Heat is a form of energy called thermal energy which flows from a higher temperature body to a lower temperature body when they are placed in contact.

Heat or thermal energy of a body is the sum of kinetic energies of all its constituent particles, on account of translational, vibrational and rotational motion.

The SI unit of heat energy is joule (J).

The practical unit of heat energy is calorie.

1 cal = 4.18 J

1 calorie is the quantity of heat required to raise the temperature of 1 g of water by 1°C.  Mechanical energy or work (W) can be converted into heat (Q) by 1 W = JQ

where J = Joule’s mechanical equivalent of heat.

J is a conversion factor (not a physical quantity) and its value is 4.186 J/cal.