Thermodynamics 2

Question 1

State the second law of thermodynamics (3)

Answer 1

-when objects of different temperatures are brought into thermal contact
-the spontaneous flow of heat that results is always from the high temperature object to the low temperature object
-spontaneous heat flow never proceeds in the reverse direction

Question 2

State what a heat engine is and provide an example (2)

Answer 2

-heat engine is a device that converts heat into work
-a classic example is the steam engine

Question 3

How does a steam engine function (4)

Answer 3

-fuel heats the water
-the vapor expands and does work against the piston
-the vapor condenses back into water again
-the cycle repeats

Question 4

What do all heat engines have in common (3)

Answer 4

-a high temperature reservoir
-a low temperature reservoir
-a cyclical engine

Question 5

Define the terms Qc and Qh in the context of a heat engine. How is the heat supplied from the hot reservoir (Qh) distributed in the engines operation (4)

Answer 5

-in heat engines, Qh represents the heat supplied from the hot reservoir
-and Qc is where the waste heat is released to the cold reservoir
-during the engines operation Qh is partially converted into work
-while the remaining heat Qc is released as waste heat

Question 6

Explain the heat transfer process in a heat engine cycle, where Qh is supplied from the hot reservoir. What happens to this heat and how is it related to the work and waste heat in the cycle (4)

Answer 6

-the heat transfer process in a heat engine cycle involves Qh being supplied from the hot reservoir
-a portion of this heat is converted into useful work
-while the rest Qc is released as waste heat to the cold reservoir
-work done = Qh - Qc

Question 7

Describe the role of the hot reservoir and the cold reservoir in a heat engines operation.
What is the outcome of the heat transfer process and how is it connected to the engine’s efficiency (3)

Answer 7

-in a heat engine the hot reservoir Qh and cold reservoir acts as a heat sink to receive waste heat
-the outcome of the heat transfer process is the production of useful work and the rejection of waste heat,
-the engines efficiency is a measure of how effectively it converts heat from the hot reservoir into useful work

Question 8

Define efficiency and state its formula (2)

Answer 8

-is the fraction of the heat supplied to the engine that appears as work
E = w/Qh or 1-(Qc/Qh)

Question 9

Why should there be a temperature difference in the engine (1)

Answer 9

In order for the engine to run there should be a temperature difference otherwise heat will not be transferred

Question 10

Explain the key components of carnots theorem regarding maximum efficiency heat engine. What role does reversibility play in achieving maximum efficiency (3)

Answer 10

-carnots theorem specifies 2 crucial conditions for a heat engine to achieve maximum efficiency between 2 constant temperature reservoirs Tc and Th

-to have max efficiency, it must be an engine in which all processes are reversible
-all reversible engines operating between the same two temperatures Tc and Th should have the same efficiency

Question 11

State the formula for maximum efficiency (1)

Answer 11

E = 1 - (Tc /Th)
Tc = tempt of cold reservoir
Th = tempt of hot reservoir

Question 12

State Carnot’s theorem (3)

Answer 12

-if an engine operating between 2 constant temperature reservoirs is to have maximum efficiency
-it must be in which all processes are reversible
-in addition all reversible engines operating between the same 2 temperatures have the same efficiency

Question 13

Can the max efficiency be 100%, if not, why ? (1)

Answer 13

-some of the heat is always wasted

Question 14

State the formula for heat discharged (1)

Answer 14

= input - output

Question 15

State the formula for the max work a heat engine can do (1)

Answer 15

= (1- Tc/Th) x Qh

Question 16

How does a refrigerator function (3)

Answer 16

-it uses work to extract heat from the cold reservoir Qc (inside of the fridge) and exhausts to the kitchen
Qh = Qc +W
-more heat is released into the surrounding of the kitchen (hot reservoir) Qh
-as a fridge operates, it warms the kitchen at the same time that it cools the food stores in it

Question 17

How does an ideal fridge function (1)

Answer 17

-it would remove most of the heat from the interior while requiring the smallest amount of work done

Question 18

State the formula for COP

Answer 18

COP = Qc / W
Bigger the value of cop the better

Question 19

How does an air conditioner function (3)

Answer 19

-air conditioner uses electrical energy to transfer the heat indoors to the outdoor envoirment cooling the indoor space
-the motors of the ac does the work
-more heat is removed to the hot reservoir (Qh) than is removed from the cold reservoir (Qc)

Question 20

How does a heat pump function (4)

Answer 20

-heat is removed from the outside air (cold reservoir)
-and is transferred into the house
-keeping it warm
-the work that the pump does contributes to a warmer house

Question 21

State the formulas used to determine how much work is needed to add HEAT Qh to a place (3)

Answer 21

W = Qh - Qc
W = Qh x ( 1 - Qc/Qh)
W = Qh x (1- Tc/Th)

Tc = tempt of cold reservoir
Th= tempt of hot reservoir
Qc = heat wasted
Qh= heat supplied

Question 22

State the third law of thermodynamics (2)

Answer 22

-it is impossible to lower the temperature of an object to absolute zero
-in a finite number of steps