State and explain the first law of thermodynamics

 The first law of thermodynamics is a form of law of conservation of energy applied to

a system. Mathematically it can be written as

\(Q=\triangle U+ W\)

where Q is heat energy, \(\triangle\)U is the change in the internal energy and W is work done.

Q is positive if heat is supplied to the system and negative if it is removed from the

system. \(\triangle\)U is positive if there is increase in internal energy and negative if there is

decrease in internal energy of the system. Work done W is positive if work is done

by the system (during expansion) and negative if work is done on the system (during

contraction). The heat energy supplied to a system can be used for increasing its

internal energy and for doing work.