Thermodynamics- Second Law

Question 1

Uses of the 2nd law of thermodynamics

Answer 1

Identify the direction of the process. Determine degree of degradation of energy during a process. Determine theoretical limits for the performance of systems. Predicting the degree of completion of chemical reactions.

Question 2

What does quality of energy refer to?

Answer 2

How useful the energy is. For example some energy is easier to transfer than others.

Question 3

What is degree of degradation of energy to do with?

Answer 3

Where not all energy is used in the way we want

Question 4

What is a thermal energy reservoir?

Answer 4

A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply finite amounts of heat without undergoing any change in temperature.

Question 5

Can work always be converted to heat directly and completely?

Answer 5

Yes but the reverse is not true

Question 6

How do heat engines work?

Answer 6

They receive heat from a high temperature source. They convert part of this heat to work (usually in the form of a rotating shaft). They reject the remaining waste heat to a low temperature sink (eg atmosphere or river). They operate on a cycle.

Question 7

What is a working fluid?

Answer 7

The fluid involved in cyclic devices to and from which heat is transferred.

Question 8

Why does internal energy not change in a heat engine?

Answer 8

It is a cyclic process so the temperature at the start equals the temperature at the end and Δu=cv Δt.
Therefore the q(net in) = W(net out)

Question 9

What is thermal efficiency η?

Answer 9

W(net out) over total heat input (Qh). W(net out)=Qh-Ql so
η=1-Ql/Qh
Ql is heat rejected to low temperature reservoir

Question 10

Typical values of thermal efficiency

Answer 10

25-50%

Question 11

What is the Kelvin-Planck statement?

Answer 11

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

Question 12

What do refrigerators do?

Answer 12

They transfer heat from a low-temperature medium to a high temperature one. They are cyclic devices

Question 13

How does a vapour-compression refrigeration cycle work?

Answer 13

Has 4 components of a compressor, condenser, expansion valve, evaporator. The working fluid is called a refrigerant. The refrigerant enters the compressor as vapour and is condensed to the condenser pressure so its temperature increases. As it flows through the coils of the condenser, it cools by rejecting heat to the surrounding medium. Then enters expansion valve where temperature and pressure drop dramatically. It then evaporated in the evaporator by absorbing heat from the refrigerated space the enters compressor again.

Question 14

What are air conditioners?

Answer 14

Basically refrigerators whose refrigerated space is a room or building instead of the food compartment.

Question 15

What is the coefficient of performance?

Answer 15

The desired output over the required input. A measure of the efficiency of the heat pump or refrigerator.

Question 16

What is the difference between a refrigerator and a heat pump?

Answer 16

They operate on the same cycle but the objective of a heat pump is to maintain a heated space at a high temperature by supplying heat to it from a low-temperature source, whereas the objective of a refrigerator is to maintain the refrigerated space at a low temperature by removing heat from it.

Question 17

Formulae for coefficient of performance (COP) of refrigerator

Answer 17

Ql/Wnet,in
Ql is heat removed from refrigerated space
But Wnet,in is Qh-Ql
So COP=Ql/(Qh-Ql)

Question 18

Can the COP of a refrigerator be greater than 1?

Answer 18

Yes because the heat removed from the refrigerated space can be greater than the work input

Question 19

Formulae for coefficient of performance of heat pumps

Answer 19

Qh/Wnet,in
Where Qh is heat supplied to heated space
But Wnet,in is Qh-Ql
So COP=Qh/(Qh-Ql)

Question 20

Can a heat pump have a COP less than 1?

Answer 20

No because it equals the COP for a refrigerator +1 and the COP for a refrigerator is always positive.

Question 21

What is the Clausius statement?

Answer 21

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.

Question 22

What is a perpetual motion machine?

Answer 22

Any device that violates either the first or second law of thermodynamics. PMM1 violates the first law (by creating energy). PMM2 violates the second law.

Question 23

What is a reversible process?

Answer 23

A process that can be reversed without leaving any trace on the surroundings. So the system and the surroundings are returned to their initial states at the need of the reverse process. It involves no irreversibilities within the system or its surroundings.

Question 24

What is an irreversible process?

Answer 24

A process that is not reversible. Even though the system can be restored to its initial state following a process, the surroundings do some work on the system for this to happen.

Question 25

Why are we interested in reversible processes?

Answer 25

They are easy to analyse. They serve as idealised models (theoretical limits) to which actual processes can be compared.

Question 26

What are irreversibilities and give some examples?

Answer 26

The factors that cause a process to be irreversible. They include friction, unrestrained expansion, mixing of 2 fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, chemical reactions.

Question 27

What is an internally reversible process?

Answer 27

A process where no irreversibilities occur within the boundaries of the system during the process. The system proceeds through a series of equilibrium states and for the reverse process, the system passes through exactly the same equilibrium states.

Question 28

What is an externally reversible process?

Answer 28

A process where no irreversibilities occur outside the system boundaries during the process

Question 29

Describe the processes in a Carnot cycle in a closed system

Answer 29

Reversible isothermal expansion: the cylinder head is in close contact with an energy source at the same temperature as the gas, gas is allowed to expand slowly and stays the same temperature (heat transferred Qh). Reversible adiabatic expansion: system is now adiabatic, gas continues to expand slowly but its temperature drops. Reversible isothermal compression: cylinder now in contact with energy sink at same colder temperature as gas, gas is compressed and stays same temperature (heat rejected Ql). Reversible adiabatic compression: system now adiabatic, gas compressed until temperature and volume back at initial states.

Question 30

Pressure against volume diagram for Carnot cycle

Answer 30

Starts near top left. Curves down. Curves down steeper. Curves back up backwards. Curves back up backwards steeper to start position. Reverse Carnot cycle graph is same starting point but goes in opposite direction (diagram looks the same). Shape is curvy rhombus. Area enclosed by rhombus represents net work done during the cycle.

Question 31

Another name for the reversed Carnot cycle

Answer 31

Carnot refrigeration cycle.

Question 32

What are the Carnot principles?

Answer 32

The efficiency of an irreversible process is always less than the efficiency of a reversible one operating between the same two reservoirs. The efficiency of all reversible heat engines operating between the same two reservoirs are the same.

Question 33

What is a thermodynamic temperature scale?

Answer 33

A temperature scale that is independent of the properties of the substances that are used to measure the temperature.

Question 34

Formula relating heat transfer from thermal reservoirs and temperature of reservoirs

Answer 34

Qh/Ql=Th/Tl
Only for reversible process
Can use this relation for COP and thermal efficiency

Question 35

What is the most efficient type of heat engine?

Answer 35

The Carnot heat engine.

Question 36

What does the quality of thermal energy depend on?

Answer 36

It’s temperature. The higher the temperature, the more of it that can be converted into work.

Question 37

How does the COP of heat pumps and refrigerators change as Tl decreases?

Answer 37

The COP decreases because it requires more work to absorb heat from lower-temperature media