An experiment is conducted to determine the emissivity of a certain material. A

Question

An experiment is conducted to determine the emissivity of a certain material. A solid sphere of diameter 0.02 m is coated with the new material and is placed in an evacuated spherical enclosure. The inside surface of the enclosure has a diameter of 0.1 m and an emissivity of 0.95. The enclosure is maintained at a temperature of 200 K at all times. The sphere has an internal heating element. Electric current is passed through the heating element. When steady operating conditions are reached, it is observed that the heater in the sphere is consuming electric power at a rate of 2 w and the temperature of the surface of the sphere is 500 K. Determine the emissivity of the coating on the sphere. (Note: Since the space between the sphere and the surrounding enclosure is evacuated, radiation heat transfer is the only significant mode of heat transfer. Also, the surface area of a sphere of radius "r" is 4 pi r2.)

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