# Heat transfer through radiation and its relation with temperature

My question is: if a body and the surroundings have the exact same temperature, will that body still lose heat in terms of radiation? If so then what exactly is heat. Heat is defined as energy flow between a system and another or between a system and its surroundings by virtue of the temperature difference. so no energy flow will occur if temperature difference between a body and its surroundings is zero. So no radiation should occur. But stefan-boltzmann implies that radiation energy is only dependent on absolute temperature of the body. Thus my confusion?

Another question: I learned that heat is transferred from a higher temperature body to a lower temperature body. Is this universal to all modes of heat transfer? If 2 bodies are seperated by a vacuum will the hot body lose heat through radiation and the cold body absorb that heat? So when both those bodies have the same temperature no radiation should occur as they are in thermal equilibrium? That being what is considered the temperature of vacuum that seperates the 2 bodies. Does it have temperature or is it undefined?

• Try to ask one question per page. This may help you to make people respond. – Ján Lalinský Feb 17 '15 at 22:55

Often we don't consider this because we're thinking about something that's a lot hotter than its surroundings. Because $T^4$ increases very rapidly with $T$, the radiative energy transfer from the environment is often small enough to be ignored in this case. In particular, for a body in space that's not exposed to sunlight, the relevant incoming radiation is the cosmic microwave background, which has a temperature of only 3 kelvin and can be ignored for most purposes.