# What is heat and heat transfer?

Physics books write that heat is the transfer of thermal energy. They also write that heat transfer is the transfer of thermal energy from a thermodynamic system of high temperature to a one of lower temperature. But in this way they are not differentiating between heat and thermal energy. Shouldn't they write that heat transfer is the transfer of the transfer of thermal energy? Otherwise we should call it thermal energy transfer rather than heat transfer. Please illustrate this point for me and thank you.

• "Physics books write that heat is the transfer of thermal energy" Are you sure they don't say that heat is the thermal energy being transferred? There is a difference, and I've only ever seen to referred to as the energy itself. – JMac Feb 11 at 12:43

Heat transfer $$\equiv$$ transfer of energy, without doing work, and without passing material to the system.

You can call this energy what you like. You can call it heat energy or thermal energy. Inside the system it takes the form of kinetic energy of the particles, and field energy of the fields. Once it has arrived in any given system, it is best to call it "internal energy".

If you want to be precise, you should say that the result of any energy transfer is that the internal energy of the system on the receiving end went up, and the internal energy of something else went down by that same amount. We use the word "heat" to distinguish the heat flow process from other types of process, where work is done or material is transferred. Work is done when a force acts and a body being acted on moves through some displacement. Material is transferred when matter moves from one location to another. Heat flow is happening when one system exchanges energy with another through many small changes at the level of its small parts (e.g. molecules), and these small motions are not being tracked in detail by a thermodynamic description.

Definitions in scientific work are those human conventions that are useful or insightful. Standard physics terminology is that "heat", when used as a noun, refers to that which has physical dimensions of energy, and is equal to the change in the internal energy of the system receiving the energy by heat flow, when that is the type of process that is happening. When we learn thermal physics, we have to learn not to think of heat as a property or as something that can be located anywhere, because it is not a function of state. (The same goes for work.) Therefore the phrase "heat transfer" should not be taken to suggest that heat can be picked up and put down. Rather it should be understood to be a short-hand, a way of saying "transfer of energy, through the process of heat flow".

• I don't understand the down vote. This answer seems to me perfectly clear. – Vincent Fraticelli Feb 11 at 12:15
• Small nitpick (this answer still seems like the correct one IMO) "heat energy" is redundant. – JMac Feb 11 at 12:39

heat is actually energy,like the energy from sun. whereas heat transfer is the passage of energy from one body to other one at high temp. to one at low temp.like water flows from high to low potential energy similarly heat transfer takes place from a body of huge to body of low internal energy(potential energy stored).

Heat is the energy that is transfered. Heat transfer is the process of transferring the transfered energy.

Authors are free to define their terms however they want. But it is essential they use their own terms consistently, and it's a very good idea that they use language that is generally accepted. Both of these precepts are sometimes violated.

Most physics texts define heat as the thermal energy that is transferred across the boundary of a system. Note carefully that heat is different than the thermal energy contained within the system as part of the system's internal energy. Heat is thermal energy that enters or leaves the system. In books that adhere to this definition, it's my opinion that the phrase heat transfer should not appear. I wouldn't know what that phrase means. If both of those terms appear in the same book then either 1.) the author is making some other distinction that he should explain or 2.) the author is inconsistent.

I'm not familiar with current engineering textbooks, but I know that in engineering circles, the term heat transfer is used. I would presume (but I don't know for certain) that the good engineering books would take care to define what that term means. It may or may not be the same thing as what physicists call heat.

• Most textbooks define heat as the energy that is transferred. This is why you often see textbooks dedicated to heat transfer. It's also why heat is measured in Joules, and not Watts. This answer seems to play into OP's confusion with poorly defined terms. I really can't find good references that call heat the transfer of energy. Every reference I see calls it the energy that is transferred. There is a difference. – JMac Feb 11 at 12:45
• @JMac Good point. I was careless in my wording. I will reword what I wrote. However, "heat transfer" is still not well defined to my way of thinking. I have taken that phrase to mean "thermal energy transfer", which if true, equates heat with thermal energy. I solve the problem by not using the phrase "heat transfer". But the phrase is in use, and I don't know what it means. – garyp Feb 11 at 16:13
• @garyp, for chemical engineers, a term that is closely related to heat is enthalpy. Enthalpy is the heat content of a given mass of a substance, based on a standard reference temperature. – David White Feb 12 at 17:46