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(iv)There is no change in internal energy in a cyclic process.

 Heat and work:
Change in internal energy is accompanied by heat and work.
Heat:It is the form of energy which brings the change in temperature. It is

denoted by q.
Exothermic Process: The process in which heat energy is released.
q = - ve

Endothermic Process: The process in which heat energy is absorbed.
q = + ve
Work:Work done is a boundary phenomenon. In adiabatic process,
w = +ve if work is done on the system.

w = -ve if work is done by the system.
 First law of Thermodynamics:It has the following statements.

(i)Energy can neither be created nor be destroyed, but it can be
transformed from one form to another.
(ii)The total energy of the universe is constant.
(iii)The total energy of an isolated system remains constant.

Mathematical form of first law of thermodynamics is,
∆U = q + w

 Adiabatic work done (W ):
ad
Consider compression of a gas in a cylinder by pressing a piston.
Let F – force applied by the piston.
P – initial pressure of the gas.
in
P – final pressure of the gas.
ex
V – initial volume of the gas.
i
V – final volume of the gas.
f
A – Area of cross section of the piston
l – distance moved by the piston
P = F/A
ex
F = P . A
ex
Adiabatic work done, W = F. l
ad
= P . A. l
ex
= P (-∆V)
ex
= - P . ∆V
ex
 Reversible work done (W ):
rev