Thermal Efficiency and Entropy Concepts

What fundamental concept, related to the Second Law of Thermodynamics, explains why a perfectly efficient engine cannot be invented?

• Conservation of mass

• Entropy

• Gravitational force

• Chemical reactions

How is the thermal efficiency of a heat engine calculated?

• Total energy input divided by useful work output

• Useful work output divided by total heat input

• Total heat output multiplied by useful work output

• Heat lost to the surroundings divided by total energy input

What is the approximate thermal efficiency of many internal combustion engines found in cars today?

• 10%

• 20%

• 40%

• 60%

What does the Second Law of Thermodynamics primarily focus on regarding energy?

• The total quantity of energy in a system.

• The creation or destruction of energy.

• The quality of energy and its tendency to become less usable.

• The conversion of mass into energy.

In the first stage of the Carnot cycle, reversible isothermal expansion, what happens to the gas?

• The gas expands, and its temperature decreases.

• The gas expands, and heat is transferred from the reservoir to maintain constant temperature.

• The gas is compressed, and its temperature increases.

• The gas is compressed, and heat is transferred to the reservoir.

During the reversible adiabatic expansion phase of the Carnot cycle, what is used to prevent heat transfer?

• A heat sink.

• A cold reservoir.

• Insulation.

• A high-temperature reservoir.

What does entropy measure in a system?

• The total amount of energy available for work.

• The thermal energy per unit temperature that is unavailable for doing work.

• The rate of heat transfer between two reservoirs.

• The efficiency of a heat engine.

According to the second law of thermodynamics, what is the overall trend of entropy in the universe?

• It always decreases.

• It remains constant.

• It always increases or stays the same.

• It fluctuates randomly.