Topic 10 - Thermal physics

Question 1

State the equation of state for an ideal gas

Answer 1

PV = nRT

Question 2

What is an ideal gas?

Answer 2

A gas that agrees with the ideal gas equation

Question 3

What are the differences between ideal gases and real gases?

Answer 3

• Real gases are affected by high pressures and densities and low temperatures whereas ideal gases are not
• Ideal gases do not liquefy at low temperatures

Question 4

What is absolute zero temperature?

Answer 4

A state at which all molecular motion has stopped, the lowest possible temperature: -273.15°C or 0 K

Question 5

What is the Kelvin scale of temperature based on?

Answer 5

The behaviour of an ideal gas (pressure is proportional to the temperature)

Question 6

Define system and surroundings

Answer 6

A system is the thing in question (in this case gases)

The surroundings are everything else outside of the system (environment)

Question 7

What is the expression for the work involved in a volume change of a gas at constant pressure?

Answer 7

Question 8

What is the first law of thermodynamics?

Answer 8

Question 9

What is the first law of thermodynamics a statement of?

Answer 9

The principle of energy conservation

Question 10

Draw the isochoric, isobaric, isothermal, and adiabatic changes of state of an ideal gas on P-V diagrams

Answer 10

Question 11

Describe the isochoric change of state of an ideal gas

Answer 11

• There is no work done by or on the gas → no change in volume

Question 12

Describe the isobaric change of state of an ideal gas

Answer 12

• Any expansion or compression occurs at constant pressure
• Occurs when gases are allowed to expand freely

Question 13

Describe the isothermal change of state of an ideal gas

Answer 13

• No change in the internal energy of the gas → temperature is constant
• The work done on the gas is all transferred away from the gas as thermal energy
• To be isothermal the change must be as slow as possible

Question 14

Describe the adiabatic change of state of an ideal gas

Answer 14

• No thermal energy is transferred between the gas and the surroundings
• All the work done by the gas is transferred from the internal energy → temperature decreases
• All the work done on the gas is transferred to the internal energy
• Adiabatic lines of P-V diagrams are steeper than isothermal lines
• The change must be as rapid as possible in a well-insulated container

Question 15

How is the work done calculated in a thermodynamic cycle?

Answer 15

By calculating the area enclosed by the cycle

Question 16

What is the second law of thermodynamics?

Answer 16

Thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature

Question 17

What is entropy?

Answer 17

A numerical expression of the degree of disorder in a system

Question 18

State the second law of thermodynamics in terms of entropy changes

Answer 18

In every process, the total entropy of any isolated system, or the universe as a whole, always increases

Question 19

What happens to the total entropy of a system and surroundings if local entropy decreases?

Answer 19

It increases.

Question 20

What do we know about overall energy and overall entropy?

Answer 20

Overall energy is always conserved and overall entropy always increases