Question

What is heat? State its S.I. unit.

Answer 1

Heat, energy that is transferred from one body to another as the result of a difference in temperature. If two bodies at different temperatures are brought together, energy is transferred in essence, heat flows from the hotter body to the colder. The S.I. unit of heat is joule.

Complete answer:
Heat is the transfer of kinetic energy from one medium or object to another, or from an energy source to a medium or object. This is the amount of heat required to raise the temperature of one pound of pure liquid water by one degree Fahrenheit. The unit of heat is the same as that of energy. The S.I. unit of heat is Joule.
The Three Mechanisms of Heat Transfer:
Conduction: Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules. Conduction occurs more readily in solids and liquids, where the particles are closer together, than in gases, where particles are further apart.
Convection: Convection is the process of heat transfer by the bulk movement of molecules within fluids such as gases and liquids. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid.
Radiation: Radiation is energy that comes from a source and travels through space at the speed of light. This energy has an electric field and a magnetic field associated with it, and has wave-like properties. You could also call radiation “electromagnetic waves”.

Note:
Primary sources can be used directly, as they appear in the natural environment: coal, oil, natural gas and wood, nuclear fuels, the sun, the wind, tides, mountain lakes, the rivers and the Earth heat that supplies geothermal energy. Geothermal systems provide the most efficient type of heating. They can cut heating bills by up to $70\%$.