First law of thermodynamics
Let us consider a gas inside a cylinder fitted with a movable frictionless piston. The walls of the cylinder are made up of non conducting material and the bottom is made up of conducting material (Fig.).
Let the bottom of the
cylinder be brought in contact with a hot body like burner. The entire heat
energy given to the
gas is not converted into work. A part of the heat energy is used up in
increasing the temperature of the gas (i.e) in increasing its internal energy
and the remaining energy is used up in pushing
the piston upwards
(i.e.) in doing work.
If ∆Q is the heat
energy supplied to the gas, U1 and U2 are initial and
final internal energies and ∆W is the work done by the system, then
∆Q = ∆W + (U2
∆Q = ∆W + ∆U
where ∆U is the change
in the internal energy of the system.
Hence, the first law of
thermodynamics states that the amount of heat energy supplied to a system is
equal to the sum of the change in internal energy of the system and the work
done by the system. This law is in accordance with the law of conservation of