How Physicists Define Heat Energy

Heat and Transfer of Energy

Illustration of an iron on a piece of clothing, with the definition of heat energy above it

Heat energy is also accurately called thermal energy or simply heat.  It is a form of energy transfer among particles in a substance (or system) by means of kinetic energy. In other words, heat is transferred from one location to another by particles bouncing into each other.

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

Heat vs. Temperature

It is crucial to understand the difference between heat and temperature. This distinction between heat and temperature is subtle but very important.

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

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

You can measure the temperature of a room by placing a thermometer in the room and measuring the ambient air temperature. You can add heat to a room by turning on a space heater. As heat is added to the room, the temperature rises.

In thermodynamics equations, heat is a quantity of energy which may be transferred between two systems. In contrast, both temperature and internal energy are static 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

The SI unit for heat is a form of energy called the joule (J). 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

Heat may be measured as a static state or as a process. A static measure of heat is temperature. Heat transfer (a process that occurs over time) may be calculated using equations or measured using calorimetry. Calculations of heat transfer are based on variations of the First Law of Thermodynamics.