Understanding Heat Radiation Part A This formula applies to _______________. Par

Question

Understanding Heat Radiation

Part A

This formula applies to _______________.

Part 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?

Part 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  on each side and painted black (therefore justifying an emissivity  near unity) emits 2.5  at a temperature of 20. 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?

Part D

As a rough approximation, the human body may be considered to be a cylinder of length L=2.0m and circumference C=0.8m. (To simplify things, ignore the circular top and bottom of the cylinder, and just consider the cylindrical sides.) If the emissivity of skin is taken to be e=0.6, and the surface temperature is taken to be T=30C, how much thermal power P does the human body radiate?

Express the power radiated numerically; give your answer to the nearest 10 W.

any object of total surface area A, Kelvin temperature T, and emissivity e any object of cross-sectional area A, Kelvin temperature T, and emissivity e any object of total surface area A, Kelvin temperature T, and emissivity any object of cross-sectional area A, Kelvin temperature T, and emissivity

Explanation / Answer

part A

any object of total surface area A, Kelvin temperature T, and emissivity e

part B

emissivity

partC

The surrounding room is near the temperature of the box and radiates about 2.5 kW of thermal energy into the box.

partD

P = sigma*A*e*T^4

P = 5.67*10^-8*2*0.8*0.6*(273+30)^4

P = 460 W

P = 0.46*10^3 W

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