# Heat Transfer black-body cube Problem

## Main Question or Discussion Point

How many days does it take for a perfect black-body cube (0.0100 m on a side, 30 degrees C) to radiate the same amount of energy that a one-hundred-watt light bulb uses in one hour?

If someone could point me in the right direction with how to go about solving this problem i would greatly appreciate it. I don't want to just be given the answer but shown how to go about solving this problem so that I can learn from the process. Thanks.

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cepheid
Staff Emeritus
Gold Member
You know the total surface area of the blackbody.

You know, from the Stefan–Boltzmann law, how much energy per unit time (power) a blackbody emits in EM radiation *per unit area.*

Therefore you know the total power of the EM radiation being emitted by this cube.

You know how much energy is emitted by the light bulb in one hour because you know its power.

Since you now know the power of the blackbody, you can calculate how much time it takes for it to radiate that amount of energy.

There you have it -- step by step.

Since the wording of the problem indicates you're expected to get an answer in days, you would expect the power of the blackbody to be much lower than that of the light bulb since it is taking such a significantly longer amount of time to radiate 100 watt-hours worth of energy.

Thank you so much for replying.