# Physics 111 Variable Variable Variable Variable e Variable

Physics 111

Variable

Variable

Variable

Variable e

Variable σ

Concept Checker �Which is the smaller temperature increment - a degree Celsius or a degree Fahrenheit? Explain.

Solution: �Answer: degree Fahrenheit �The degree Fahrenheit is the smaller increment. After all, there are 180 of these Fahrenheit divisions between the normal freezing point and the normal boiling point of water; there are only 100 of the Celsius divisions between these two temperatures. If more Fahrenheit divisions can befit between these two divisions than Celsius divisions, then the Fahrenheit divisions must be smaller.

Practice Problem 1 �At what rate is heat lost through a 2. 0 m × 4. 5 m rectangular glass windowpane that is 2. 5 cm thick when the inside temperature is 30 °C and the outside temperature is 25 °C? The thermal conductivity for glass is 0. 80 W/(m · C°).

Solution: �Q/t = k A delta T / L �Q/t =. 8 * 10 * (30 -25) /. 025 1600 W =

Practice Problem 2: �Assuming a filament in a 150 W light bulb acts like a perfect blackbody, what is the temperature of the hottest portion of the filament if it has a surface area of 7. 6 x 10− 5 m 2? The Stefan-Boltzmann constant is 5. 67 x 10− 8 W/(m 2 · K 2)

Solution 2: �Q = e ổ T^4 At �T^4 = Q/t / eo. A = P / eo. A = 150 / 1 (5. 67 x 10− 8 ) *(7. 6 x 10− 5 ) T = 2429 K

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