Heat is a form of energy which is transferred between a system and its surrounding as a result of temperature difference only. Once it is transferred, it becomes the internal energy of the receiving body. When we say that a body is heated, it means its molecules begin to move with greater kinetic energy.

Heat is meaningful only as long as the energy is being transferred. Expression like "heat in a body " or "heat of a body" is meaningless.

S.l unit of heat energy is Joule (J). Another common unit of heat energy is calorie (cal).

1 calorie = 4.18 J

`text(1 calorie :)` The amount of heat needed to increase the temperature of 1 gm of water from `14.5^oC` to `15.5^oC` at one atmospheric pressure is 1 calorie.

`text(Mechanical Equivalent of Heat :)`

If mechanical work W produces the same temperature change as heat H, then we can write,

`W=JH`

Where, J is called mechanical equivalent of heat. J is expressed in joule/calorie.

Consider the system to be a certain mass of gas contained in a cylinder with a movable piston as shown in Fig. Experience shows there are two ways of changing the state of the gas (and hence its internal energy).

One way is to put the cylinder in contact with a body at a higher temperature than that of the gas.

The other way is to push the piston down i.e. to do work on the system, which again results in increasing the internal energy of the gas.

In short, heat and work are two different modes of altering the state of a thermodynamic system and changing its internal energy.

Heat and work in thermodynamics are not state variables. They are modes of energy transfer to a system resulting in change in its internal energy.

We know that every bulk system consists of a large number of molecules.

Internal energy is simply the sum of the kinetic energies and potential energies of these molecules.

Internal energy is the sum of molecular kinetic and potential energies in the frame of reference relative to which the centre of mass of the system is at rest.

Thus, it includes only the (disordered) energy associated with the random motion of molecules of the system. We denote the internal energy of a system by U.

U is simply a macroscopic variable of the system.

The important thing about internal energy is that it depends only on the state of the system, not on how that state was achieved.

Internal energy U of a system is an example of a thermodynamic -state variable- - its value depends only on the given state of the system.

Thus, the internal energy of a given mass of gas depends on its state described by specific values of pressure, volume and temperature.

If we neglect the small intermolecular forces in a gas, the internal energy of a gas is just the sum of kinetic energies associated with various random motions of its molecules.