# Test Prep for AP® Courses

### 14.1Heat

1

An ice cube is placed in a cup of hot water. Which of the following statements correctly describes energy transfer at the molecular level?

1. Kinetic energy is transferred only from the hot water molecules to the water molecules in the ice.
2. Kinetic energy is transferred only from the water molecules in the ice to the hot water molecules.
3. Kinetic energy is transferred mostly from the hot water molecules to the water molecules in the ice.
4. Kinetic energy is transferred mostly from the water molecules in the ice to the hot water molecules.
2

The molecular description of heat transfer from higher to lower temperatures applies to spontaneous processes, that is, processes in which no energy is added to or removed or from the systems by work or heat. Refrigeration is an example of work being done to remove energy from air within a given space, and thus lower the temperature of the air. Assume a typical kitchen refrigerator, where the air inside the unit forms the system with a temperature of 25 °C, and the walls are kept at a constant temperature of 10 °C. In terms of molecules and average kinetic energy, describe how the air is made colder.

3

Suppose particle a with mass m is moving to the right at speed v. It then experiences an elastic collision with particle b, also with mass m, sitting at rest. The particles then move with speeds va and vb, respectively, at angles $θ a θ a$ and $θ b θ b$ relative to the horizontal, as shown in the diagrams. What are the expressions for va and vb, expressed in terms of v and $θ b ? θ b ?$

Figure 14.36
4

Suppose particle a with mass m is moving to the right at speed v. It then experiences an elastic collision with particle b, also with mass m, sitting at rest. The collision may be head-on, so that particle b moves to the right as a result; it may be such a glancing collision that particle b is barely impacted; or it may be somewhere in between. Use conservation of energy to show that the final energies of the two particles will each be in the range $0≤E≤ 1 2 m v 2 . 0≤E≤ 1 2 m v 2 .$ How does this result help explain why a system, such as an ideal gas, approaches thermal equilibrium as a result of particle collisions?

### 14.5Conduction

5

Two objects made of the same material, initially at different temperatures, are brought into thermal contact with each other, but are otherwise thermally isolated from their surroundings. Which of the following best describes the average kinetic energy of the molecules making up the cooler of the two objects, compared to that of the hotter, and how that average kinetic energy is changing with time?

1. lower, decreasing
2. lower, increasing
3. higher, decreasing
4. higher, increasing
6

For the experiment that you devised Example 14.1, which variables can be changed, and how should they be changed, so as to shorten the time in which a measurement is made?

1. Use a smaller quantity of ice (smaller m).
2. Use containers with greater thickness (larger d).
3. Use containers with smaller surface areas (smaller A).
4. Use a lower ambient temperature outside the container (smaller T2).
7

With reference to Example 14.1, why does the change in the temperature of the ice indicate that it has entirely melted?

### 14.5Conduction

8

Which of the following correctly describes the rate of conductive heat transfer for a substance?

1. The rate decreases with increased surface area and decreased thickness.
2. The rate increases with increased temperature difference and surface area.
3. The rate increases with decreased temperature difference and increased thickness.
4. The rate decreases with decreased temperature difference and increased surface area.
9

You wish to design a saucepan that has the same rate of thermal conduction as a pan made of silver. You need to use a less costly material, and limit the size of the pan so that the surface area in contact with a range heating element is no more than 50 percent greater than that of the hypothetical silver pan. Explain what other factor(s) can be adjusted, and by how much, so that your design will be successful. Use Table 14.3 to obtain thermal conductivity values for different substances.

### 14.6Convection

10

Under which two conditions would convection in a fluid be greatest?

1. The gravitational acceleration is large, and the fluid density varies greatly for a given temperature change.
2. The gravitational acceleration is small, and the fluid density varies greatly for a given temperature change.
3. The gravitational acceleration is large, and the fluid density varies slightly for a given temperature change.
4. The gravitational acceleration is small, and the fluid density varies slightly for a given temperature change.
11

Sea breezes occur along coastlines, and consist of cool air moving toward the shore from the ocean. However, this only occurs during the day, and is a stronger effect when the air temperature on the land is greatest and the air temperature above the water is coldest. At night, the breezes are reversed, moving from the land toward the ocean. Taking into consideration the specific heat capacities of water and sand, which is about the same as that of concrete, explain how sea breezes form during the day and change direction at night.