Calculate the rate of steady heat transfer from a 8 m-by-10 m roof of a building during a cold winter day. The roof temperature can be assumed to be constant at 12°C. The air temperature outside the building is -5°C. The sky temperature is -17°C. The solar irradiation at the location is 500 W/m². The convection coefficient can be assumed to be 8 W/m².°c. The emissivity of the roof is 0.75, and solar absorptivity is 0.4. Neglect any conduction via the walls to the ground. 2173 W

Calculate the rate of steady heat transfer from a 8 m-by-10 m roof of a building during a cold winter day. The roof temperature can be assumed to be constant at 12°C. The air temperature outside the building is -5°C. The sky temperature is -17°C. The solar irradiation at the location is 500 W/m². The convection coefficient can be assumed to be 8 W/m².°c. The emissivity of the roof is 0.75, and solar absorptivity is 0.4. Neglect any conduction via the walls to the ground. 2173 W

College Physics

1st Edition

ISBN:9781938168000

Author:Paul Peter Urone, Roger Hinrichs

Publisher:Paul Peter Urone, Roger Hinrichs

Chapter14: Heat And Heat Transfer Methods

Section: Chapter Questions

Problem 58PE: (a) Calculate the rate of heat transfer by radiation from a car radiator at 110C into a 50.0C...

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Unreasonable Results A meteorite 1.20 cm in diameter is so hot immediately after penetrating the atmosphere that it radiates 20.0 kW of power. (a) What is its temperature, if the surroundings are at 20.0C and it has an emissivity of 0.800? (b) What is unreasonable about this result? (c) Which premise or assumption is responsible?
The 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?
(a) What is the rate of heat conduction through the 3.00-cm-thick fur of a large animal having a I .40-m surface area? Assume that the animal's skin temperature is 32.0 , that the air temperature is 5.00 , and that has the same thermal conductivity as air. (b) What food intake will the animal need in one day to replace this heat transfer?
A person inhales and exhales 2.00 L of 37.0C air, evaporating 4.00102g of water from the lungs and breathing passages with each breath. (a) How much heat transfer occurs due to evaporation in each breath? (b) What is the rate of heat transfer in watts if the person is breathing at a moderate rate of 18.0 breaths per minute? (c) If the inhaled air had a temperature of 20.0C, what is the rate of heat transfer for warming the air? (d) Discuss the total rate of heat transfer as it relates to typical metabolic rates. Will this breathing be a major form of heat transfer for this person?
The 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?
(a) Calculate the rate of heat transfer by radiation from a car radiator at 110C into a 50.0C environment, if the radiator has an emissivity of 0.750 and a 1.20m2 surface area. (b) Is this a significant fraction of the heat transfer by an automobile engine? To answer this, assume a horsepower of 200 hp (1.5 kW) and the efficiency of automobile engines as 25%.
In 1986, a gargantuan iceberg broke away from the Ross Ice Shelf in Antarctica. It was approximately a rectangle 160 km long, 40.0 km wide, and 250 m thick. (a) What is the mass of this iceberg, given that the density of ice is 917kg/m3 ? (b) How much heat transfer (in joules) is needed to melt it? (c) How many years would it take sunlight alone to melt ice this thick, if the ice absorbs an average of 100W/m2, 12.00 h per day?
At our distance from the Sun, the intensity of solar radiation is 1 370 W/m2. The temperature of the Earth is affected by the greenhouse effect of the atmosphere. This phenomenon describes the effect of absorption of infrared light emitted by the surface so as to make the surface temperature of the Earth higher than if it were airless. For comparison, consider a spherical object of radius r with no atmosphere at the same distance from the Sun as the Earth. Assume its emissivity is the same for all kinds of electromagnetic waves and its temperature is uniform over its surface. (a) Explain why the projected area over which it absorbs sunlight is r2 and the surface area over which it radiates is 4r2. (b) Compute its steady-state temperature. Is it chilly?
A granite ball of radius 2.00 m and emissivity 0.450 is heated to 135C. (a) Convert the given temperature to Kelvin. (b) What is the surface area of the ball? (c) If the ambient temperature is 25.0C, what net power does the ball radiate?
(a) Calculate the rate of heat conduction through a double-paned window that has a 150-m2 area and is made of two panes of 0.800 cm-thick glass separated by a 1.00 cm air gap. The inside surface temperature is 15.0 C, while that on the outside is 10.0 OC. (Hint: There are identical temperature drops across the two glass panes. First find these and then the temperature drop across the air gap. This problem ignores the increased heat transfer in the air gap due to convection.) (b) Calculate the rate of heat conduction through a 1.60-cm-thick window of the same area and with the same temperatures. Compare your answer with that for part (a).
(a) At what temperature do the Fahrenheit and Celsius scales have the same numerical value? (b) At what temperature do me Fahrenheit and Kelvin scales have the same numerical value?
The 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 sin face 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 37C and the exterior temperature is 0C. Assume that both a protective layer of clothing and an insulating layer of unmoving air a absent, and a body area of 2.0 m2.