Physics Chapter 3

Question 1

Define thermal equilibrium

Answer 1

A system is at thermal equilibrium when the temperatures inside the system are constant

Question 2

Definition of temperature?

Answer 2

Temperature is a measure of the average kinetic energy and (indicates direction of heat flow.)

Question 3

Heat

Answer 3

A transfer of thermal energy as a result of difference in temperatures

Question 4

Is the potential energy of liquids and solids positive or negative?

Answer 4

Negative

Question 5

State the ideal gas law

Answer 5

PV=nRT

Question 6

Definition of ideal gas

Answer 6

A gas that behaves all gas laws

Question 7

Internal energy of a substance

Answer 7

Sum of the potential and kinetic energies of the molecules in a substance

Question 8

Specific latent heat of fusion

Answer 8

The energy absorbed when a unit mass of solid melts to become liquid at constant temperature

Question 9

Why doesn’t temperature change during a phase change? (use the example liquid to gas) (3 marks)

Answer 9

The energy supplied is used to break the intermolecular bonds and separate the molecules and
is not used to increase their average kinetic energy.
As temperature is a measure of average kinetic energy, the temperature does not increase.

Question 10

Which one is greater: specific latent heat of vaporisation or that of fusion?

Answer 10

PE increase for vaporisation is larger than fusion, so slh of vaporisation is higher (this is just memorised don’t ask me why)

Question 11

How does evaporation decrease the temperature of a substance?

Answer 11

The molecules with higher energy escape the water, so the molecules left behind have a lower energy. Thus, the temperature decreases.

Question 12

State, in terms of the molecular positions, what happens when ice melts (What is the structure before and after?)

Answer 12

In ice, molecules vibrate about a fixed point
The temperature difference between the ice and the surroundings means that energy is transferred to the ice, separating the ice molecules.
When it becomes liquid water, the molecules are able to change position.

Question 13

Assumptions made when we assume a gas to be ideal gas (Assumptions of kinetic theory)

Answer 13

Including but not limited to:

1. Molecules do not have any volume
2. Molecules move at a range of speeds
3. Intermolecular forces are negligible
4. All collisions are elastic
5. Duration of collisions is negligible compared to time between collisions

Question 14

Explain, with reference to the kinetic model of an ideal gas, what happens to the pressure of a gas when the temperature is increased and volume remains constant? [3]/[4]

Answer 14

Since temperature is a measure of average KE of particles, the KE of gas particles increases when temperature increases. [1]
(The average speed of molecules increases,) so the molecules collide with the walls of the container more frequently. [1]
Hence, the rate of change of momentum of the particles increases. [1]
By P = F/A, the pressure increases as force acting on the walls increases. [1]

Question 15

Difference between real gas and ideal gas

Answer 15

1. A real gas can be liquified but an ideal gas cannot
2. A real gas does not obey Boyle’s law for all values of pressure
3. A real gas does not obey Charle’s law for all values of temperature

Question 16

State the equation relating average KE of a gas and the temperature of the gas

Answer 16

E = (3/2)(kT)
Where k = Boltzmann’s constant = R/Na
Where R = Gas Constant, Na = Avogadro’s number

Question 17

For an ideal gas, state and explain how the internal energy and absolute temperature are related

Answer 17

1. Absolute temperature is a measure of the average kinetic energy of a gas.
2. Hence the internal energy is proportional to the temperature of the gas
   (Do NOT mention potential energy! Ideal gas!!)

Question 18

For a gas with constant pressure, explain why the volume will increase when temperature is raised (Without reference to pV=nRT (3)
(The gas is held in a container with a moveable piston)

Answer 18

1. As the temperature increases, the average kinetic energy increases as well, so as molecules collide with the piston they transfer more momentum to the piston
2. This means to keep pressure constant the frequency of collisions must decrease
3. Therefore the volume increases

Question 19

Distinguish between thermal energy (heat) and temperature [3]

Answer 19

Thermal energy is energy transfer between different objects as a result of temperature difference. [1]
Temperature is a measure of average kinetic energy of the particles of an object. [1]
Temperature is measured in K and thermal energy is measured in J. [1]

Question 20

Explain, in terms of the energy of its molecules, why the temperature of a pure substance does not change during melting.

Answer 20

The energy absorbed by the substance is used to break the bonds between the molecules. [1]
The PE of the molecules increases [1]
but the KE of the molecules does not change. [1]

Question 21

Define thermal capacity and specific heat capacity. [2]

Answer 21

Thermal capacity:
It is the energy required to increase the temperature of a substance by 1 K

Specific heat capacity:
It is the energy required to increase the temperature of a substance per 1 kg by 1 K

Question 22

Define the specific latent heat of fusion of a substance. [2]

Answer 22

It the energy absorbed per 1 kg of the substance [1]
when it melts (or freezes) at constant temperature. [1]

Question 23

Distinguish between internal energy and thermal energy (heat).

Answer 23

Internal energy:
It is the sum of potential energy and kinetic energy of all particles of a substance. [1]

Thermal energy:
It is energy transfer between different objects as a result of temperature difference. [1]

Question 24

State the difference between evaporation and boiling with reference to temperature and the surface area of a liquid. [2]

Answer 24

Temperature: Evaporation occurs at any temperature but boiling occurs at a definite temperature.

Surface Area:
Evaporation occurs at the surface of the liquid but boiling occurs throughout the liquid.

Question 25

Outline how a temperature scale is constructed.

Answer 25

Put a blank thermometer in pure melting ice. Mark the thermometer as the lower fixed point. Then put the thermometer in boiling water and mark it as the upper fixed point. [1]
Mark the thermometer between the lower and upper fixed points by 100 equal divisions. [1]

Question 26

A gas in a cylinder is compressed isothermally by the piston so that the volume of the gas is reduced.
Explain why the compression must be carried out slowly. [2]

Answer 26

A constant temperature is required as it is compressed isothermally.
A slow compression allows time for the internal energy to leave from the gas. [1]