The heat energy of a substance is determined by how active its atoms and molecules are. A hot object is one whose atoms and molecules are excited and show rapid movement. A cooler object’s molecules and atoms will be less excited and show less movement. When these guys are in the excited state, they take up a lot of space because they’re moving around so fast. When the atoms and molecules settle down, or cool down, they take up less space.
In physics and thermodynamics, heat is energy transferred from one body or thermodynamic system to another due to thermal contactwhen the systems are at different temperatures. It is also often described as the process of transfer of energy between physical entities. In this description, it is an energy transfer to the body in any other way than due to work performed on the body.In engineering, energy transfer by heat between objects is classified as either thermal conduction, first described scientifically by Joseph Fourier, by fluid convection, which is the mixing of hot and cold fluid regions due to pressure differentials, and by thermal radiation, the transmission of electromagnetic radiation described by black body theory.
Thermodynamically, energy can only be transferred by heat between objects, or regions within an object, with different temperatures, a consequence of the zeroth law of thermodynamics. This transfer happens spontaneously only in the direction to the colder body, as per thesecond law of thermodynamics. The transfer of energy by heat from one object to another object with an equal or higher temperature can happen only with the aid of a heat pump via mechanical work or by using mirrors or lenses to focus electromagnetic radiation which thereby increase its energy flux density.
A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Heat is also often referred to as thermal energy, although many definitions require this thermal energy to be in transfer between two systems to be called heat, otherwise, many sources prefer to continue to refer to the internal quantity as thermal energy.