Question 1

Which statement best describes the first law of thermodynamics as applied to a heat engine?

Accepted Answer

The change in internal energy of the system equals the sum of work done and heat transferred: $$\Delta E_{int} = W + Q.$$

Question 2

A geothermal power plant uses a hot reservoir at $$122^\circ \mathrm{C}$$ and a cold reservoir at $$5^\circ \mathrm{C}. $$What is the theoretical maximum efficiency of this heat engine?

Accepted Answer

$$e_{max} = 1 - \frac{T_C}{T_H} = 1 - \frac{278}{395} \approx 0.30$$

Question 3

Which of the following best explains why no heat engine can be 100% efficient?

Accepted Answer

Because the second law of thermodynamics requires that some energy be exhausted to the cold reservoir, increasing entropy.

Question 4

A heat pump is used to heat a house. The inside temperature is $$21^\circ \mathrm{C}$$ and the outside is $$8^\circ \mathrm{C}. $$What is the maximum theoretical coefficient of performance (COP) for heating?

Accepted Answer

$$COP_{max} = \frac{T_H}{T_H - T_C} = \frac{294}{294 - 281} \approx 22.6$$

Question 5

Which process will result in an increase in the entropy of a system?

Accepted Answer

Allowing a gas to expand freely into a vacuum.

Question 6

A refrigerator is a type of heat pump. When used for cooling, how is its coefficient of performance (COP) defined?

Accepted Answer

$$COP_{cooling} = \frac{Q_C}{W}$$

Question 7

Which of the following statements about entropy is correct?

Accepted Answer

Entropy is maximized when a system is in thermal equilibrium.

Question 8

A hybrid vehicle is more efficient than a conventional vehicle because:

Accepted Answer

It transforms some kinetic energy into chemical energy in a battery, which can be reused for motion.

Question 9

If you add 11 kJ of heat to a house using a heat pump with a coefficient of performance (COP) of 3.2, how much electric energy is required?

Accepted Answer

$$W = \frac{Q_H}{COP} = \frac{11\,\mathrm{kJ}}{3.2} \approx 3.44\,\mathrm{kJ}$$

Question 10

Which law of thermodynamics explains why heat spontaneously flows from a hot object to a cold object?

Accepted Answer

The second law of thermodynamics.

Question 11

A heat engine absorbs $$Q_H$$ from a hot reservoir and exhausts $$Q_C to a $$cold reservoir. What is the work output $$W$$?

Accepted Answer

$$W = Q_H - Q_C$$

Question 12

Which of the following is a consequence of the second law of thermodynamics?

Accepted Answer

Transformation of energy into thermal energy is irreversible.

Question 13

What is the meaning of the requirement $$\Delta G \u003c 0$$ for a spontaneous chemical reaction?

Accepted Answer

The reaction will only occur if the free energy decreases, reflecting an increase in total entropy.

Question 14

Which of the following best describes thermal equilibrium?

Accepted Answer

Two systems are at the same temperature and no net energy is transferred.

Question 15

Why does the entropy of the environment increase when a living organism grows and becomes more ordered?

Accepted Answer

Because the organism exchanges energy with its environment, increasing the environment's entropy.

Question 16

Which of the following is true about the Kelvin temperature scale?

Accepted Answer

Absolute zero is $$0\,\mathrm{K}$$, where the kinetic energy of atoms is zero.

Question 17

A heat engine receives 16 MJ from a hot reservoir and produces 1.8 MJ of electricity. What is its actual efficiency?

Accepted Answer

$$e = \frac{1.8\,\mathrm{MJ}}{16\,\mathrm{MJ}} = 0.1125$$

Question 18

Which of the following is a correct statement about heat ($$Q) in $$thermodynamics?

Accepted Answer

Heat is energy transferred due to a temperature difference.

Question 19

If a system undergoes a free expansion (no heat or work exchanged), what happens to its entropy?

Accepted Answer

Entropy increases because the energy is more dispersed.

Thermal Energy, Heat, and Thermodynamics: Key Concepts from Lecture 19 (Sections 11.5-11.8)

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