Deck 11: Energy in Thermal Processes

Question 1

The first experiment, which systematically demonstrated the equivalence of mechanical energy and heat, was performed by:

A)Joule.
B)Boltzmann.
C)Thompson, Count Rumford.
D)Kelvin.

Answer 1

Joule.

Question 2

Who demonstrated that when heat is gained or lost by a system during some process, the gain or loss can be accounted for by an equivalent quantity of mechanical work done on the system?

A)Joule
B)Boltzmann
C)Thompson, Count Rumford
D)Kelvin

Answer 2

Joule

Question 3

A 10-kg piece of aluminum (which has a specific heat of 900 J/kg* $\degree$ C) is warmed so that its temperature increases by 5.0 C $\degree$ .How much heat was transferred into it?

A)4.5 * 10⁴ J
B)9.0 * 10⁴ J
C)1.4 * 10⁵ J
D)2.0 * 10⁵ J

Answer 3

4.5 * 10⁴ J

Question 4

Which of the following statements is true?

A)A hot object contains a lot of heat.
B)A cold object contains only a little heat.
C)Objects do not contain heat.
D)Statements a and b are true.

Question 5

A swimming pool heater has to be able to raise the temperature of the 40 000 gallons of water in the pool by 10.0 C $\degree$ .How many kilowatt-hours of energy are required? (One gallon of water has a mass of approximately 3.8 kg and the specific heat of water is 4 186 J/kg* $\degree$ C.)

A)1 960 kWh
B)1 770 kWh
C)330 kWh
D)216 kWh

Question 6

An inventor develops a stationary cycling device by which an individual, while pedaling, can convert all of the energy expended into heat for warming water.What minimum power must be generated if 300 g water (enough for 1 cup of coffee) is to be heated in 10 min from 20 $\degree$ C to 95 $\degree$ C? (1 cal = 4.186 J, the specific heat of water is 4 186 J/kg*(\degree\)C)

A)9 400 W
B)590 W
C)160 W
D)31 W

Question 7

On a sunny day at the beach, the reason the sand gets so hot and the water stays relatively cool is attributed to the difference in which property between water and sand?

A)mass density
B)specific heat
C)temperature
D)thermal conductivity

Question 8

Arrange from smallest to largest: the BTU, the joule, and the calorie.

A)BTU, J, cal
B)J, cal, BTU
C)cal, BTU, J
D)J, BTU, cal

Question 9

A 3.00-g lead bullet is traveling at a speed of 240 m/s when it embeds in a wood post.If we assume that half of the resultant heat energy generated remains with the bullet, what is the increase in temperature of the embedded bullet? (specific heat of lead = 0.030 5 kcal/kg* $\degree$ C, 1 kcal = 4 186 J)

A)113 $\degree$ C
B)137 $\degree$ C
C)226 $\degree$ C
D)259 $\degree$ C

Question 10

Heat flow occurs between two bodies in thermal contact when they differ in what property?

A)mass
B)specific heat
C)density
D)temperature

Question 11

Marc attaches a falling 500-kg object with a rope through a pulley to a paddle wheel shaft.He places the system in a well-insulated tank holding 25 kg of water.When the object falls, it causes the paddle wheel to rotate and churn the water.If the object falls a vertical distance of 100 m at constant speed, what is the temperature change of the water? (1 kcal = 4 186 J, the specific heat of water is 4 186 J/kg* $\degree$ C, and g = 9.8 m/s²)

A)19 600 C $\degree$
B)4 700 C $\degree$
C)4.7 C $\degree$
D)0.8 C $\degree$

Question 12

A solar heated house loses about 5.4 * 10⁷ cal through its outer surfaces on a typical 24-h winter day.What mass of storage rock is needed to provide this amount of heat if it is brought up to initial temperature of 62 $\degree$ C by the solar collectors and the house is maintained at 20 $\degree$ C? (Specific heat of rock is 0.21 cal/g* $\degree$ C.)

A)163 kg
B)1 230 kg
C)6 100 kg
D)12 700 kg

Question 13

A waterfall is 145 m high.What is the increase in water temperature at the bottom of the falls if all the initial potential energy goes into heating the water? (g = 9.8 m/s², c_w = 4 186 J/kg* $\degree$ C)

A)0.16 $\degree$ C
B)0.34 $\degree$ C
C)0.69 $\degree$ C
D)1.04 $\degree$ C

Question 14

Sea breezes that occur near the shore are attributed to a difference between land and water with respect to what property?

A)mass density
B)coefficient of volume expansion
C)specific heat
D)emissivity

Question 15

An inventor develops a stationary cycling device by which an individual, while pedaling, can convert all of the energy expended into heat for warming water.How much mechanical energy is required to increase the temperature of 300 g of water (enough for 1 cup of coffee) from 20 $\degree$ C to 95 $\degree$ C? (1 cal = 4.186 J, the specific heat of water is 4 186 J/kg* $\degree$ C)

A)94 000 J
B)22 000 J
C)5 400 J
D)14 J

Question 16

A 0.2-kg aluminum plate, initially at 20 $\degree$ C, slides down a 15-m-long surface, inclined at a 30 $\degree$ angle to the horizontal.The force of kinetic friction exactly balances the component of gravity down the plane so that the plate, once started, glides down at constant velocity.If 90% of the mechanical energy of the system is absorbed by the aluminum, what is its temperature increase at the bottom of the incline? (Specific heat for aluminum is 900 J/kg* $\degree$ C.)

A)0.16 C $\degree$
B)0.07 C $\degree$
C)0.04 C $\degree$
D)0.03 C $\degree$

Question 17

How many calories are equal to one BTU? (One calorie = 4.186 J, one BTU = 1 054 J.)

A)0.252
B)3.97
C)252
D)397

Question 18

Of the following systems, which contains the most heat?

A)100 kg of water at 80 $\degree$ C
B)250 kg of water at 40 $\degree$ C
C)600 kg of ice at 0 $\degree$ C
D)Systems do not contain heat.

Question 19

Calories of the food type are equal to which of the following?

A)4.186 J
B)4 186 J
C)1 BTU
D)1054 J

Question 20

If heat is flowing from a table to a block of ice moving across the table, which of the following must be true?

A)The table is rough and there is friction between the table and ice.
B)The ice is cooler than the table.
C)The ice is changing phase.
D)All three are possible, but none is absolutely necessary.

Question 21

Which of the following best describes a substance in which the temperature remains constant while at the same time it is experiencing an inward heat flow?

A)gas
B)liquid
C)solid
D)substance undergoing a change of state

Question 22

Iced tea is made by adding ice to 1.8 kg of hot tea, initially at 80 $\degree$ C.How many kg of ice, initially at 0 $\degree$ C, are required to bring the mixture to 10 $\degree$ C? (L_f = 3.33 * 10⁵ J/kg, c_w = 4 186 J/kg* $\degree$ C)

A)1.8 kg
B)1.6 kg
C)1.4 kg
D)1.2 kg

Question 23

As I use sandpaper on some rusty metal, the sandpaper gets hot because:

A)heat is flowing from the sandpaper into the metal.
B)heat is flowing from the metal into the sandpaper.
C)frictional processes increase the internal energy of the sandpaper.
D)heat is flowing from my hand into the sandpaper.

Question 24

Twenty grams of a solid at 70 $\degree$ C is place in 100 grams of a fluid at 20 $\degree$ C.Thermal equilibrium is reached at 30 $\degree$ C.The specific heat of the solid:

A)is equal to that of the fluid.
B)is less than that of the fluid.
C)is more than that of the fluid.
D)cannot be compared to that of a material in a different phase.

Question 25

A machine gear consists of 0.10 kg of iron and 0.16 kg of copper.How much total heat is generated in the part if its temperature increases by 35 C $\degree$ ? (Specific heats of iron and copper are 450 and 390 J/kg* $\degree$ C, respectively.)

A)910 J
B)3 800 J
C)4 000 J
D)4 400 J

Question 26

A solar heating system has a 25.0% conversion efficiency; the solar radiation incident on the panels is 1 000 W/m².What is the increase in temperature of 30.0 kg of water in a 1.00-h period by a 4.00-m²-area collector? (c_w = 4 186 J/kg* $\degree$ C)

A)14.3 $\degree$ C
B)22.4 $\degree$ C
C)28.7 $\degree$ C
D)44.3 $\degree$ C

Question 27

A 0.003 0-kg lead bullet is traveling at a speed of 240 m/s when it embeds in a block of ice at 0 $\degree$ C.If all the heat generated goes into melting ice, what quantity of ice is melted? (L_f = 80 kcal/kg, the specific heat of lead = 0.03 kcal/kg*(\degree\)C, and 1 kcal = 4 186 J)

A)1.47 * 10^-2 kg
B)5.8 * 10^-4 kg
C)3.2 * 10^-3 kg
D)2.6 * 10^-4 kg

Question 28

Find the final equilibrium temperature when 10.0 g of milk at 10.0 $\degree$ C is added to 160 g of coffee at 90.0 $\degree$ C.(Assume the specific heats of coffee and milk are the same as water and neglect the heat capacity of the container.) c_water = 1.00 cal/g* $\degree$ C = 4186 J/kg* $\degree$ C

A)85.3 $\degree$ C
B)77.7 $\degree$ C
C)71.4 $\degree$ C
D)66.7 $\degree$ C

Question 29

A 2.00-kg copper rod is 50.00 cm long at 23 $\degree$ C.If 40 000 J are transferred to the rod by heat, what is its change in length? c_copper = 387 J/kg*(\degree\)C and a_copper = 17 * 10^-6/ $\degree$ C.

A)0.022 cm
B)0.044 cm
C)0.059 cm
D)More information is needed.

Question 30

If a 1000-kg car was moving at 30 m/s, what would be its kinetic energy expressed in the unusual (for kinetic energy) units of calories? (1 cal = 4.186 J)

A)3.0 * 10⁴
B)9.0 * 10⁵
C)3.8 * 10⁶
D)1.1 * 10⁵

Question 31

A 50-g cube of ice, initially at 0.0 $\degree$ C, is dropped into 200 g of water in an 80-g aluminum container, both initially at 30 $\degree$ C.What is the final equilibrium temperature? (Specific heat for aluminum is 900 J/kg* $\degree$ C, the specific heat of water is 4 186 J/kg* $\degree$ C, and L_f = 3.33 * 10⁵ J/kg.)

A)17.9 $\degree$ C
B)9.5 $\degree$ C
C)12.1 $\degree$ C
D)20.6 $\degree$ C

Question 32

A slice of bread contains about 100 kcal.If specific heat of a person were 1.00 kcal/kg* $\degree$ C, by how many $\degree$ C would the temperature of a 70.0-kg person increase if all the energy in the bread were converted to heat?

A)2.25 $\degree$ C
B)1.86 $\degree$ C
C)1.43 $\degree$ C
D)1.00 $\degree$ C

Question 33

125 g of dry ice (solid CO₂) is dropped into a beaker containing 500 g of 66 $\degree$ C water.The dry ice converts directly to gas, leaving the solution.When the dry ice is gone, the final temperature of the water is 29 $\degree$ C.What is the heat of vaporization of solid CO₂? (c_water = 1.00 cal/g* $\degree$ C)

A)37 cal/g
B)74 cal/g
C)111 cal/g
D)148 cal/g

Question 34

A 120-g block of copper is taken from a kiln and quickly placed into a beaker of negligible heat capacity containing 300 g of water.The water temperature rises from 15 $\degree$ C to 35 $\degree$ C.Given c_Cu = 0.10 cal/g* $\degree$ C, and c_water = 1.00 cal/g*(\degree\)C, what was the temperature of the kiln?

A)500 $\degree$ C
B)360 $\degree$ C
C)720 $\degree$ C
D)535 $\degree$ C

Question 35

A hot (70 $\degree$ C) lump of metal has a mass of 250 g and a specific heat of 0.25 cal/g* $\degree$ C.John drops the metal into a 500-g calorimeter containing 75 g of water at 20 $\degree$ C.The calorimeter is constructed of a material that has a specific heat of 0.10 cal/ g* $\degree$ C.When equilibrium is reached, what will be the final temperature? c_water = 1.00 cal/g* $\degree$ C.

A)114 $\degree$ C
B)72 $\degree$ C
C)64 $\degree$ C
D)37 $\degree$ C

Question 36

An 80.0-g piece of copper, initially at 295 $\degree$ C, is dropped into 250 g of water contained in a 300-g aluminum calorimeter; the water and calorimeter are initially at 10.0 $\degree$ C.What is the final temperature of the system? (Specific heats of copper and aluminum are 0.092 0 and 0.215 cal/g* $\degree$ C, respectively.c_w = 1.00 cal/g*(\degree\)C)

A)12.8 $\degree$ C
B)16.5 $\degree$ C
C)28.4 $\degree$ C
D)32.1 $\degree$ C

Question 37

A piece of copper of mass 100 g is being drilled through with a ¹/₂" electric drill.The drill operates at 40.0 W and takes 30.0 s to bore through the copper.If all the energy from the drill heats the copper, find the copper's increase in temperature.c_copper = 387 J/kg* $\degree$ C.

A)40.6 C $\degree$
B)34.7 C $\degree$
C)31.0 C $\degree$
D)27.3 C $\degree$

Question 38

What is the temperature increase of 4.0 kg of water when heated by an 800-W immersion heater for 10 min? (c_w = 4 186 J/kg* $\degree$ C)

A)56 $\degree$ C
B)51 $\degree$ C
C)29 $\degree$ C
D)14 $\degree$ C

Question 39

A puddle holds 150 g of water.If 0.50 g of water evaporates from the surface, what is the approximate temperature change of the remaining water? (L_v = 540 cal/g)

A)+1.8 C $\degree$
B)-1.8 C $\degree$
C)+0.18 C $\degree$
D)-0.18 C $\degree$

Question 40

In cloud formation, water vapor turns into water droplets which get bigger and bigger until it rains.This will cause the temperature of the air in the clouds to:

A)get warmer.
B)get cooler.
C)will not affect the temperature of the air in the clouds.
D)There is no air in clouds.

Question 41

A 2.0-m² Thermopane window is constructed, using two layers of glass 4.0 mm thick, separated by an air space of 5.0 mm.If the temperature difference is 20 $\degree$ C from the inside of the house to the outside air, what is the rate of heat flow through this window? (Thermal conductivity for glass is 0.84 J/s*m*(\degree\)C and for air 0.023 4 J/s*m* $\degree$ C.)

A)7 700 W
B)1 900 W
C)547 W
D)180 W

Question 42

The surfaces of a Dewar flask are silvered for the purpose of minimizing heat transfer by what process?

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

Question 43

A windowpane is half a centimeter thick and has an area of 1.0 m².The temperature difference between the inside and outside surfaces of the pane is 15 $\degree$ C.What is the rate of heat flow through this window? (Thermal conductivity for glass is 0.84 J/s*m* $\degree$ C.)

A)50 000 J/s
B)2 500 J/s
C)1 300 J/s
D)630 J/s

Question 44

The thermal conductivity of aluminum is 238 J/s*m* $\degree$ C and of copper is 397 J/s*m* $\degree$ C.A rod of each material is used as a heat conductor.If the rods have the same geometry and are used between the same temperature differences for the same time interval, what is the ratio of the heat transferred by the aluminum to the heat transferred by the copper?

A)0.599
B)1.67
C)0.359
D)2.78

Question 45

I take 1.0 kg of ice and dump it into 1.0 kg of water and, when equilibrium is reached, I have 2.0 kg of ice at 0 $\degree$ C.The water was originally at 0 $\degree$ C.The specific heat of water = 1.00 kcal/kg* $\degree$ C, the specific heat of ice = 0.50 kcal/kg* $\degree$ C, and the latent heat of fusion of water is 80 kcal/kg.The original temperature of the ice was:

A)one or two degrees below 0 $\degree$ C.
B)-80 $\degree$ C.
C)-160 $\degree$ C.
D)The whole experiment is impossible.

Question 46

How much heat energy is required to vaporize a 1.0-g ice cube at 0 $\degree$ C? The heat of fusion of ice is 80 cal/g.The heat of vaporization of water is 540 cal/g, and c_water = 1.00 cal/g* $\degree$ C.

A)620 cal
B)720 cal
C)820 cal
D)1 kcal

Question 47

The emissivity of an ideal reflector has which of the following values?

A)0
B)1
C)100
D)infinity

Question 48

100 g of liquid nitrogen at its boiling point of 77 K is stirred into a beaker containing 500 g of 15 $\degree$ C water.If the nitrogen leaves the solution as soon as it turns to gas, how much water freezes? The heat of vaporization of nitrogen is 48 cal/g and that of water is 80 cal/g.

A)none
B)29 g
C)68 g
D)109 g

Question 49

How much heat energy must be removed from 100 g of oxygen at 22 $\degree$ C to liquefy it at -183 $\degree$ C? (The specific heat of oxygen gas is 0.218 cal/g* $\degree$ C, and its heat of vaporization is 50.9 cal/g.)

A)13 700 cal
B)9 560 cal
C)4 320 cal
D)2 160 cal

Question 50

If cooking is done using an aluminum pan over an electric burner, which of the following will not promote the rate of heat flow from burner to food?

A)increase pan bottom thickness
B)increase pan bottom area
C)increase burner temperature
D)decrease height of pan sides

Question 51

The filament temperature of a light bulb is 2 000 K when the bulb delivers 40 W of power.If its emissivity remains constant, what power is delivered when the filament temperature is 2 500 K?

A)105 W
B)62 W
C)98 W
D)50 W

Question 52

If one's hands are being warmed by holding them to one side of a flame, the predominant form of heat transfer is what process?

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

Question 53

A 5-g lead bullet traveling in 20 $\degree$ C air at 300 m/s strikes a flat steel plate and stops.What is the final temperature of the lead bullet? (Assume the bullet retains all heat.) The melting point of lead is 327 $\degree$ C.The specific heat of lead is 0.128 J/g* $\degree$ C.The heat of fusion of lead is 24.5 J/g.

A)227 $\degree$ C
B)260 $\degree$ C
C)293 $\degree$ C
D)327 $\degree$ C

Question 54

Dmitri places one end of a copper rod in a heat reservoir and the other end in a heat sink.By what factor is the rate of heat flow changed when the temperature difference between the reservoir and sink is tripled?

A)0.33
B)1/9
C)3.0
D)9.0

Question 55

How does the heat energy from the sun reach us through the vacuum of space?

A)conduction
B)radiation
C)convection
D)none of the above choices are valid

Question 56

Which one of the following processes of heat transfer requires the presence of a fluid?

A)conduction
B)radiation
C)convection
D)none of the above choices are valid

Question 57

Which of the following involves the greatest heat transfer?

A)One gram of steam at 100 $\degree$ C changing to water at 100 $\degree$ C.
B)One gram of ice at 0 $\degree$ C changing to water at 0 $\degree$ C.
C)One gram of water cooling from 100 $\degree$ C to 0 $\degree$ C.
D)One gram of ice heating from -100 $\degree$ C to 0 $\degree$ C.

Question 58

Carly places one end of a steel bar in a Bunsen flame and the other end in an ice cube.By what factor is the rate of heat flow changed when the bar's cross-sectional area is doubled?

A)2
B)1/2
C)4.0
D)1/4

Question 59

The use of fiberglass insulation in the outer walls of a building is intended to minimize heat transfer through the wall by what process?

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

Question 60

When a wool blanket is used to keep warm, what is the primary insulating material?

A)wool
B)air
C)the trim around the edge of the blanket
D)a thin layer of aluminum foil (usually not apparent) inside the blanket

Question 61

I place a 500-g ice cube (initially at 0 $\degree$ C) in a Styrofoam box with wall thickness 1.0 cm and total surface area 600 cm².If the air surrounding the box is at 20 $\degree$ C and after 4 hours the ice is completely melted, what is the conductivity of the Styrofoam material? (L_f = 80 cal/g)

A)9.6 * 10^-5 cal/s*cm* $\degree$ C
B)2.8 * 10^-6 cal/s*cm* $\degree$ C
C)1.15 * 10^-2 cal/s*cm* $\degree$ C
D)2.3 * 10^-4 cal/s*cm* $\degree$ C

Question 62

Inside a house, stepping on a tile floor barefooted may feel almost cold, but stepping on carpet in an adjacent room feels comfortably warm.Why is this?

A)It's because the tile is below room temperature while the carpet is at room temperature.
B)It's because the tile is at room temperature while carpet is normally warmer.
C)It's because the thermal conductivity of tile is less than that of carpet.
D)It's because the thermal conductivity of carpet is less than that of tile.

Question 63

Which type of heating causes sunburn?

A)conduction
B)convection
C)radiation
D)all of the above

Question 64

Consider two different rods.The greatest thermal conductivity will be in the rod with:

A)electrons that are freer to move from atom to atom.
B)the greater specific heat.
C)the greater cross-sectional area.
D)the greater length.

Question 65

A metal bar is used to conduct heat.When the temperature at one end is 100 $\degree$ C and at the other is 20 $\degree$ C, heat is transferred at a rate of 16 J/s.If the temperature of the hotter end is reduced to 80 $\degree$ C, what will be the rate of heat transfer?

A)4 J/s
B)8 J/s
C)9 J/s
D)12 J/s

Question 66

The surface of the Sun has a temperature of about 5 800 K.If the radius of the Sun is 7 * 10⁸ m, determine the power output of the sun.(Take e = 1, and s = 5.67 * 10^-8 W/m²*K⁴).

A)3.95 * 10²⁶ W
B)5.17 * 10²⁷ W
C)9.62 * 10²⁸ W
D)6.96 * 10³⁰ W

Question 67

An object at 27 $\degree$ C has its temperature increased to 37 $\degree$ C.The power then radiated by this object increases by how many percent?

A)3.3
B)14
C)37
D)253

Question 68

The tungsten filament of a light bulb has an operating temperature of about 2 100 K.If the emitting area of the filament is 1.0 cm², and its emissivity is 0.68, what is the power output of the light bulb? (s = 5.67 * 10^-8 W/m²*K⁴)

A)100 W
B)75 W
C)60 W
D)40 W

Question 69

Pennies used to be made of copper, but now they are made of copper-coated zinc.If one were to do a precise calorimetry experiment to determine the specific heat of the new pennies, what would the result be?

A)It would be that of copper since copper is on the outside.
B)It would be that of zinc since zinc is in the center.
C)It would be the sum of the copper and zinc specific heats.
D)It would be between that of copper and that of zinc, depending on coating thickness.

Question 70

In a greenhouse, electromagnetic energy in the form of visible light enters the glass panes and is absorbed and then reradiated.What happens to this reradiated electromagnetic radiation from within the greenhouse?

A)100% returns to the atmosphere.
B)It's partially blocked by glass.
C)It's transformed into ultraviolet upon striking the glass.
D)It's reflected as visible light upon striking the glass.

Question 71

In winter, light-colored clothes will keep you warmer than dark-colored clothes if:

A)you are warmer than your surroundings.
B)you are at the same temperature as your surroundings.
C)you are cooler than your surroundings.
D)you are standing in sunlight.

Question 72

A silver bar of length 30 cm and cross-sectional area 1.0 cm² is used to transfer heat from a 100 $\degree$ C reservoir to a 0 $\degree$ C block of ice.How much ice is melted per second? (For silver, k = 427 J/s*m* $\degree$ C.For ice, L_f = 334 000 J/kg.)

A)4.2 g/s
B)2.1 g/s
C)0.80 g/s
D)0.043 g/s

Question 73

What temperature increase is necessary to increase the power radiated from an object by a factor of 8?

A)8 K
B)2 K
C)100%
D)about 68%

Question 74

Of the planets with atmospheres, which is the warmest?

A)Venus
B)Earth
C)Mars
D)Jupiter

Question 75

Which of the following produces greenhouse gases?

A)burning fossil fuel
B)digestive processes in cows
C)automobile pollution
D)all of the above

Question 76

The inside of a house is at 20 $\degree$ C on an early morning when the temperature outside is 15 $\degree$ C.The next morning the inside temperature is the same but the outside temperature is now 10 $\degree$ C.How much does the energy per unit time lost by conduction through the walls, windows, doors, etc., change for the house from the first morning to the second one?

A)Since the inside temperature stays the same, the loss is the same both days.
B)The loss doubles.
C)The loss halves.
D)The loss increases by 5/288 since we need to use the Kelvin scale for this calculation.

Question 77

At high noon, the sun delivers 1 000 W to each square meter of a blacktop road.What is the equilibrium temperature of the hot asphalt, assuming its emissivity e = 1? (s = 5.67 * 10^-8 W/m²*K⁴) .

A)75 $\degree$ C
B)84 $\degree$ C
C)91 $\degree$ C
D)99 $\degree$ C

Question 78

Carbon dioxide and water molecules in the atmosphere will absorb:

A)infrared light.
B)visible light.
C)ultraviolet light.
D)radio waves.

Question 79

A metal bar is used to conduct heat.When the temperature at one end is 100 $\degree$ C and at the other is 20 $\degree$ C, heat is transferred at a rate of 16 J/s.The bar is then stretched uniformly to twice its original length.If again it has ends at 100 $\degree$ C and 20 $\degree$ C, at what rate will heat be transferred between it ends?

A)4 J/s
B)8 J/s
C)16 J/s
D)32 J/s

Question 80

Two stars, A and B, have the same emissivity, but the radii and surface temperatures are different with R_A = 0.5 R_B, and T_A = 2 T_B.Assuming the temperature of space to be negligible, which star radiates the most energy per unit time?

A)Star A
B)Star B
C)Both radiate the same amount of energy per unit time.
D)More information is needed in order to make a determination.