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.