Question 1

Entropy is

Accepted Answer

Entropy is a measure of the disorder or randomness of a system. As the disorder increases, the entropy of the system also increases. Therefore, option C is the correct answer. Entropy is not another term for heat (option A), and it is not necessarily conserved in thermal processes (option B). However, it is true that entropy never locally decreases in any process (option D), which is known as the second law of thermodynamics.

Question 2

Which of these is characteristic of an ideal (Carnot) heat engine?

Accepted Answer

An ideal (Carnot) heat engine operates between two temperatures and is reversible, meaning it can run forward as an engine or in reverse as a refrigerator. In its ideal, reversible process, it does not lead to an increase in entropy in the universe, which is a theoretical construct. In reality, all real processes cause entropy to increase, but the Carnot cycle represents an idealized limit where this does not occur.

Question 3

A heat engine that would take in 1000 J of heat from a reservoir at 500 K and exhaust 500 J to a reservoir at 300 K, converting the other 500 J of heat to work, is

Accepted Answer

The efficiency of the engine would be 50%, which is greater than the maximum efficiency allowed by the Carnot cycle for these temperatures. The Carnot efficiency is 1 - (300/500) = 0.4 or 40%, so the engine described is not thermodynamically possible.

Question 4

Suppose 150 Joules of work is done on a system and that 300 Joules of heat are delivered to the system. Which of these is predicted by the first law of thermodynamics?

Accepted Answer

The first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. In this case, the heat added to the system is 300 Joules and the work done on the system is 150 Joules. Therefore, the change in internal energy of the system is 300 Joules - 150 Joules = 150 Joules.

Question 5

A reversible engine is one that

Accepted Answer

A reversible engine is defined by its ability to operate in a cycle that, if reversed, leaves no change in the entropy of the system and its surroundings, thus causing no increase in net entropy.

Question 6

Which of these is impossible, by the second law of thermodynamics?

Accepted Answer

The second law of thermodynamics states that heat flows from hot objects to cold objects, and this process cannot be reversed without the input of external energy. Therefore, it is impossible for an engine to generate electricity by extracting heat from cold water and dumping waste heat to warmer air without the input of external energy. This violates the second law of thermodynamics.

Question 7

What method does not directly use heat for generating electricity?

Accepted Answer

Hydroelectric dams do not directly use heat to generate electricity. They rely on the force of falling water to turn turbines and generate power.

Question 8

An inventor has created a heat engine that extracts energy from geothermal hotspots and geysers like Old Faithful at Yellowstone National Park. She claims to turn all of the geothermal energy into work. Such a device is

Accepted Answer

The claim to turn all of the geothermal energy into work violates Kelvin's statement of the second law of thermodynamics, which states that it is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.

Question 9

Entropy and temperature measure different properties of a sample of a substance, because temperature gauges average kinetic energy of molecules or atoms, but

Accepted Answer

Temperature is a measure of the average kinetic energy of the molecules or atoms in a substance, while entropy is a measure of the disorderliness or unruliness of a system. Low entropy substances can have a temperature, and the entropy of an isolated system can decrease if energy is added to the system. Therefore, the best choice is D, which correctly describes the difference between temperature and entropy.

Question 10

An amount of water forms a single hexagonally symmetric snowflake. Elsewhere the same amount of water is a droplet of liquid water. Which state has higher entropy?

Accepted Answer

The droplet of liquid water has higher entropy because in its liquid state, water molecules have more freedom to move and arrange themselves in various configurations, leading to a higher degree of disorder compared to the structured, fixed arrangement of molecules in a snowflake.

Question 11

A heat engine takes in 500 J of energy from the hot reservoir in one cycle while performing 100 J of work. The amount of heat transferred to the cold reservoir in the same time is

Accepted Answer

According to the first law of thermodynamics, the total energy input to the engine (500 J) is used to perform work (100 J) and to transfer heat to the cold reservoir (let's call this Qc). Therefore, we can write:
500 J = 100 J + Qc
Solving for Qc, we get:
Qc = 400 J
So the correct answer is (B), 400 J.

Question 12

A heat pump is capable of delivering more energy to the home than goes into the operation of the pump itself, when conditions are favorable. Which of the following statements is correct?

Accepted Answer

The functioning of a heat pump involves transferring heat from the outdoors to the inside of a building, meaning it is capable of transferring some energy from the surroundings. This is why it is able to deliver more energy to the home than goes into the operation of the pump itself. A heat pump does not violate the second law of thermodynamics as it does not create energy out of nothing.

Question 13

Electric heaters can operate at efficiencies very close to 100%. This does not contradict the fact that Carnot engines are the most efficient heat engines because

Accepted Answer

Electric heaters convert electrical energy directly into heat without performing mechanical work in the process, unlike heat engines which convert heat into work. Therefore, the efficiency of electric heaters is not limited by the Carnot efficiency, which applies to heat engines that operate between two temperatures.

Question 14

A refrigerator transfers heat from a colder body to a hotter body while energy is used to operate the device. This is

Accepted Answer

The statement is consistent with Clausius's statement of the second law, which states that heat cannot spontaneously flow from a colder body to a hotter body. However, a refrigerator can transfer heat from a colder body to a hotter body by using external energy, such as electricity. This process is not a violation of the second law because the net entropy of the system (the refrigerator and its surroundings) increases.

Question 15

It is impossible to construct an engine that is more than 100% efficient. The physical law covering this case is

Accepted Answer

The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transformed. This implies that no engine can be more than 100% efficient, as it cannot produce more energy than it consumes.

Question 16

The efficiency of a Carnot engine would be increased if

Accepted Answer

The answer of The efficiency of a Carnot engine would...

Question 17

Some people claim that the theory of evolution violates the second law of thermodynamics, since that theory holds that order develops out of disorder, corresponding to a decrease in entropy. This claim

Accepted Answer

The claim is incorrect because the second law of thermodynamics applies only to closed systems, not to open ones like Earth. Evolution does not violate the second law because the local decrease in entropy due to organisms creating order is more than compensated for by the increase in entropy in the surrounding environment, such as through the consumption of food and the release of waste heat.

Question 18

If, for an ideal heat engine, Q_H , Q_C, and W are, respectively, the heat absorbed, the heat exhausted, and the work done per cycle, which of these is true?

Accepted Answer

In an ideal heat engine, the heat exhausted (Q_C) cannot be more than the heat absorbed (Q_H) because some of the absorbed heat is converted into work (W).

Question 19

An engine that decreases the total entropy in the universe is

Accepted Answer

The second law of thermodynamics states that the total entropy of an isolated system always increases over time, and it is impossible for any engine to decrease the total entropy in the universe. Therefore, option D is the correct answer. Options A, B, and C are all devices that move heat from one place to another and do not violate the second law of thermodynamics, but they do not decrease the total entropy of the universe.

Question 20

It is an advantage for an automobile engine to run at a high temperature because

Accepted Answer

A higher temperature difference between the engine and its surroundings leads to higher efficiency as per the Carnot cycle. Therefore, it is an advantage for an automobile engine to operate at a high temperature.

Quiz 11: Heat Engines and the Second Law of Thermodynamics