How Physicists Define Heat Energy

Heat is a measure of the kinetic energy of particles of matter.
Heat is a measure of the kinetic energy of particles of matter. Tim Robberts / Getty Images

Heat energy (or thermal energy or simply heat) is a form of energy transfer among particles in a substance (or system) by means of kinetic energy of those particles. In other words, under kinetic theory, the heat is transferred by particles bouncing into each other.

In physical equations, the amount of heat transferred is usually denoted by the symbol Q.

Heat vs. Temperature

Note this crucial component to the above definition:

Heat always refers to the transfer of energy between systems (or bodies), not to energy contained within the systems (or bodies).

This distinction between heat and temperature is subtle, but very important. Heat refers to the total energy of the molecular motion or kinetic energy of a material. Temperature, on the other hand, is a measure of the average or apparent energy of molecular motion. In other words, heat is energy, while temperature is a measure of energy. Adding heat will increase a body's temperature, while removing heat will lower the temperature. In thermodynamics equations, heat is a quantity of energy which may be transferred between two systems. In contrast, both temperature and internal energy are state functions. Heat is measurable (as temperature), but it is not a material.

Example: The iron is hot, so it's reasonable to say it must have a lot of heat in it. Reasonable, but wrong.

It's more appropriate to say that it has a lot of energy in it (i.e. it has a high temperature), and touching it will cause that energy to transfer to your hand ... in the form of heat.

Units of Heat

As a form of energy, the SI unit for heat is the joule (J), though heat is frequently also measured in the calorie (cal), which is defined as "the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius." Heat is also sometimes measured in "British thermal units" or Btu.

Sign Conventions for Heat Energy Transfer

Heat transfer may be indicated by either a positive or negative number. Heat that is released into the surroundings is written as a negative quantity (Q < 0). When heat is absorbed from the surroundings, it is written as a positive value (Q > 0).

A related term is heat flux, which is the rate of heat transfer per unit cross-section area. Heat flux may be given in units of watts per square meter or joules per square meter.

Measuring Heat

A static measure of heat is temperature. Heat transfer may be calculated using equations or measured using calorimetry. Calculations of heat transfer are based on variations of the First Law of Thermodynamics.

Edited by Anne Marie Helmenstine, Ph.D.

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Jones, Andrew Zimmerman. "How Physicists Define Heat Energy." ThoughtCo, Aug. 10, 2017, Jones, Andrew Zimmerman. (2017, August 10). How Physicists Define Heat Energy. Retrieved from Jones, Andrew Zimmerman. "How Physicists Define Heat Energy." ThoughtCo. (accessed March 18, 2018).