Question

Second Law of Thermodynamics

Learning Goal:

To understand the implications of the second law of thermodynamics.

The second law of thermodynamics explains the direction in which the thermodynamic processes tend to go. That is, it limits the types of final states of the system that naturally evolve from a given initial state. The second law has many practical applications. For example it explains the limits of efficiency for heat engines and refrigerators. To develop a better understanding of this law, try these conceptual questions.

Part A

The thermodynamic processes that occur in nature ____________.

The thermodynamic processes that occur in nature ____________.

	convert thermal energy into mechanical energy
	lead to a more ordered state
	cannot be reversed
	do not conserve energy

Part B

According to the second law of thermodynamics, it is impossible for ____________.

According to the second law of thermodynamics, it is impossible for ____________.

	heat energy to flow from a colder body to a hotter body
	an ideal heat engine to have the efficiency of 99%
	an ideal heat engine to have non-zero power.
	a physical process to yield more energy than what is put in

Part C

If the coefficient of performance of a refrigerator is 1, which the following statements is true?

If the coefficient of performance of a refrigerator is 1, which the following statements is true?

	The temperature outside equals the temperature inside of the refrigerator.
	The rate at which heat is removed from the inside equals the rate at which heat is delivered outside.
	The power consumed by the refrigerator equals the rate at which heat is removed from the inside.
	The power consumed by the refrigerator equals the rate at which heat is delivered to the outside.

Part D

To increase the efficiency of an ideal heat engine, one must increase which of the following?

To increase the efficiency of an ideal heat engine, one must increase which of the following?

	the amount of heat consumed per second
	the temperature of the cold reservoir
	the temperature of the hot reservoir
	the size of the cold reservoir
	the size of the hot reservoir

Part E

How would you increase the coefficient of performance of an ideal refrigerator?

How would you increase the coefficient of performance of an ideal refrigerator?

	Increase the mechanical work input.
	Decrease the outside temperature.
	Decrease the inside temperature.
	Increase the outside temperature.

Part F

Why must every heat engine have a cold reservoir?

Why must every heat engine have a cold reservoir?

	Because it is impossible for even a perfect engine to convert heat entirely into mechanical work.
	Because the cold reservoir keeps the engine from overheating.
	Because the cold reservoir keeps the engine from overcooling.
	Because the cold reservoir increases the power of the engine.

Answer 1

Concepts and reason

The concept required to solve the given problem is second law of thermodynamics.

Use the properties of second law of thermodynamics and then find out the correct option.

Fundamentals

Second law of thermodynamics: It states that total entropy can remain uniform in ideal cases where the system is in a steady state.

(A)

A thermodynamic process is one which converts the heat energy into the mechanical energy and some of the heat into the surroundings.

During the conversion of heat energy into the mechanical energy, the total heat energy cannot be converted. Some energy will always be lost to the surrounding. During the thermodynamic process, total energy is conserved.

The thermodynamic process that occurs in nature is irreversible process and cannot be reversed back.

(B)

According to the properties of second law of thermodynamics, it is possible to transfer the heat from colder body to the hotter body without the help of any external agency. No ideal heat engine exists with efficiency about 100%. From the law of conservation of energy, it is impossible to get more output than input under any physical process.

(C)

The coefficient of performance of a refrigerator is,

$COP= $

Here, Q_C is the quantity of heat removed from the cold reservoir and W is the work done.

Substitute for W in above equation.

Here, T_C is the temperature of cold body and T_H is the temperature of cold body and Q_H is the heat supplied to the hot reservoir.

Substitute 1 for COP, then

It implies that the heat removed from the inside is equal to the power consumed by the refrigerator.

(D)

The efficiency of a heat engine is,

From the equation, to increase the efficiency of a heat engine, either the temperature of the cold reservoir will be decrease or the temperature of the hot reservoir will be increase.

(E)

The coefficient of performance of a refrigerator is,

Here, Q_C is the quantity of heat removed from the cold reservoir and W is the work done.

Substitute for W in above equation.

Here, T_C is the temperature of cold body and T_H is the temperature of cold body and Q_H is the heat supplied to the hot reservoir.

To increase the value of COP, the value of T_H will be decrease.

(F)

Heat engine can never convert total heat energy into mechanical energy. So therefore, the heat engine must have a cold reservoir because it is impossible for even a perfect engine to convert heat entirely into mechanical work.

Ans: Part A

The thermodynamic process that occurs in nature cannot be reversed.