# 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. Nov 12, 2015 at 21:37

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.

I agree that heat is the flow of thermal energy, as is cold, and that such a phenomenon does not flow. Also, heat and cold are subjective concepts. If you are the subject then "heat" means a flow of thermal energy in your direction and cold the opposite. If the object from or to where the thermal energy is flowing is the subject, then it is the other way around. "Heat coming from a source" is colloquial for "thermal energy is coming from a source". This can be in the form of contact heat , such as hot air or hot metal, or thermal radiation.

First some clarification of the terms :-

1. HEAT FLOW :- Heat does not flows. It's a misconception to think that heat flows. In mid-nineteenth century physicist attempted to understand heat by thinking of it as some kind of massless fluid called 'caloric' which turns out to be wrong. That misconception of treating heat as fluid has seeped throughout the history of science.

2. then what is HEAT? :- If you think heat is a form of energy then it's a mistake. 'Heat' is name of a process it is not a thing. The transfer of energy by virtue of temperature difference is called heat.

Hence the question that which way heat "flows" is not valid. Because heat doesn't flows at all. What important is that there is difference in temperature and the hot object cools down and cold object heats up. Because it happens in such a mystical manners we got thinking that the hot body had something which "flowed" to the colder one.

• Heat is a coarse grained energy transport. Heat flow (more accurately - diffusion) modeled many times by Fick's law - with well defined heat current density. Heat flow is essential ingredient of various processes as the Nernst and Seebeck effects. Jun 22, 2020 at 18:53
• The heat flux density is a current in that it's the thermal analogue of the electric current density. Jun 22, 2020 at 19:09
• @Alexander it's fine to take heat as some sort of flow to model out some phenomena but if u r trying understand what heat is.. Then that won't do.. Also heat does not flows. Jun 23, 2020 at 17:56
• @Ayussh00 In what sense it doesn't flow? Sure it's not a flow of real particles. However, then any quasiparticle theory, as most condensed matter theories, doesn't have flow either. By flow most people mean that you can define continuous in space mean current density. For example see en.wikipedia.org/wiki/Thermal_conduction Jun 23, 2020 at 18:45
• @Alexander At microscopic scale heat is molecular agitation and in this sense it does not flows. Jun 24, 2020 at 14:32