Heat is a form of energy that is transferred between systems or objects due to a difference in temperature. It flows spontaneously from a hotter body to a cooler one until thermal equilibrium is reached, meaning both bodies reach the same temperature
. At the microscopic level, heat corresponds to the kinetic energy of particles—atoms, molecules, or ions—in matter. These particles are always in motion, and their motion constitutes thermal energy. When heat is transferred, it involves the transfer of this kinetic energy from faster-moving (hotter) particles to slower-moving (cooler) ones
. Heat transfer can occur through several mechanisms:
- Conduction: Direct transfer of energy through particle collisions in solids or between objects in contact.
- Convection: Transfer of heat by the movement of fluids (liquids or gases).
- Radiation: Transfer of energy through electromagnetic waves without requiring a medium
Heat is measured in units of energy such as joules (J), calories (cal), or British thermal units (BTU), distinct from temperature, which measures the average kinetic energy of particles in a system
. In summary, heat is the energy in transit due to temperature differences, involving the transfer of microscopic kinetic energy that causes changes in temperature or phase of substances
