Quiz on 2nd Law of Thermodynamics

Which statement best describes the 2nd Law of Thermodynamics?

• Energy cannot be created or destroyed.

• The total entropy of an isolated system never decreases.

• Heat always flows from a hot object to a cold object.

• The internal energy of a system remains constant during a process.

Answer explanation

The 2nd Law of Thermodynamics states that the total entropy of an isolated system never decreases, meaning systems naturally progress towards disorder. This distinguishes it from other laws that focus on energy conservation.

Entropy is a measure of:

• Internal energy of a system.

• Disorder or randomness of a system.

• Temperature of a system.

• Pressure of a system.

Answer explanation

Entropy quantifies the disorder or randomness in a system. A higher entropy indicates greater disorder, making 'Disorder or randomness of a system' the correct choice, while the other options relate to different thermodynamic properties.

The efficiency of a heat engine is given by:

• (Output work) / (Input work)

• (Input work) / (Output work)

• (Heat added) / (Temperature)

• (Temperature) / (Heat added)

Answer explanation

The efficiency of a heat engine is defined as the ratio of output work to input work. Therefore, the correct choice is (Output work)/(Input work), which measures how effectively the engine converts energy.

A heat engine absorbs 500 J of heat and performs 200 J of work. What is its efficiency?

• 0.40

• 0.25

• 0.50

• 0.75

Answer explanation

Efficiency is calculated as work done divided by heat absorbed. Here, efficiency = 200 J / 500 J = 0.40. Thus, the correct answer is 0.40.

According to the Kelvin-Planck statement of the 2nd Law of Thermodynamics, it is impossible to:

• Convert mechanical energy into thermal energy.

• Achieve 100% efficiency in a heat engine.

• Transfer heat from a cold reservoir to a hot reservoir without external work.

• Decrease the entropy of an isolated system.

Answer explanation

The Kelvin-Planck statement asserts that it is impossible to transfer heat from a cold reservoir to a hot reservoir without external work, highlighting the directionality of heat flow in thermodynamic processes.

The Clausius statement of the 2nd Law of Thermodynamics deals with the impossibility of:

• Perpetual motion machines of the first kind.

• Perpetual motion machines of the second kind.

• Heat flow from a cold body to a hot body without any external work.

• Achieving 100% efficiency in a heat engine.

Answer explanation

The Clausius statement asserts that heat cannot spontaneously flow from a colder body to a hotter body without external work, highlighting the directionality of heat transfer in thermodynamic processes.

The Carnot engine operates between two reservoirs at temperatures T1 and T2. What is the efficiency of the Carnot engine?

• (T2 - T1)/T2

• (T1 - T2)/T1

• 1 - (T1/T2)

• 1 - (T2/T1)

Answer explanation

The efficiency of a Carnot engine is given by the formula 1 - (T1/T2), where T1 is the temperature of the hot reservoir and T2 is the temperature of the cold reservoir. Thus, the correct choice is 1 - (T1/T2).

Which of the following is a reversible process?

• Melting of ice cubes in a glass of water.

• Mixing of two different gases.

• Expansion of a gas against zero pressure.

• Friction between two surfaces.

Answer explanation

The expansion of a gas against zero pressure is reversible because it can return to its original state without any energy loss. In contrast, melting ice, mixing gases, and friction involve irreversible changes.

Which law states that "The entropy of a perfect crystal approaches zero as the temperature approaches absolute zero"?

• Zeroth Law of Thermodynamics

• First Law of Thermodynamics

• Second Law of Thermodynamics

• Third Law of Thermodynamics

Answer explanation

The Third Law of Thermodynamics states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero, highlighting the behavior of systems at very low temperatures.

Which process violates the 2nd Law of Thermodynamics?

• Heat flowing from a hot reservoir to a cold reservoir.

• Spontaneous heat transfer from a cold object to a hot object.

• A refrigerator cooling its interior.

• A Carnot engine operating in a cycle.

Answer explanation

The 2nd Law of Thermodynamics states that heat cannot spontaneously flow from a cold object to a hot object. Therefore, the process of spontaneous heat transfer from a cold object to a hot object violates this law.