How do we know which direction heat flows? From cold to hot, or from hot to cold?

I'm trying to figure out which direction the heat flows. After many google searches I've determined it flows from hot (higher energy) to cold (lower energy).

My question tho is, how do we experimentally prove that it's indeed the case? Can it be "seen" flowing somehow?

• Note: energy flows via heat in the direction of higher temperature to lower temperature. This is not the same thing as higher energy to lower energy. – march Nov 12 '15 at 21:37

3 Answers

The definition of "Heat" is energy transferred due to thermal contact.

We know hot things cool down when they touch cold things, and cold things heat up when they touch hot things. So, that means the hot thing has to be losing energy, and the cold thing has to be gaining energy. That means that heat is flowing from hot to cold.

Start by defining the temperature of an object, which is usually done by construction of suitable measurement devices as thermometers, whose working principles will not be discussed here (as it would take us far from the point).

Once you have a thermometer you can measure the temperatures of two different objects, say $T_1$ and $T_2$. It is an experimental fact that, after putting in contact two such objects with $T_1 > T_2$, the system will reach an equilibrium state with the two bodies having the same temperature (that, again, can be measured via a thermometer).

When such happens, it is possible to experimentally build machines (see Carnot and similars) capable of doing mechanical work whilst operating between two sources at different temperatures. This is interpreted as the fact that thermal differences force the exchange of thermal energy, which can be quantified by the amount of mechanical work such machines can perform. By definition, thermal energies exchanges are called heat in the standard terminology.

Since heat is what determines a change in thermal energy, it must flow from high thermal energy object to low thermal energy object.