Question 1

Which property of a material determines how efficiently it absorbs and emits thermal radiation, as illustrated by the lampblack-coated and silver-coated blocks in the first image?

Accepted Answer

Emissivity ($$\varepsilon)$$

Question 2

A blackbody is defined as an object with an emissivity ($$\varepsilon) of 1. $$What does this mean in terms of its ability to absorb and emit radiation?

Accepted Answer

It absorbs and emits all incident electromagnetic radiation.

Question 3

The Stefan-Boltzmann law gives the power radiated by an object as $$P = \varepsilon \sigma A T^4$. If the temperature of an object doubles, by what factor does the power radiated increase?

Accepted Answer

It increases by a factor of 16.

Question 4

In the second and fourth images, thermal cameras are used to visualize heat. What physical principle allows these cameras to detect temperature differences?

Accepted Answer

They detect infrared radiation emitted by objects.

Question 5

A household radiator with an area of $$1.65\ 	ext{m}^2$$ and emissivity $$\varepsilon = 0.53 is at$$ $$53.0^\circ	ext{C}. $$What is the correct formula to calculate the power it emits?

Accepted Answer

$$P = \varepsilon \sigma A (T + 273.15)^4$

Question 6

In the third image, a glowing horseshoe is being forged. Why does it appear bright and emit visible light?

Accepted Answer

It is emitting thermal radiation due to its high temperature.

Question 7

A spacecraft is covered with a shiny metallic foil, as shown in the fifth image. What is the primary purpose of this covering?

Accepted Answer

To minimize heat transfer by radiation.

Question 8

If the walls of a room are at $$22.5^\circ	ext{C}$$ and a radiator is at $$53.0^\circ	ext{C}$$, what is the net power radiated from the radiator?

Accepted Answer

$$P_{net} = \varepsilon \sigma A (T_{rad}^4 - T_{wall}^4)$$

Question 9

Which of the following surfaces would be the best emitter of thermal radiation?

Accepted Answer

A lampblack-coated surface

Question 10

A furnace opening acts as a blackbody. If its area is $$150\ 	ext{mm}^2$$ and its temperature is $$1250^\circ	ext{C}$$, which equation would you use to calculate the power radiated?

Accepted Answer

$$P = \sigma A (T + 273.15)^4$

Question 11

If a surface has an emissivity of $$\varepsilon = 0.53$$, what fraction of incident radiation does it absorb?

Accepted Answer

53%

Question 12

Which of the following statements about heat transfer by radiation is correct?

Accepted Answer

All objects emit electromagnetic radiation, regardless of temperature.

Question 13

A muffin tray is shown in a thermal image with some muffins hotter than others. What could explain this temperature difference?

Accepted Answer

Different emissivities of the muffin surfaces.

Question 14

Which factor does NOT affect the rate of heat transfer by radiation according to the Stefan-Boltzmann law?

Accepted Answer

Thermal conductivity

Question 15

A surface with low emissivity is used in spacecraft design. What is the main advantage of this?

Accepted Answer

It decreases heat emission, helping to maintain internal temperature.

Question 16

If the power radiated by a surface is $$P = 560\ 	ext{W}$$ and the power absorbed is $$P = 380\ 	ext{W}$$, what is the net power radiated?

Accepted Answer

$$180\ 	ext{W}$$

Question 17

Which of the following is a real-world application of the Stefan-Boltzmann law?

Accepted Answer

Designing thermal insulation for spacecraft

Question 18

Why does a lampblack-coated block heat up faster than a silver-coated block when exposed to sunlight?

Accepted Answer

Lampblack has higher emissivity and absorbs more radiation.

Question 19

Which constant is used in the Stefan-Boltzmann law, and what is its value?

Accepted Answer

$$\sigma = 5.6704 	imes 10$$^{-8}$$\ 	ext{W} \cdot 	ext{m}^{-2} \cdot 	ext{K}^{-4}$$

Heat Transfer by Radiation and the Stefan-Boltzmann Law

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