Deck 11: Heat

Question 1

Explain how the specific heats of materials are measured using the technique of calorimetry.

Answer 1

The specific heat of a substance can be determined by measuring the masses and temperature changes of the objects involved. Typically a substance of known mass and temperature is put into a quantity of water in a calorimeter. The water is at a different temperature from that of the substance, usually a lower one. The principle of the conservation of energy is then applied to determine the substance's specific heat. This method is called the method of mixtures.

Question 2

Why do you suppose water has been such a popular heat transfer material (steam heat, hot water systems, etc)?

Answer 2

It is readily available, but more importantly, it has a relatively large heat capacity and latent heat values. Hence, it can store large quantities of heat to be transferred elsewhere.

Question 3

Glass has more than three times larger thermal conductivity than air. Yet glass-wool (glass fibers and air) air spaces. Why is glass-wool better although glass has larger conductivity than air?

Answer 3

Convection is very important, and the glass fibers reduce the convecting air currents.

Question 4

It was once thought that a "black hole" absorbed all the radiation falling upon it and radiated none. If this were true, what would one infer the temperature of a black hole to be? Actually Stephen Hawking discovered that black holes have a VERY small probability of radiating. Modifying your first answer, would you now infer a large or small temperature for a black hole.?

Question 5

The mechanical equivalent of heat was first determined by

A) Lord Kelvin.
B) Con Edison.
C) Stefan and Boltzmann.
D) Count Rumford.
E) James Joule.

Question 6

Which of the following represents the "Mechanical Equivalent of Heat"?

A) 1 ft-lb = 1.36 Joules
B) 1.00 J = 0.239 cal
C) 1 cal = 3.97 × 10^-3 BTU
D) 1. eV = 1.6 × 10^-19 J
E) 1. J = 1 kg.m² / s²

Question 7

Which of the following is the smallest unit of heat energy?

A) calorie
B) Joule
C) kilocalorie
D) Btu

Question 8

A substance with a greater specific heat ________ has a larger heat capacity.

A) seldom
B) never
C) sometimes
D) always

Question 9

A thermally isolated system is made up of a hot piece of aluminum and a cold piece of copper; the aluminum and the copper are in thermal contact. The specific heat capacity of aluminum is more than double that of copper. Which object experiences the greater magnitude gain or loss of heat during the time the system takes to reach thermal equilibrium?

A) the copper
B) the aluminum
C) Neither; both experience the same size gain or loss of heat.
D) It is impossible to tell without knowing the masses.

Question 10

A thermally isolated system is made up of a hot piece of aluminum and a cold piece of copper; the aluminum and the copper are in thermal contact. The specific heat capacity of aluminum is more than double that of copper. Which object experiences the greater temperature change during the time the system takes to reach thermal equilibrium?

A) the aluminum
B) Neither; both experience the same size temperature change.
C) It is impossible to tell without knowing the masses.
D) the copper

Question 11

It is a well-known fact that water has a higher specific heat capacity than iron. Now, consider equal masses of water and iron that are initially in thermal equilibrium. The same amount of heat, 30. calories, is added to each. Which statement is true?

A) They are no longer in thermal equilibrium; the water is warmer.
B) They are no longer in thermal equilibrium; the iron is warmer.
C) They remain in thermal equilibrium.
D) It is impossible to say without knowing the exact mass involved and the exact specific heat capacities.

Question 12

The heat required to change a substance from the solid to the liquid state is referred to as the

A) heat of freezing.
B) heat of vaporization.
C) heat of melting.
D) heat of fusion.

Question 13

Steam will burn the skin more severely than boiling water because of water's relatively large

A) latent heat.
B) emmisivity.
C) conductivity.
D) specific heat.
E) compressibility.

Question 14

A substance with a greater specific heat will ________ have a large latent heat.

A) always
B) never
C) sometimes

Question 15

The specific heat of ice is about ________ the specific heat of liquid water.

A) half
B) the same as
C) one tenth
D) 3 times greater than
E) double

Question 16

When a vapor condenses into a liquid, its temperature

A) rises.
B) falls.
C) stays the same.
D) insufficient information, cannot answer

Question 17

At higher altitudes water boils at ________ temperature as/than at sea level.

A) a higher
B) the same
C) a lower

Question 18

Phase changes occur with an attendant

A) temperature remaining constant.
B) convection.
C) temperature increase.
D) temperature decrease.

Question 19

At higher altitudes it takes ________ time to bring water to a boil on a stove.

A) more
B) the same
C) less

Question 20

Which of the following best explains why sweating is important to humans in maintaining suitable body temperature?

A) Moisture on the skin increases thermal conductivity, thereby allowing heat to flow out of the body more effectively.
B) Functioning of the sweat gland absorbs energy that otherwise would go into heating the body.
C) Evaporation of moisture from the skin extracts heat from the body.
D) The high specific heat of water on the skin absorbs heat from the body.

Question 21

In a liquid at a given temperature, the molecules are moving in every direction, some fast, some slowly. Electrical forces of adhesion tend to hold them together. However, occasionally one molecule gains enough energy (as a result of collisions) so that it pulls loose from its neighbors and escapes from the liquid. Which of the following can best be understood in terms of this phenomenon?

A) When you step out of a swimming pool and stand in the wind, you will get colder than you would if you stayed out of the wind.
B) If snow begins to fall when you are skiing, you will feel colder than you did before it started to snow.
C) A hot water bottle will do a better job of keeping you warm than will a rock of the same mass heated to the same temperature.
D) When a large steel suspension bridge is built, gaps are left between the girders.
E) Increasing the atmospheric pressure over a liquid will cause the boiling temperature to decrease.

Question 22

A chunk of ice (T = -20°C) is added to a thermally insulated container of cold water (T = 0°C). What happens in the container?

A) Some of the water freezes and the chunk of ice gets larger.
B) The ice melts until thermal equilibrium is established.
C) The water cools down until thermal equilibrium is established.

Question 23

You blow on food to cool it. Blowing increases

A) radiation.
B) the specific heat.
C) convection.
D) the latent heat.
E) conduction.

Question 24

Which of the following would be the best radiator of thermal energy?

A) a shiny surface
B) a metallic surface
C) a perfectly white surface
D) a perfectly black surface
E) styrofoam

Question 25

By what primary heat transfer mechanism does one end of an iron bar become hot when the other end is placed in a flame?

A) natural convection
B) forced convection
C) conduction
D) radiation

Question 26

A large fan placed in a closed room is effective by enhancing

A) radiation.
B) conduction.
C) convection.
D) sublimation.

Question 27

By what primary heat transfer mechanism does the sun warm the Earth?

A) convection and conduction
B) radiation
C) convection
D) conduction

Question 28

In the vacuum of space, one gets rid of unwanted heat from spacecraft by utilizing

A) radiation.
B) convection.
C) conduction.
D) evaporation.

Question 29

A layer of insulating material with thermal conductivity K is placed on a layer of another material of thermal conductivity 2K. The layers have equal thickness. What is the effective thermal conductivity of the composite sheet?

A) 1.5K
B) 2K/3
C) 3K
D) K/5
E) K/3

Question 30

Consider two neighboring rectangular houses built from the same materials. One of the houses has twice the length, width, and height of the other. Under identical climatic conditions, what would be true about the rate that heat would have to be supplied to maintain the same inside temperature on a cold day? Compared to the small house, the larger house would need heat supplied at

A) 8 times the rate.
B) twice the rate.
C) 4 times the rate.
D) 16 times the rate.
E) 6 times the rate.

Question 31

If you double the absolute temperature of an object, it will radiate energy

A) 8 times faster.
B) 16 times faster.
C) 4 times faster.
D) 2 times faster.

Question 32

Gasoline yields 4.8 × 10⁷ joules per kg when burned. The density of gasoline is approximately the same as that of water, and 1 gal = 3.8 L. How much energy does your car use on a trip of 100 mi if you get 25 mi per gallon?

Question 33

The water flowing over Niagara Falls drops a distance of 50. m. Assuming that all the gravitational energy is converted to thermal energy, by what temperature does the water rise?

Question 34

0.45 kg of a metal at 90.° C is added to 0.40 kg of water at 20.0° C.
(a) If the final temperature of the mixture is 26.° C, what is the specific heat of the metal?
(b) specific heat determined in part "a"given the same data?

Question 35

A 100. gram steel bullet moves at 250. m/s just before imbedding itself into a thick anchored wood post. If 40.% of the initial kinetic energy goes into heating the bullet (steel c = 460.J/kg-C)
(a) how hot does the bullet become if it initially is at 25.°C?
(b) what would be the bullet's temperature if it had been twice as massive ( 2 times 100. grams)?

Question 36

How much heat would be required to melt 1.0 kg of LEAD, initially at room temperature? Assume room temperature to be 20° C.
(c = 920 J/kg-C°
_Lf = 0.25 × 10⁵ J/kg
LV = 8.67 × 10⁵ J/kg
melts at 328.°C
boils at 1744.°C)

Question 37

The heat of fusion of ice is 80. kcal/kg-C°. When 50. g of ice at 0°C is added to 50. g of water at 25°C, what is the final temperature?

Question 38

The heat of fusion of lead is 5.9 kcal/kg, the heat of vaporization is 207. cal/kg, and its melting point is 328°C. How much heat is required to melt 50. g of lead initially at 23°C? (The specific heat of lead is 0.031 kcal/kg-C°. )

Question 39

Ice cream, when eaten, gives about 2100. Calories/kg to the body. On the other hand, the body must give back energy to warm it up to body temperature (37°C). Assuming ice cream has similar thermal properties to water:
(a) 1/2 kg produces how much energy?
(b) 1/2 kg absorbs how much energy to reach 37°C?
(c) What is the net gain or loss (eating 1/2 kg ice cream)?

Question 40

Consider a light bulb, in empty space, radiating 200. watts with its filament heated to 2800.°C. Assuming it is a good approximation to a black body.
(a) What is the area of the filament?
(b) If placed in an oven, with walls at 800.°C, what is its net power radiated now?

Question 41

1500 J of work is equivalent to how much heat?

A) 6,279,000 cal
B) 358 cal
C) 6.279 kcal
D) 358 kcal

Question 42

14.5 kcal of heat is equivalent to how much work?

A) 60.7 J
B) 3.46 J
C) 60700 J
D) 3460 J

Question 43

What is the power output of a hot air furnace that produces heat at the rate of 160,000. BTU/hr?

A) 1.6 kW
B) 7.6 kW
C) 47 kW
D) 80 kW
E) 22 kW

Question 44

How much heat is needed to raise the temperature of 200 g of lead (c = 0.11 kcal/kg . °C) by 10 C°?

A) 20 kcal
B) 330 cal
C) 2000 cal
D) 33.0 cal

Question 45

If 40 kcal of heat is added to 2.0 kg of water, what is the resulting temperature change?

A) 0.05 C°
B) 80 C°
C) 20 C°
D) 40 C°

Question 46

A 2.0 kg aluminum block is originally at 10°C. If 140 kJ of heat is added to the block, what is its final temperature?

A) 45°C
B) 65°C
C) 88°C
D) 78°C

Question 47

150 kcal of heat raises the temperature of 2.0 kg of material by 400 F°. What is the material's specific heat capacity?

A) 1.35 kcal/kg . °C
B) 0.19 kcal/kg . °C
C) 0.75 kcal/kg . °C
D) 0.34 kcal/kg. °C

Question 48

50 g of lead (c = 0.11 kcal/kg. °C) at 100°C is put into 75 g of water at 0°C. What is the final temperature of the mixture?

A) 50°C
B) 6.8°C
C) 25°C
D) 2°C

Question 49

A 0.600-kg piece of metal is heated to 100°C and placed in an aluminum can of mass 0.200-kg which contains 0.500 kg of water initially at 17.3°C. The final equilibrium temperature of the mixture is 20.2°C, what is the specific heat of the metal?

A) 450 J/kg . C°
B) 900 J/kg . C°
C) 140 J/kg . C°
D) 270 J/kg . C°

Question 50

A 0.10 kg piece of copper, initially at 95°C, is dropped into 0.20 kg of water contained in a 0.28-kg aluminum can; the water and aluminum are initially at 15°C. What is the final temperature of the system?

A) 23.7°C
B) 17.8°C
C) 18.3°C
D) 19.2°C

Question 51

If 50 g of material at 100°C is mixed with 100 g of water at 0°C, the final temperature is 40°C. What is the specific heat of the material?

A) 7.5 kcal/kg . °C
B) 0.75 kcal/kg . °C
C) 1.33 kcal/kg . °C
D) 0.33 kcal/kg . °C

Question 52

Two equal mass objects (which are in thermal contact) make up a system that is thermally isolated from its surroundings. One object has an initial temperature of 100°C and the other has an initial temperature of 0°C. What is the equilibrium temperature of the system, assuming that no phase changes take place for either object? (The hot object has a specific heat capacity that is three times that of the cold object.)

A) 19°C
B) 25°C
C) 23°C
D) 50°C
E) 75°C

Question 53

How much heat must be removed from steam to change it to liquid?

A) 1.8 kcal/g
B) 540 cal/g
C) 600 cal/g
D) 1 kcal/g

Question 54

A block of ice at 0°C is added to a 150. g aluminum calorimeter cup that holds 200. g of water at 10°C. If all but 2. g of ice melt, what was the original mass of the block of ice?

A) 38.8 g
B) 62.2 g
C) 47.6 g
D) 31.1 g
E) 42.0 g

Question 55

8.0 grams of water initially at 100°C are poured into a cavity in a very large block of ice initially at 0°C How many g of ice melt before thermal equilibrium is attained?

A) 100 g
B) 10 g
C) 900 g
D) 500 g
E) an unknown amount; it cannot be calculated without first knowing the mass of the block of ice

Question 56

Which of the following walls (identical areas) would be the best thermal INSULATOR? (thermal conductivities, k, are given in J/m-s-C°)

A) 20. feet of iron (k = 46.)
B) 5.0 inches of concrete (k = 1.3)
C) 1.0 inch thick wood (k = 0.12)
D) 6.0 inches of solid glass (k = 0.84)

Question 57

The thermal conductivity of aluminum is twice that of brass. Two rods (one aluminum and the other brass) are joined together end to end in excellent thermal contact. The rods are of equal lengths and radii. The free end of the brass rod is maintained at 0°C and the aluminum's free end is heated to 200°C. If no heat escapes from the sides of the rods, what is the temperature at the interface between the two metals?

A) 76°C
B) 92°C
C) 133°C
D) 148°C
E) 155°C

Question 58

An aluminum kettle (mass = 100 g) of pure water (mass = 60 g) is at room temperature (20°C). The kettle and its contents are placed on a 1000-Watt electric burner and heated to boiling. Assuming that all the heat from the burner heats the kettle and its contents and that a negligible amount of water evaporates before boiling begins, calculate the amount of time required to bring the water to a boil.

A) 4.0 min
B) 8.1 min
C) 3.5 min
D) 4.9 min
E) 7.3 min

Question 59

Most stars radiate much like black bodies. Consider two stars having the same surface temperature, the second having 10 times the diameter of the first. The power radiated per unit area is

A) 10⁴ times larger for the 2nd.
B) 10 times larger for the 2nd.
C) 10² times larger for the 2nd.
D) the same for both.
E) 10⁸ times larger for the 2nd.

Question 60

What temperature exists inside a solar collector (effective collection area of 15 m²) on a bright sunny day when the outside temperature is +20.°C? Assume that the collector is thermally insulated, that the sun irradiates the collector with a power per unit area of 600 W/m², and that the collector acts as a perfect black body.

A) 73°C
B) 154°C
C) 107°C
D) 33°C
E) 93°C

Question 61

A lamp radiates 75. J/s when the room is set on 27°C but it radiates 90. J/s when the room temperature drops to 10. C. What is the temperature of the lamp?

A) 86.° C
B) 97.° C
C) 110.° C
D) 97. K
E) 86. K