During Cyclic Thermodynamics process the net heat supplied to the system by the surroundings is equal to the net work done by the system on the surroundings.

Or

Heat and work are mutually convertible.

Or

It is impossible to construct a perpetual motion machine of first kind.

i.e.                         cyclic ∫dQ = cyclic ∫dW

Where Q represents the heat supplied and W represents the work done.

The law is not proved analytically but it is proved experimentally.