Does a difference in temperature between an object and its surroundings cause a net radiation of IR rays between them?

Question

I know that when an object has more thermal energy than its surroundings, it will transmit thermal energy to its surroundings by conduction until it reaches thermal equilibrium with its surroundings. What I’m wondering is whether some objects can also partly reach thermal equilibrium by transferring thermal energy by radiating IR radiation.

I know that good absorbers of radiation are good emitters of radiation, and that generally, an object would be absorbing and emitting equal quantities of energy per second, meaning that it would stay at the same temperature. However, am I right that if an object was hotter than its surroundings, it would emit slightly more IR radiation than it was absorbing, meaning that its temperature would decrease until it was in thermal equilibrium with its surroundings? Equally, am I right that if that object was cooler than its surroundings, it would be absorbing more IR radiation per second than it was emitting per second, meaning that its temperature would increase?

Finally, if the above is all true, then would it be true to say that the greater the temperature difference between this object and its surroundings, then the faster the rate of thermal energy transfer via IR rays, just as with thermal energy transfer via conduction? (Meaning that the rate of net emission of IR rays by an object that was hotter than its surroundings would be gradually decreasing as the temperature of that object got closer and closer to the temperature of its surroundings?) Thanks so much.

Answer 1

Yes, your reasoning is correct. The main ways to transfer thermal energy are conduction, convection and radiation.

Every body emits thermal radiation with a radiative flux proportional to the fourth power of the temperature. $$\Phi \propto T^4$$

The proportionality constant depends on the emissivity of the material and the shape of the radiating surface. If two bodies have different temperatures, there will be a radiative flux between them

$$\Phi \propto T_1^4-T_2^4$$

Given enough time, the two bodies will reach thermal equilibrium by radiation in the same way they do by conduction. Keep in mind that the formula above is only valid if the distance between the two bodies is far greater than the dominant wavelength. Otherwise near-field effects must be taken in consideration.