Definition: Heat energy (or just heat) is a form of energy which transfers among particles in a substance (or system) by means of kinetic energy of those particle. In other words, under kinetic theory, the heat is transfered by particles bouncing into each other.
In physical equations, the amount of heat transferred is usually denoted with 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 can be very confusing, because we're used to in day-to-day conversation talking about heat as if it's contained in something. This distinction between heat and temperature is subtle, but very important.
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.Also Known As: thermal energy
