Topic 3 - Thermal physics

Question 1

What is the role of temperature?

Answer 1

Temperature determines the direction of thermal energy transfer between two objects

Question 2

What is thermal equilibrium?

Answer 2

When temperatures of two objects have stopped changing and both objects are at the same temperature, the objects have reached thermal equilibrium

Question 3

What is the relationship between the Kelvin and Celsius scales?

Answer 3

T(K) = t(°C) + 273

Question 4

What is the internal energy of a substance?

Answer 4

The total potential energy and random kinetic energy of the molecules of the substance

Question 5

What is the reason for kinetic energy in molecules?

Answer 5

Their random/translational/rotational motion

Question 6

What is the reason for potential energy of molecules?

Answer 6

The forces between the molecules

Question 7

What is meant by thermal energy?

Answer 7

Non-mechanical transfer of energy between a system and its surroundings

Question 8

What are the three ways in which thermal energy is transferred?

Answer 8

1. Conduction
2. Convection
3. Radiation

Question 9

What is the mole?

Answer 9

The amount of substance that contains as many particles as there are atoms in 12 g of carbon-12

Question 10

What is the Avogadro constant (N_A)?

Answer 10

The number of particles in a mole

N_A = 6.02 X 10²³ mol^–1

Question 11

What is molar mass?

Answer 11

The mass that contains one mole of its particles

Unit: gmol^–1

Question 12

What is specific heat capacity?

Answer 12

The amount of energy needed to raise the temperature of 1 kg of the substance by 1 K

Unit: Jkg^–1K^–1

Question 13

What is the equation for thermal energy?

Answer 13

Q = cm∆T

Question 14

What is thermal capacity (C)?

Answer 14

The amount of energy needed to raise its temperature by 1 K

Unit: JK^–1 or J°C^–1

Question 15

What are the physical differences between the solid, liquid, and gaseous phases of substances?

Answer 15

Solids

• The molecules are held close together by strong forces
• The molecules vibrate about their positions

Liquids

• Molecules still vibrate
• Forces between some molecules are overcome, allowing them to move around a little
• Molecules are still almost as close together as in solids
• No regularity in the arrangement, constantly changing

Gases

• Forces between molecules are usually negligible
• Molecules move independently in random directions

Question 16

What are the possible phase changes?

Answer 16

1. Melting
2. Freezing
3. Evaporating
4. Boiling
5. Condensing

Question 17

Why does the temperature not change during phase changes?

Answer 17

During melting or boiling there is no incerase in the average random kinetic energies of the molecules.

A change in temperature would involve a change in energy.

Question 18

What is the difference between evaporation and boiling?

Answer 18

Evaporation occurs when some molecules gain enough kinetic energy to escape from the surface of the liquid.

Boiling occurs at a precise temperature, the temperature at which the molecules have enough kinetic energy to form bubbles inside the liquid.

Question 19

Where do evaporation and boiling occur?

Answer 19

Evaporation occurs only at the surface of the liquid whereas boiling occurs everywhere in the liquid

Question 20

What is specific latent heat (L)?

Answer 20

The amount of energy transferred when one kilogram of the substance changes phase at a constant temperature

Unit: Jkg^–1

Q = mL

Question 21

What is pressure?

Answer 21

The force per unit area

Unit: Pa or Nm^–2

Question 22

What are the assumptions of the kinetic model of an ideal gas?

Answer 22

1. Contains a very large number of identical molecules
2. The volume of the molecules is negligible
3. The molecules are in completely random motion
4. There are negligible forces between the molecules, except when they collide. (any changes of internal energy are in the form of changes of random kinetic energy)
5. All collisions are elastic

Question 23

What is temperature?

Answer 23

A measure of the average random kinetic energy of the molecules

Question 24

What are the laws for the behaviour of an ideal gas?

Answer 24

1. Boyle’s law
2. Charles’ law
3. Pressure law

Question 25

What is Boyle’s law?

Answer 25

The smaller the volume, the higher the pressure

Question 26

What is Charles’ law?

Answer 26

The higher the temperature of an ideal gas, the larger the volume

Question 27

What is pressure law?

Answer 27

The higher the temperature of an ideal gas, the higher the pressure.