# Heat and Energy Practice

In usually warm climates that experience an occasional hard freeze, fruit growers spray the fruit trees with water, hoping that a layer of ice will form on the fruit. Why is this advantageous?

Answer: Ice acts as an insulator

Concrete has a higher specific heat than does soil. Use this fact to partially explain why cities have higher average temperatures than the surrounding countryside. Would you expect evening breezes to blow from city to country or from country to city? Explain.

Answer: Concrete releases its excess heat at night, causing the warmer air over the city to rise. Cooler air from the country comes in to fill the space so the breezes blow from country into city.

A 20 g aluminum cup (specific heat = 900 J/kgC) contains 800 g of water (specific heat = 4186 J/kgC) in thermal equilibrium with the cup at 80°C. The combination of cup and water is cooled uniformly so that the temperature decreases by 1.5°C per minute. At what rate is energy being removed? Express your answer in watts.

A student drops 2 metallic objects into a 120 g steel container holding 150 g of water at 25°C. One object is a 200g cube of copper that is initially at 85°C and the other is a chunk of aluminum that is initially at 5.0°C. To the student’s surprise, the water reaches a final temperature of 25°C, precisely where it started. What is the mass of the aluminum chunk?

Steam at 100°C is added to ice at 0°C.

A. Find the amount of ice melted and the final temperature when the mass of steam is 10 g and the mass of ice is 50 g.

Answer: A. All ice melts, Tƒ= 40°C

B. Repeat with steam of mass 1.0 g and ice of mass 50 g.

Answer: B. 8.0 g melt, Tƒ= 0°C

The bottom of a copper kettle has a 10 cm radius and is 2.00 mm thick. The temperature of the outside surface is 102°C and the water inside the kettle is boiling at 1 atm of pressure. Find the rate at which energy is being transferred through the bottom of the kettle.

A large, hot pizza floats in outer space. It is 70 cm in diameter and 2.0 cm thick, with a temperature of 100°C. Assume its emissivity is 0.8. What is the order of magnitude of its rate of energy loss?

Water is being boiled in an open kettle that has a 0.500 cm thick circular aluminum bottom with a radius of 12.0 cm. If the water boils away at a rate of 0.500 kg/min what is the temperature of the lower surface of the bottom of the kettle? (Assume that the top surface of the bottom of the kettle is at 100°C.)

A bar of gold is in thermal contact with a bar of silver of the same length and area. One end of the compound (the Au end) is maintained at 80.0°C. The opposite end (the Ag end) is maintained at 30.0°C. When the energy flow reaches steady state, find the temperature at the junction.

Imagine you have 1 kg each of iron, glass, and water, and that all three samples are at 10°C.

A. Rank the samples from lowest to highest temperature after 100 J of energy is added to each by heat.

Answer: A. Water, glass, iron

B. Rank them from least to greatest amount of energy transferred by heat if enough energy is transferred so that each increases in temperature by 20°C.

Answer: B. iron, glass, water

The specific heat of substance A is greater than that for substance B. If equal amounts of energy are added by heat to both these substances, the one reaching the higher temperature, assuming no melting, freezing, or evaporation occurs, will be:
A. substance A
B. substance B

C. there will be no difference in the final temperatures

D. could be either A or B

Answer: D. – No information about mass is given.

An amount of energy is added to ice, raising its temperature from-10°C to -5°C. A larger amount of energy is added to the same mass of liquid water, raising its temperature from 15°C to 20°C. From these results, we can conclude that
A. overcoming the latent heat of fusion of ice requires an input of energy

B. the latent heat of fusion of ice delivers some energy to the system

C. the specific heat of ice is less than that of water

D. the specific heat of ice is greater than that of water.

Star A has twice the radius and twice the absolute temperature of star B. What is the ratio of the power output of star A to that of star B due to electromagnetic radiation? The emissivity of both stars can be assumed to be 1.
A. 4
B. 8
C. 16
D. 32
E. 64