Thermal Physics

Question 1

What is internal energy?

Answer 1

The sum of the random distribution of the kinetic and potential energies of its molecules.

Question 2

When is the internal energy of an object increased? (State two ways)

Answer 2

1. When energy is transferred to the object by heating

2. When work is done on it (e.g. work done by electricity)

Question 3

State the first law of thermodynamics

Answer 3

The change of internal energy of an object = the total energy transfer due to work done and heating

Question 4

What is thermal equilibrium?

Answer 4

When no overall heat transfer occurs between two objects at the same temperature

Question 5

How do you convert a value of temperature from the Celsius scale (in degrees Celsius) to the absolute scale (in kelvin)?

Answer 5

Temperature in kelvin = temperature in degrees Celsius + 273.15

Question 6

What is meant by the term ‘absolute zero’?

Answer 6

Absolute zero is the lowest possible temperature an object can have. It is 0 kelvins and has minimum internal energy.

Question 7

What is the triple point of water?

Answer 7

273.16K, the temperature at which ice, water and water vapour co-exist in thermodynamic equilibrium.

Question 8

What is a closed system?

Answer 8

A closed system is one which doesn’t allow any transfer of matter in or out. (The total internal energy is constant)

Question 9

What is the specific heat capacity of a substance?

Answer 9

The energy needed to raise the temperature of 1kg of the substance by 1K (without a change of state).

Q = mc(T2 - T1)

Question 10

What kind of energy changes when a substance changes state?

Answer 10

When a substance changes state:

• its internal energy changes
• the potential energies of the particles change but NOT the kinetic energy (or temperature) of the particles

Question 11

What happens in the inversion tube experiment?

Answer 11

• gravitational potential energy is converted into internal energy
• for ‘n’ inversion, the loss of GPE = mgLn where L is the length of the tube
• therefore as internal energy = mc(T2 - T1), mgLn = mc(T2 - T1)

Question 12

What equation gives you the energy needed to heat a liquid in a calorimeter?

Answer 12

Q = E = Pt = (IV)t

Therefore IVt = mc(T2-T1) of the liquid + mc(T2-T1) of the calorimeter

Question 13

What is the specific latent heat (of fusion or vaporisation)?

Answer 13

The quantity of thermal energy required to change the state of 1kg of a substance.

Question 14

What is the latent heat of fusion and when is it released?

Answer 14

The energy needed to melt a solid at its melting point. The latent heat is released when a liquid solidifies.

Question 15

What is the latent heat of vaporisation and when is it released?

Answer 15

The energy needed to vaporise a liquid. The latent heat of vaporisation is released when a vapour condenses.

Question 16

What is sublimation?

Answer 16

The change of state when a solid changes to a vapour directly.

Question 17

What are the three empirical gas laws?

Answer 17

1. Boyle’s law: pV = constant at a constant temperature. Pressure and volume are inversely proportional.
2. Charles’ law: V/T = constant at a constant pressure. Volume and absolute temperature are directly proportional.
3. The Pressure Law: p/T = constant at a constant volume. Pressure and absolute temperature are directly proportional.

Question 18

What is an ideal gas and what are the two ideal gas equations?

Answer 18

An ideal gas is a gas that obeys Boyle’s law.

• pV = nRT
• pV = NkT

Where R is the molar gas constant and k is the Boltzmann constant

Question 19

What is an isothermal change?

Answer 19

Any change at a constant temperature.

Question 20

What is an isobaric change?

Answer 20

Any change at a constant pressure.

Question 21

What is the molar mass of a substance?

Answer 21

The mass of one mole of a gas.

Equal to the relative molecular mass of that substance.

E.g. Molar mass of helium (4, 2 He) is 4g, 0.004kg (its relative molecular mass = 4)

Question 22

What is the work done in changing the volume of gas at a constant pressure?

Answer 22

Work done = p x change in volume

Question 23

What is ‘Brownian Motion’?

Answer 23

• the random and unpredictable motion of a particle
• such as a smoke particle
• caused by molecules of the surrounding substance colliding at random with the particle
• provided evidence for the existence of atoms and supported kinetic theory

Question 24

What are the 5 simplifying assumptions used in kinetic theory?

Answer 24

1. Molecules are point molecules and the volume of each molecule is negligible.
2. The motion of the molecules move randomly and follow Newton’s laws (Brownian motion).
3. The molecules do not attract each other and always move in straight lines.
4. All collisions are perfectly elastic (no loss of kinetic energy).
5. Any forces acting during collisions last for less time than the time between collisions.

Question 25

What is the internal energy of an ideal gas made from?

Answer 25

Internal energy is kinetic energy of the atoms for an ideal gas.