Topic 3: Thermal Physics

Question 1

Define temperature.

Answer 1

A measure of the average random Ek of the molecules in an object. It is a scalar quantity.

Question 2

Define internal energy.

Answer 2

Internal energy is the sum of the total intermolecular potential energy of the molecules and the total random kinetic energy of the molecules.

Question 3

What are the two types of kinetic energy of molecules?

Answer 3

Translational (whole molecules moving in a certain direction).
Rotational (molecules rotating about an axis).

Question 4

Define thermal energy/heat.

Answer 4

The flow of energy from one object to another due to temperature difference.

Question 5

Describe the phases of matter.

Answer 5

Solid:
Fixed volume and shape. Strong forces between molecules. Molecules vibrate about a mean position.

Liquid:
Fixed volume and variable shape (assumes that of the container). Strong forces between molecules. Molecules vibrate.

Gas:
Volume and shape fill container. Very weak forces between molecules. Molecules move independently in all directions.

Question 6

Describe the differences between boiling and evaporating.

Answer 6

Boiling takes place throughout a liquid and always occurs at the same temperature for that specific liquid while evaporation occurs at the surface of a liquid and can happen at all temperatures.

Question 7

Define thermal capacity.

Answer 7

The amount of energy required to raise the temperature of an object by 1K. Used for objects. Measured in J K-1 or J °C-1.

Question 8

Define specific heat capacity.

Answer 8

The amount of energy required to raise the temperature of the unit mass of a substance by 1K. Used for substances. Measured in J kg-1 K-1.

Question 9

Why do substances have different specific heat capacities?

Answer 9

Because different substances have different molecular structures, forces between molecules and densities.

Question 10

Define specific latent heat.

Answer 10

The amount of energy per unit mass of a substance absorbed or released during change of phase without a change in temperature. Measured in J kg-1.

Solid <-> Liquid (Fusion)
Liquid <-> Gas (Vaporisation)

Question 11

Why does the temperature not change during a phase change?

Answer 11

Because the molecules in the substance move further apart as bonds between molecules are broken (or closer together if bonds are formed) but their speeds does not change. The molecules therefore gain or lose potential energy but NOT kinetic energy. Temperature is dependent on the average kinetic energy of the molecules and because this does not change, the temperature remains constant.

Question 12

Compare the latent heat of vaporisation with the latent heat of fusion.

Answer 12

The latent heat of vaporisation is larger than the latent heat of fusion for any substance. This is because when changing from a solid to a liquid the molecules are only moved slightly further apart. However, the change from a liquid to a gas requires the molecules to be moved far apart so they are completely separated to form a gas. This requires more energy therefore the latent heat of vaporisation is larger than that of fusion.

Question 13

Define mole.

Answer 13

The amount of a substance that contains the same number of elementary entities as the number of atoms in 0.012 kg of 12C.

Question 14

Define Avogadro’s constant.

Answer 14

The number of atoms in 0.012 kg of 12C.
6.022 x 10²³

Question 15

Define molar mass.

Answer 15

The mass of one mole of a substance.

Question 16

What is an ideal gas?

Answer 16

A gas that follows the three gas laws for all values of p, V and T.

Question 17

What is assumed for an ideal gas?

Answer 17

• They undergo perfectly elastic collision between themselves and the walls of their container (no kinetic energy is lost in collisions).
• They have no intermolecular forces between them (therefore internal energy = Ek).
• The particles have negligible volume.

Question 18

When is a real gas approximate to an ideal gas?

Answer 18

At conditions of low pressure, low density and moderate temperature.

Question 19

How can you change pressure?

Answer 19

By changing the number of collisions per unit area, the number of collisions per unit time and the force per collision.

Question 20

Give the Pressure Law.

Answer 20

At a constant volume, pressure is proportional to temperature (in K). Key word: isovolumetric.

Question 21

Describe the Pressure Law.

Answer 21

When there is an increase in the temperature of a gas, the average kinetic energy of the molecules increases. If the volume stays constant, this increases the number of collisions per unit time and the average force per collision between the gas molecules and the container. Therefore, there is an increase in the total force on the container so the pressure increases.

P ∝ T

Question 22

Give Charles’ Law.

Answer 22

At a constant pressure, volume is proportional to temperature (in K). Key word: isobaric.

Question 23

Describe Charles’ Law.

Answer 23

When there is an increase in the temperature of a gas, the average kinetic energy of the molecules increases. If the volume also increases there is a decrease in the number of collisions per unit time per unit area but there is a greater average force per collision between the gas molecules and the container. The average force per unit area of the container remains constant, so the pressure remains constant.

Question 24

Give Boyle’s Law.

Answer 24

At a constant temperature, pressure is inversely proportional to volume. Key word: isothermic.
P∝1/V

Question 25

Describe Boyle’s Law.

Answer 25

If the temperature of the gas stays constant there is no change in the average kinetic energy of the gas molecules. If the volume of the gas increases while the temperature remains constant, the average force per collision is unchanged but there is a decrease in the number of collisions per unit time between the gas molecules and the container. This means there is a decrease in the total force on the container so there is a decrease in pressure.

Question 26

What are the pressure and volume of an ideal gas dependent on?

Answer 26

The number of gas molecules present (N).
The average random kinetic energy per molecule (Ek).

Question 27

What are the units used in the pV = nRT equation?

Answer 27

V in m³
p in Pa
T in K