Thermodynamics Quiz

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Questions and Answers

What is the limitation of the 1st law of thermodynamics?

Heat flow from cold to hot is possible

Kinetic energy can be transformed into potential energy

Heat can be completely converted into work

Heat is not completely converted into work (correct)

What is the main idea behind the 2nd law of thermodynamics?

Heat can flow from cold to hot

Heat can flow from hot to cold

Heat cannot flow from cold to hot (correct)

Heat can flow in both directions

What is the importance of the 2nd law of thermodynamics in engineering systems?

To determine the theoretical limits of refrigerators

To determine the theoretical limits of heat engines

To determine the theoretical limits of both heat engines and refrigerators (correct)

To determine the practical limits of both heat engines and refrigerators

What is the direction of change according to the 2nd law of thermodynamics?

Processes occur in a certain direction Signup and view all the answers

What is the difference between the 1st and 2nd laws of thermodynamics?

The 1st law deals with energy quantity, while the 2nd law deals with energy quality Signup and view all the answers

What is the Kelvin-Planck statement of the 2nd law of thermodynamics?

No heat engine can have 100% efficiency Signup and view all the answers

What is the Clausius statement of the 2nd law of thermodynamics?

Heat cannot flow from a cold body to a hot body Signup and view all the answers

What is the purpose of the 2nd law of thermodynamics in refrigeration systems?

To determine the theoretical limits of refrigerators Signup and view all the answers

What is the direction of heat energy flow between two bodies at different temperatures?

From the body at high temperature to the body at low temperature Signup and view all the answers

What is the main implication of the Kelvin-Planck statement?

It is impossible to build a heat engine that has 100% thermal efficiency Signup and view all the answers

What is the main idea behind the Clausius statement?

Heat can never flow from a colder body to a hotter body Signup and view all the answers

What is the relationship between the Kelvin-Planck statement and the Clausius statement?

They are equivalent Signup and view all the answers

What is entropy in the context of thermodynamics?

A measure of the disorder of a system Signup and view all the answers

What is the significance of the change in entropy?

The change in entropy is more important than its absolute value Signup and view all the answers

What is the main difference between a heat engine and a heat pump?

A heat engine produces work, while a heat pump consumes work Signup and view all the answers

What is the ultimate goal of a heat engine?

To produce work from the heat energy transferred Signup and view all the answers

What is the unit of change in entropy?

JK-1 Signup and view all the answers

What is the expression for the change in entropy when a system absorbs thermal energy Q at constant absolute temperature T?

ΔS = Q/T Signup and view all the answers

What is the change in entropy of a hot reservoir when it loses 100 J of heat at 400 K?

-0.25 JK-1 Signup and view all the answers

What is the expression for the differential entropy change of an ideal gas?

ds = cvdT + RdV Signup and view all the answers

What is the expression for the entropy change of an ideal gas for a constant volume process?

ΔS = cv ln(T2/T1) Signup and view all the answers

What is the expression for the entropy change of an ideal gas for a constant pressure process?

ΔS = cp ln(T2/T1) Signup and view all the answers

What is the expression for the entropy change of an ideal gas for an isothermal process?

ΔS = R ln(V2/V1) Signup and view all the answers

What is the change in entropy of a cold reservoir when it absorbs 85 J of heat at 300 K?

0.283 JK-1 Signup and view all the answers

Study Notes

Heat Transfer and Temperature

When two bodies at different temperatures are brought into contact, heat energy flows from the body at high temperature to the body at low temperature.
Heat energy never flows from lower temperature to higher temperature without applying external work.

Kelvin-Plank Statement

It is impossible to construct a heat engine that operates on a cycle to receive heat from a single reservoir and produce an equivalent amount of work.
It implies that it is impossible to build a heat engine that has 100% thermal efficiency.

Clausius Statement

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.
Heat cannot itself flow from a colder body to a hotter body.

Equivalence of the Two Statements

The violation of one statement leads to a violation of the other statement, i.e., they are equivalent.
Violation of the Kelvin-Plank statement leads to a violation of the Clausius statement, and vice versa.

Entropy

Is a thermodynamic function of the state of the system.
Can be interpreted as the amount of order or disorder of a system.
As with internal energy, it is the change in entropy that is important, not its absolute value.

Change in Entropy

The change in entropy (ΔS) of a system when an amount of thermal energy (Q) is added to a system by a reversible process at constant absolute temperature (T) is given by: ΔS = Q/T.
The units of the change in entropy are J/K.

Example

A heat engine removes 100 J each cycle from a heat reservoir at 400 K and exhausts 85 J of thermal energy to a reservoir at 300 K. The change in entropy for each reservoir is: ΔS (hot) = -0.25 J/K, ΔS (cold) = 0.283 J/K.

Entropy of Ideal Gas

An expression for the entropy change of an ideal gas can be obtained from: ds = cv dT / T + R dV / V.
The entropy change for a process is obtained by integrating this relation between the end states.
For constant volume process, ΔS = cv ln(T2/T1).
For constant pressure process, ΔS = cp ln(T2/T1).
For isothermal process, ΔS = R ln(V2/V1).

First Law of Thermodynamics

According to the First Law, work can be completely converted into heat or heat can be completely converted into work.
Potential energy can be transformed into kinetic energy or kinetic energy can be transformed into potential energy.
Heat flows from hot to cold or from cold to hot.
Gas expands from high pressure to low pressure or from low pressure to high pressure.

Limitations of the First Law of Thermodynamics

Heat is not completely converted into work.
Kinetic energy cannot be transformed into potential energy.
Heat flow from cold to hot is not possible.
Gas expands from low pressure to high pressure is not possible.
The First Law does not help to predict whether a certain process is possible or not.
A process can proceed in a particular direction only, but the First Law does not provide information about the direction.
The First Law does not provide sufficient conditions for a certain process to take place.

Second Law of Thermodynamics

Heat cannot flow itself from a cold body to a hot body.
The Second Law of Thermodynamics is also used to determine the theoretical limits for the performance of mostly used engineering systems like heat engines and refrigerators.

Direction of Change

The Second Law of Thermodynamics asserts that processes occur in a certain direction, and that energy has quality as well as quantity.

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Description

This quiz covers the basics of thermodynamics, including heat transfer, temperature, and the Kelvin-Planck statement.