Heat Engines and Refridgerators

Question 1

What (in simple terms) is a heat engine?

Answer 1

A device that converts heat into work

Question 2

What are the three components of a steam engine?

Answer 2

Boiler (heat in), engine (work out), condenser (heat out).

Question 3

What is a heat reservoir?

Answer 3

A system with a very large heat capacity so it can absorb or release hear without its own temperature changing.

Question 4

What is the efficiency of a heat engine?

Answer 4

The ratio of the work done by the engine to the heat absorbed from the high temperature reservoir.

𝜀 =𝑊/𝑄ℎ

Question 5

What is the efficiency of a heat engine given by in terms of Qc and Qh?

Answer 5

1-(Qc/Qh)

Question 6

What does the second law of thermodynamics state for heat engines?

Answer 6

It is impossible for a heat engine working in a cycle to produce only the effect of absorbing heat from a single reservoir and performing an equivalent amount of work.

Question 7

What (in simple terms) is a refridgerator?

Answer 7

A device that uses work to absorb heat from a cold reservoir and release heat to a hot reservoir. (Essentially a heat engine in the opposite direction).

Question 8

What is the coefficient of performance of a refrigerator?

Answer 8

The ratio of the heat absorbed from the cold reservoir to the work done on the refrigerator.

COP = Qc/W

Question 9

What does the second law of thermodynamics state for refridgerators?

Answer 9

It is impossible for a refrigerator working in a cycle to produce only the effect of absorbing heat from a cold object and releasing the same amount of heat to a hot object.

Question 10

What are the steps of The Stirling Cycle?

Answer 10

1 → 2 isometric (constant 𝑉) heating at 𝑉𝑙𝑜𝑤
2 → 3 isothermal expansion at 𝑇ℎ
3 → 4 isometric cooling at 𝑉ℎ𝑖𝑔ℎ
4 → 1 isothermal compression at 𝑇c

Question 11

What are the steps of the Stirling Cycle as a refrigerator?

Answer 11

Heat is absorbed from the cold reservoir in stage 1 → 4 and released to it in
stage 2 → 1. Heat is released to the hot reservoir in stage 3 → 2 and absorbed
from it in stage 4 → 3. Work is done on the system in stage 3 → 2 and by the
system in stage 1 → 4.

Question 12

Is the Stirling Cycle reversible?

Answer 12

It is not reversible because the values f Qh and Qc are not the same when operating as a heat engine and as a refrigerator

Question 13

What are the steps of the Carnot Cycle?

Answer 13

1 → 2 quasi-static adiabatic compression
2 → 3 isothermal expansion at 𝑇ℎ
3 → 4 quasi-static adiabatic expansion
4 → 1 isothermal compression at 𝑇c

Question 14

What are the steps of the Carnot Cycle if it is used in heat engine?

Answer 14

heat is absorbed from the hot reservoir in stage 2 → 3 and released to the cold reservoir in stage 4 → 1. Work is done by the system in stages 2 → 3 and 3 → 4, and work is done on the system in stages 4 → 1 and 1→2.

Question 15

What are the steps of the Carnot Cycle if it is used in Refrigerator?

Answer 15

Heat is absorbed from the cold reservoir in stage 1 → 4 and released to the hot reservoir in stage 3 → 2. Work is done on the system in stages 4 → 3 and 3 → 2, and work is done by the system in stages 2 →1 and 1 → 4.

Question 16

Is the Carnot cycle reversible?

Answer 16

Yes, because each step is reversible.

Question 17

What are the conditions for a process to be reversible?

Answer 17

1) No mechanical energy is transformed into internal thermal energy by friction or other dissipative forces
2) Heat transfer only occurs between objects with an infinitesimal difference in temperature
3) The process must be quasi-static

Question 18

What does Carnot’s theorem state?

Answer 18

No engine operating between two given heat reservoirs can be more efficient than a reversible engine working between these two reservoirs.

Question 19

What is the equation that relates Qc/Qh and Tc/Th?

Answer 19

Qc/Qh = Tc/Th

Question 20

What does the third law of thermodynamics state?

Answer 20

It is impossible to reduce the temperature of any system or part of a system to absolute zero in a finite number of operations.

Question 21

What equation defines the change in entropy of a system?

Answer 21

ds = dQrev/T

Question 22

Where is the entropy of an isolated system at maximum?

Answer 22

At equilibrium

Question 23

What is the equation of entropy as a function of the state?

Answer 23

ΔS = Cvln(T2/T1) + nRln(V2/V1)