Heat energy is a form of energy that is transferred from one body to another as a result of a difference in temperature. It is the result of the movement of tiny particles such as atoms, molecules, or ions in solids, liquids, and gases. Heat energy can be transferred from one object to another, and the transfer or flow due to the difference in temperature between the two objects is called heat. Heat energy can be expressed in several different physical concepts, including internal energy, enthalpy, and thermal energy.
Thermal energy refers to the energy contained within a system that is responsible for its temperature. Heat is the flow of thermal energy. In thermodynamics, heat is energy transferred to or from a thermodynamic system by mechanisms other than thermodynamic work or transfer of matter, such as conduction, radiation, and friction. Heat refers to a quantity transferred between systems, not to a property of any one system, or "contained" within it.
Heat energy can be measured in units of work, such as joules, foot-pounds, kilowatt-hours, or calories. The two units of heat most commonly used are the calorie and the British thermal unit (BTU) . Heat energy can be transferred through conduction, convection, and radiation. For example, when you put an ice cube in a glass of lemonade, the lemonade (which is warmer) will transfer some of its heat energy to the ice, eventually melting it and reaching a state of thermal equilibrium.
