacity is calculated as: c = Q / mΔ𝜭
• The energy input will be used, but the temperature change of the water will be lower than it should be due to the escaped energy - therefore c will be too high.

Question 14

What is Avogadro’s constant? (in words)

Answer 14

The number of atoms there are in one mole of a substance.

Question 15

What are the key assumptions in the kinetic theory of gases?

Answer 15

• There are a large number of molecules in random, rapid motion.
• Particles are negligibly small compared to the total volume of the gas.
• All collisions are perfectly elastic.
• The time taken for a collision is negligibly small compared with the time between collisions.
• Between collisions there are no forces between particles.

Question 16

Why do gases exert a pressure on the container they’re in?

Answer 16

• Gas particles collide with the surfaces of the container.
• The container exerts a force on the particles to change their direction. The particles exert an equal and opposite force on the container.
• Pressure is force applied (in total, by all particles) per unit area.

Question 17

What is an ideal gas?

Answer 17

A gas where:
* The gas molecules don’t interact with each other.
* The molecules are thought to be perfectly spheres.

Question 18

What is the ideal gas equation?

Answer 18

pV=nRT

Where p = pressure, V = volume, n = number of moles, R = the ideal gas constant, T = absolute temperature

Question 19

What is Boyle’s law?

Answer 19

Pressure is inversely proportional to volume, providing temperature is constant.
i.e. pV = constant

Question 20

Assuming constant volume, how are the pressure and temperature of a gas related?

Answer 20

They’re directly proportional.
ie. P/T = constant

Question 21

Use the kinetic theory of gases to explain why a temperature increase leads to an increase in pressure.

Answer 21

● A temperature increase means the particles have more kinetic energy.
● More kinetic energy means a greater change in momentum during collisions with the container. There are also more frequent collisions.
● Change in momentum is proportional to force applied, and therefore to pressure as well.

Question 22

What equation links N, V, p, m and c?

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Answer 22

pV = 1/3 Nmc²
Where p = pressure, V = volume, N = number of particles, m = mass of a particle, ‘c’ = mean square speed.

Question 23

What is meant by the root mean square speed?

Answer 23

The square root of the mean of the squares of the speeds of the molecules.

Question 24

What does the area under a Maxwell-Boltzmann curve represent?

Answer 24

The total number of particles

Question 25

How does the Maxwell-Boltzmann curve change if the temperature of a gas is increased?

Answer 25

The average particle speed, and maximum particle speed both increase (curve shifts right).
The curve becomes lower and more spread out.

Question 26

What are the units of the Boltzmann coefficient?

Answer 26

J/K

Question 27

The average kinetic energy of a particle in an ideal gas is equal to what?

Answer 27

1.5 kT

Question 28

True or false: ‘The internal energy of an ideal gas is proportional to absolute temperature’

Answer 28

True.
In an ideal gas there is no ‘potential energy’ component in the internal energy. This means the internal energy is proportional to the kinetic energy (which is, in turn, dependent on temperature).