Q: A 68 kg person climbs a flight of stairs that is 2.7 m high. If the person uses 7200 J of chemical energy to do this, what is the efficiency of this process?

$$\text{Efficiency} = \frac{1800\,\text{J}}{7200\,\text{J}} = 0.25 = 25\%$$

Q: A blender motor does 10,000 J of work on food in 40 seconds, and 2,000 J of heat is transferred from the food to the surroundings. What is the increase in the thermal energy of the food?

$$\Delta E_{th} = W + Q = 10,000\,\text{J} + (-2,000\,\text{J}) = 8,000\,\text{J}$$

Q: A soda contains 40 g of sugar. If 1 g of sugar provides 17 kJ of energy, how many Calories does the soda provide? ($$1\,\text{Cal} = 4.19\,\text{kJ})$$

$$\text{Calories} = \frac{40 \times 17\,\text{kJ}}{4.19\,\text{kJ}} \approx 162$$

Q: Which of the following is the correct expression for the efficiency of a process?

$$\text{Efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}$$

Question 1

Which of the following statements best describes the first law of thermodynamics for a system in which only thermal energy changes?

Accepted Answer

The change in thermal energy is equal to the sum of work and heat transferred: $$\Delta E_{th} = W + Q$$

Question 2

A 68 kg person climbs a flight of stairs that is 2.7 m high. If the person uses 7200 J of chemical energy to do this, what is the efficiency of this process?

Accepted Answer

$$	ext{Efficiency} = \frac{1800\,	ext{J}}{7200\,	ext{J}} = 0.25 = 25\%$$

Question 3

Which of the following is the correct formula for the work done by a constant force $$F$$ acting at an angle $$	heta to $$the direction of displacement $$d$$?

Accepted Answer

$$W = Fd \cos 	heta$$

Question 4

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 of this engine?

Accepted Answer

$$e_{max} = 1 - \frac{T_C}{T_H} = 1 - \frac{300}{400} = 0.25 = 25\%$$

Question 5

Which of the following statements about heat engines is true?

Accepted Answer

No heat engine can convert 100% of heat into useful work.

Question 6

A blender motor does 10,000 J of work on food in 40 seconds, and 2,000 J of heat is transferred from the food to the surroundings. What is the increase in the thermal energy of the food?

Accepted Answer

$$\Delta E_{th} = W + Q = 10,000\,	ext{J} + (-2,000\,	ext{J}) = 8,000\,	ext{J}$$

Question 7

Which of the following best describes the difference between temperature and thermal energy?

Accepted Answer

Temperature is a measure of the average kinetic energy of the atoms, while thermal energy is the total energy of all the atoms.

Question 8

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

Accepted Answer

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

Question 9

Which of the following is the correct expression for the efficiency of a process?

Accepted Answer

$$	ext{Efficiency} = \frac{	ext{useful energy output}}{	ext{total energy input}}$$

Question 10

Which temperature scale is defined so that absolute zero is 0 and all temperatures are positive?

Accepted Answer

Kelvin scale

Question 11

A perfectly elastic collision occurs between two objects. Which of the following is true?

Accepted Answer

Both mechanical energy and momentum are conserved.

Question 12

Which of the following equations correctly relates Celsius and Kelvin temperatures?

Accepted Answer

$$T(K) = T(\degree C) + 273$$

Question 13

When heat $$Q is $$added to a sample of ideal gas and the gas expands against a piston doing work $$W$$, what happens to the temperature of the gas?

Accepted Answer

The temperature of the gas increases.

Question 14

Which of the following is a correct statement about the direction of heat flow?

Accepted Answer

Heat always flows spontaneously from a hotter object to a colder object.

Question 15

A compact fluorescent bulb uses 15 W of power to produce 3.0 W of visible-light energy. What is its efficiency?

Accepted Answer

$$	ext{Efficiency} = \frac{3.0\,	ext{W}}{15\,	ext{W}} = 0.20 = 20\%$$

Question 16

Which of the following is the correct formula for gravitational potential energy?

Accepted Answer

$$U_g = mgh$$

Question 17

If two containers of the same gas have different masses and temperatures, which container has the largest average kinetic energy per molecule?

Accepted Answer

The container with the highest temperature.

Question 18

Metabolizing one molecule of glucose releases $$4.7 	imes 10$$^{-19}$$\,	ext{J}$$, and each ATP molecule releases $$5.1 	imes 10$$^{-20}$$\,	ext{J}. If 38 $$ATP molecules are produced per glucose, what is the efficiency of this energy conversion?

Accepted Answer

$$	ext{Efficiency} = \frac{38 	imes 5.1 	imes 10$$^{-20}$$\,	ext{J}}{4.7 	imes 10$$^{-19}$$\,	ext{J}} \approx 0.41 = 41\%$$

Energy, Work, and the First Law of Thermodynamics (Lecture 18: Sections 11.1–11.5)

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