Q: Which of the following best describes the energy transformation when you walk at a constant speed on level ground?

$$E_{\text{chem}} \rightarrow E_{\text{th}}$$

Q: A 68 kg person climbs 2.7 m of stairs. What is the change in gravitational potential energy?

$$\Delta U_g = (68\,\text{kg})(9.8\,\text{m/s}^2)(2.7\,\text{m}) = 1800\,\text{J}$$

Q: A heat engine operates between a hot reservoir at $$T_h = 400\,\text{K}$$ and a cold reservoir at $$T_c = 300\,\text{K}. $$What is the maximum theoretical efficiency?

$$e_{\text{max}} = 1 - \frac{T_c}{T_h} = 1 - \frac{300}{400} = 0.25 = 25\%$$

Q: A heat engine absorbs 500 J of heat from a hot reservoir and exhausts 300 J to a cold reservoir. How much work does it do?

$$W = Q_h - Q_c = 500\,\text{J} - 300\,\text{J} = 200\,\text{J}$$

Question 1

A 75 W incandescent bulb and a 15 W fluorescent bulb both produce 3.0 W of visible light energy. What is the efficiency of the incandescent bulb for converting electric energy into light?

Accepted Answer

$$e = \frac{3.0}{75} = 0.04 = 4\%$$

Question 2

A machine uses 1000 J of electric energy to raise a heavy mass, increasing its potential energy by 300 J. What is the efficiency of this process?

Accepted Answer

$$e = \frac{300}{1000} = 0.30 = 30\%$$

Question 3

Which of the following best describes the energy transformation when you walk at a constant speed on level ground?

Accepted Answer

$$E_{	ext{chem}} ightarrow E_{	ext{th}}$$

Question 4

A 68 kg person climbs 2.7 m of stairs. What is the change in gravitational potential energy?

Accepted Answer

$$\Delta U_g = (68\,	ext{kg})(9.8\,	ext{m/s}^2)(2.7\,	ext{m}) = 1800\,	ext{J}$$

Question 5

Which of the following statements about entropy is correct?

Accepted Answer

The entropy of an isolated system either increases or remains constant.

Question 6

A refrigerator is an example of which of the following?

Accepted Answer

Heat pump

Question 7

Which of the following increases when the temperature of an ideal gas is increased?

Accepted Answer

The thermal energy of the gas and the average kinetic energy of the gas atoms

Question 8

A heat engine operates between a hot reservoir at $$T_h = 400\,	ext{K}$$ and a cold reservoir at $$T_c = 300\,	ext{K}. $$What is the maximum theoretical efficiency?

Accepted Answer

$$e_{	ext{max}} = 1 - \frac{T_c}{T_h} = 1 - \frac{300}{400} = 0.25 = 25\%$$

Question 9

A heat engine absorbs 500 J of heat from a hot reservoir and exhausts 300 J to a cold reservoir. How much work does it do?

Accepted Answer

$$W = Q_h - Q_c = 500\,	ext{J} - 300\,	ext{J} = 200\,	ext{J}$$

Question 10

A heat pump moves 900 J of heat from the cold outdoors into a house using 300 J of electric energy. What is its coefficient of performance (COP) for heating?

Accepted Answer

$$COP = \frac{Q_h}{W} = \frac{900}{300} = 3$$

Question 11

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

Accepted Answer

Heat energy spontaneously flows only from hot to cold.

Question 12

A 12 oz can of soda contains 40 g of sugar. If 1 g of sugar contains 17 kJ of energy, how many Calories does the soda provide? ($$1\,	ext{Cal} = 4.19\,	ext{kJ})$$

Accepted Answer

$$	ext{Calories} = \frac{40 	imes 17}{4.19} \approx 162$$

Question 13

A blender motor runs at a power of 250 W for 40 s, and 2000 J of heat leaves the food to the surroundings. By how much does the thermal energy of the food increase?

Accepted Answer

$$\Delta E_{th} = W + Q = (250 	imes 40) + (-2000) = 10,000 - 2000 = 8000\,	ext{J}$$

Question 14

Which temperature change is the largest?

Accepted Answer

An increase of $$1^\circ	ext{C} or$$ $$1\,	ext{K} ($$they are the same and larger than $$1^\circ	ext{F})$$

Question 15

A heat engine absorbs 16 MJ from a hot reservoir at $$T_h = 395\,	ext{K}$$ and exhausts heat to a cold reservoir at $$T_c = 278\,	ext{K}. $$What is the maximum possible efficiency?

Accepted Answer

$$e_{max} = 1 - \frac{278}{395} = 0.30 = 30\%$$

Question 16

Which of the following best explains why hybrid vehicles are more efficient than conventional vehicles?

Accepted Answer

Hybrid vehicles convert kinetic energy into chemical energy in a battery during braking, which can be reused, avoiding irreversible loss to thermal energy.

Chapter 11: Using Energy – Energy Transformations, Efficiency, and Thermodynamics

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