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Chapter 20 Solutions
Physics for Scientists and Engineers, Volume 1, Chapters 1-22
- Star A has twice the radius and twice the absolute surface temperature of star B. The emissivity of both stars can be assumed to be 1. What is the ratio of the power output of star A to that of star B? (a) 4 (b) 8 (c) 16 (d) 32 (e) 64arrow_forwardThe surface area of an unclothed person is 1.50 m2, and his skin temperature is 33.0C. The person is located in a dark room with a temperature of 20.0C, and the emissivity of the skin is e = 0.95. (a) At what rate is energy radiated by the body? (b) What is the significance of the sign of your answer?arrow_forwardThe thermal conductivities of human tissues vary greatly. Fat and skin have conductivities of about 0.20 W/m · K and 0.020 w/m · K respectively, while other tissues inside the body have conductivities of about 0.50 W/m · K. Assume that between the core region of the body and the skin surface lies a skin layer of 1.0 mm, fat layer of 0.50 cm, and 3.2 cm of other tissues. (a) Find the R-factor for each of these layers, and the equivalent R-factor for all layers taken together, retaining two digits. m² - K/W Rskin m² . K/W Rfat m² - K/W Rtissue |m² - K/W R (b) Find the rate of energy loss when the core temperature is 37°C and the exterior temperature is 0°C. Assume that both a protective layer of clothing and an insulating layer of unmoving air are absent, and a body area of 2.0 m2.arrow_forward
- A ceramic mug is at a temperature of 343 K (about 160°F). The mug has a surface area of about 0.03 m² and ceramic has an emissivity of about 0.9. If the ambient temperature of the room is 293 K (about 70°F), what is the net rate at which energy is lost from the mug by radiation?arrow_forwardConsider a person standing in a room at 20°C with an exposed surface area of 1.7 m2. The deep body temperature of the human body is 37°C, and the thermal conductivity of the human tissue near the skin is about 0.3 W/m K. The body is losing heat at a rate of 150 W by natural convection and radiation to the surroundings. Taking the body temperature 0,5 cm beneath the skin to be 37°C, determine the skin temperature of the person.arrow_forwardThe thermal conductivities of human tissues vary greatly. Fat and skin have conductivities of about 0.20 W/m . K and 0.020 W/m . K, respectively, while other tissues inside the body have conductivities of about 0.50 W/m . K. Assume that between the core region of the body and the skin surface lies a skin layer of 1.0 mm, fat layer of 0.50 cm, and 3.2 cm of other tissues. (a) Find the R-factor for each of these layers, and the equivalent R-factor for all layers taken together, retaining two digits. (b) Find the rate of energy loss when the core temperature is 37°C and the exterior temperature is 0°C. Assume that both a protective layer of clothing and an insulating layer of unmoving air are absent, and a body area of 2.0 m2.arrow_forward
- A classroom has dimensions 8.00 m x 10.00 m x 3.00 m. A 1000 W electric space heater is being used to warm the room from 5.00°C to 20.00°C on a cold morning. If the density of air is 1.29 kg/m°, and the specific heat capacity of air is 1004 J/(kg-K), how long will it take to heat the room? Assume no loss of thermal energy to the surroundings. A) 1.30 minutes B) 241 minutes C) 45.3 minutes O D) 77.7 minutesarrow_forwardA flat-plate solar collector is used to heat water by having water flow through tubes attached at the back of the thin solar absorber plate. The absorber plate has a surface area of 2 m2 with emissivity and absorptivity of 0.9. The surface temperature of the absorber is 35°C, and solar radiation is incident on the absorber at 500 W/m2 with a surrounding temperature of 0°C. Convection heat transfer coefficient at the absorber surface is 5 W/m2∙K, while the ambient temperature is 25°C. Net heat rate absorbed by the solar collector heats the water from an inlet temperature (Tin) to an outlet temperature (Tout). If the water flow rate is 5 g/s with a specific heat of 4.2 kJ/kg∙K, determine the temperature rise of the water.arrow_forwardThe tungsten filament of a light bulb has an operating temperature of about 2100 K.If the emitting area of the filament is 1 cm2, and its emissivity (ε) is 0.68, what is the power output of the light bulb?arrow_forward
- The side surfaces of a 3-m-high cubic industrial furnace burning natural gas are not insulated, and the temperature at the outer surface of this section is measured to be 110°C. The temperature of the furnace room, including its surfaces, is 30°C, and the emissivity of the outer surface of the furnace is 0.7. It is proposed that this section of the furnace wall be insulated with glass wool insulation (k = 0.038 W/m·K) wrapped by a reflective sheet (« = 0.2) in order to reduce the heat loss by 90 percent. Assuming the outer surface temperature of the metal section still remains at about 110°C, determine the thickness of the insulation that needs to be used. The furnace operates continuously throughout the year and has an efficiency of 78 percent. The price of the natural gas is $1.10/therm (1 therm = 105,500 kJ of energy content). If the installation of the insulation will cost $550 for materials and labor, determine how long it will take for the insulation to pay for itself from the…arrow_forwardIn an electrically heated home, the temperature of the ground in contact with a concrete basement wall is 10.3 oC. The temperature at the inside surface of the wall is 18.1 oC. The wall is 0.14 m thick and has an area of 6.5 m2. Assume that one kilowatt hour of electrical energy costs $0.10. How many hours are required for one dollar's worth of energy to be conducted through the wall?arrow_forwardHomes are often insulated with fiberglass insulation in their walls and ceiling. The thermal conductivity of fiberglass is 0.040 W/m·K. Suppose that the total surface area of the walls and roof of a windowless house is 370 m2 and that the thickness of the insulation is 10 cm. At what rate does heat leave thehouse on a day when the outside temperature is 30°C colder than the inside temperature?arrow_forward
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