In this lesson, we will learn how to use the Stefan–Boltzmann law to calculate the intensity of thermal radiation emitted by an object of a given temperature.

Q1:

Two spacecraft in orbit around Earth, shadowed from the Sun, are positioned with their sides parallel to each other but several meters apart, as shown in the diagram. The sides are of equal area and made of the same substance. At an instant 𝑡, there is a net rate of energy transfer between the two sides of 155 J/s. The power emitted, 𝑃, by the side with the lower temperature is 105 W. What is the ratio of the temperature of the higher temperature side to the lower temperature side?

Q2:

A rectangular-prism-shaped object has a temperature difference 𝑇−𝑇 across two of its parallel faces, as shown in the diagram. The perpendicular distance between the faces is 24 cm. The face at temperature 𝑇 has an emissivity of 0, as do all the other faces except the opposite face, which has an emissivity of 1. The object is in thermal equilibrium with its surroundings and is absorbing no infrared radiation. The thermal conductivity of the object is 150 W/m⋅K. The temperature 𝑇 is 60∘C. Find the value of 𝑇 in degrees Celsius. Use a value of 5.67×10/⋅WmK for the Stefan-Boltzmann constant. Answer to three significant figures.

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