14.7 Radiation  (Page 6/14)

When watching a daytime circus in a large, dark-colored tent, you sense significant heat transfer from the tent. Explain why this occurs.

Satellites designed to observe the radiation from cold (3 K) dark space have sensors that are shaded from the Sun, Earth, and Moon and that are cooled to very low temperatures. Why must the sensors be at low temperature?

Why are cloudy nights generally warmer than clear ones?

Why are thermometers that are used in weather stations shielded from the sunshine? What does a thermometer measure if it is shielded from the sunshine and also if it is not?

On average, would Earth be warmer or cooler without the atmosphere? Explain your answer.

At what net rate does heat radiate from a $\text{275}{\text{-m}}^2$ black roof on a night when the roof’s temperature is $\text{30.}\mathrm{0º}\text{C}$ and the surrounding temperature is $\text{15.}\mathrm{0º}\text{C}$ ? The emissivity of the roof is 0.900.

(a) Cherry-red embers in a fireplace are at $\text{850º}\text{C}$ and have an exposed area of $0\text{.}\text{200}{\text{ m}}^2$ and an emissivity of 0.980. The surrounding room has a temperature of $\text{18}\text{.}\mathrm{0º}\text{C}$ . If 50% of the radiant energy enters the room, what is the net rate of radiant heat transfer in kilowatts? (b) Does your answer support the contention that most of the heat transfer into a room by a fireplace comes from infrared radiation?

Radiation makes it impossible to stand close to a hot lava flow. Calculate the rate of heat transfer by radiation from $1\text{.}\text{00}{\text{ m}}^2$ of $\text{1200º}\text{C}$ fresh lava into $\text{30}\text{.}\mathrm{0º}\text{C}$ surroundings, assuming lava’s emissivity is 1.00.

(a) Calculate the rate of heat transfer by radiation from a car radiator at $110°\text{C}$ into a $\mathrm{50.0º}\text{C}$ environment, if the radiator has an emissivity of 0.750 and a $1.20{\mathrm{-m}}^2$ surface area. (b) Is this a significant fraction of the heat transfer by an automobile engine? To answer this, assume a horsepower of $200\text{hp}\left(1.5\text{kW}\right)$ and the efficiency of automobile engines as 25%.

Find the net rate of heat transfer by radiation from a skier standing in the shade, given the following. She is completely clothed in white (head to foot, including a ski mask), the clothes have an emissivity of 0.200 and a surface temperature of $\text{10}\text{.}\mathrm{0º}\text{C}$ , the surroundings are at $-\text{15}\text{.}\text{0ºC}$ , and her surface area is $1\text{.}\text{60}{\text{m}}^2$ .

Suppose you walk into a sauna that has an ambient temperature of $\text{50}\text{.0ºC}$ . (a) Calculate the rate of heat transfer to you by radiation given your skin temperature is $\text{37}\text{.0ºC}$ , the emissivity of skin is 0.98, and the surface area of your body is $1\text{.50}{\text{m}}^2$ . (b) If all other forms of heat transfer are balanced (the net heat transfer is zero), at what rate will your body temperature increase if your mass is 75.0 kg?