Thermal Physics

Question 1

Thermal equilibrium

Answer 1

When two objects in thermal contact are in thermal equilibrium, there is no net heat transfer between them. They are at the same temperature.

Question 2

Ideal gas

Answer 2

An ideal gas is a hypothetical gas that obeys the equation of state pV = nRT for an ideal gas perfectly for ALL pressure p, volume V, amount of substance n and temperature T.

Question 3

Internal energy of a substance

Answer 3

The internal energy of a substance is the sum of the kinetic energy due to the random motion of the molecules and the potential energy due to the intermolecular forces of attraction.

Question 4

Internal energy of an ideal gas

Answer 4

The internal energy of an ideal gas is the sum of the kinetic energy due to the random motion of the molecules only.

Question 5

First Law of Thermodynamics

Answer 5

The First Law of Thermodynamics states that the INCREASE in the internal energy of a system is equal to the sum of the THERMAL ENERGY SUPPLIED TO the system and the EXTERNAL WORK DONE ON the system.

Question 6

Specific heat capacity

Answer 6

The quantity of thermal energy required per unit mass per UNIT TEMPERATURE rise.

Question 7

Specific latent heat of fusion

Answer 7

The quantity of thermal energy required per unit mass when the substance changes from solid state to liquid state without a change in temperature.

Question 8

Specific latent heat of vaporisation

Answer 8

The quantity of thermal energy required per unit mass when the substance changes from liquid state to gaseous state without a change in temperature.

Question 9

Isothermal process

Answer 9

The temperature of the system remains constant. => Constant internal energy of an ideal gas (pV is constant)

Question 10

Isobaric process

Answer 10

The pressure of the system remains constant => temperature changes (because it is on a different isotherm) and volume changes

Question 11

Isochoric/Isovolumetric process

Answer 11

The volume of the system remains constant => (different isotherm means) temperature changes, pressure also changes

Question 12

Adiabatic process

Answer 12

There is no heat exchange between the system and the environment, that is, no heat supplied to or removed from the system.

(In an adiabatic expansion, since no heat is supplied to the system for it to do work, it USES ITS INTERNAL ENERGY, thus temperature decreases. Pressure also decreases.)

The path for adiabatic changes are steeper than isotherms as pressure changes more rapidly than volume.

Question 13

How to obtain pV = 1/3Nm<c^2>

Answer 13

1. Define parameters: A cubical box of sides l, containing N molecules of gas
2. Change in momentum of molecule = change in momentum of wall so its twice
3. Time travelled before further collision = distance/ speed
4. Use N2L and N3L to obtain rate of change of momentum of wall due to one molecule
5. Add up all the rates of change of momentum
6. Pressure = Force/Area by considering volume shift to other side of eqn
7. Convert to 3D