phys124s11-hw09

phys124s11-hw09 - HW 9 Solutions Understanding Heat...

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1 HW 9 Solutions Understanding Heat Conduction Conduction--the flow of heat from a hotter object to a cooler object or from a hotter region to a cooler region of the same object--is the most common mechanism of heat transfer. The formula governing this is CH H C C dQ T T kA dt L ⎛⎞ = ⎜⎟ ⎝⎠ . a) This formula applies to ________________. any object of cross-sectional area ; length ; and thermal conductivity any object of cross-sectional area ; length ; and thermal resistivity a plate of surface area ; long dimension ; and thermal conductivity a plate of surface area ; long dimension ; and thermal resistivity Imagine that you are applying the heat conduction formula to a rod that is held at a high temperature on one end and at a low temperature on the other end. Indicate whether the following statements are true or false. b) The quantity CH dQ dt is the rate of heat transfer from the hot to the cold end of the rod. true false c) The quantity CH dQ dt is the rate of heat added to the hot end of the rod to maintain its temperature. true false d) The quantity CH dQ dt is the rate of heat removed from the cold end of the rod to maintain its temperature. true false e) The quantity CH dQ dt is the total amount of heat transferred from the hot to the cold end of the rod.
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2 true false This is just a rate, for the total amount of heat we should integrate vs. time. f) In the SI system of units, what are the units of the quantity CH dQ dt ? Joul/s=Watt Understanding Heat Radiation Every object at absolute (Kelvin) temperature T will radiate electromagnetic waves. This radiation is typically in the infrared for objects at room temperature, with some visible light emitted for objects heated above 1000 K. The formula governing the rate of energy radiation from a surface is given by 4 PeT σ = , where P is the thermal power. a) This formula applies to _______________. any object of total surface area , Kelvin temperature , and emissivity any object of cross-sectional area , Kelvin temperature , and emissivity any object of total surface area , Kelvin temperature , and emissivity any object of cross-sectional area , Kelvin temperature , and emissivity b) If you wanted to find the area of the hot filament in a light bulb, you would have to know the temperature (determinable from the color of the light), the power input, the Stefan-Boltzmann constant, and what property of the filament? thermal radiation emissivity length c) If you calculate the thermal power radiated by typical objects at room temperature, you will find surprisingly large values, several kilowatts typically. For example, a square box that is 1 m on each side and painted black (therefore justifying an emissivity e near unity) emits 2.5 kW at a temperature of 20 0 C. In reality the net thermal power emitted by such a box must be much smaller than this, or else the box would cool off quite quickly. Which of the following alternatives seems to explain this conundrum best?
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This note was uploaded on 09/15/2011 for the course APHY 124 taught by Professor Eins during the Spring '11 term at SUNY Albany.

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phys124s11-hw09 - HW 9 Solutions Understanding Heat...

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