Heat – or thermal energy – is always transferred from hot to cold. More precisely, energy is transferred faster from hot to cold than it is from cold to hot.
If a hot cup of tea is left for a while, the tea goes cold, or the tea, the cup and the surroundings are all in thermal equilibrium. There are FOUR ways in which this happens:
1. The thermal energy from the tea is conducted through the walls of the cup. The tea and the cup are in CONTACT.
2. The hot outer surface of the cup radiates heat into the air, heating it up. Electromagnetic / infrared radiation leaves the surface(s).
3. Convection currents are set up in the surrounding air around the cup, the hot air rising above the cup and being replaced by colder air from below. Convection in all fluids is concerned with movement of the fluids.
4. The molecules on the surface of the tea escape from the surface by evaporation – blowing over the surface makes this happen faster – the faster moving molecules are blown out of the way – and the tea will cool more quickly.
To think about: design a teacup which will keep the tea hottest for the longest time.