Consider a cylinder with a movable piston contains a gas.
The gas does work in moving the piston.
The work done by the system against Constant pressure P is
\(\triangle\) W = Force\(\times\)distance
= Pressure \(\times\)Area\(\times\)Distance
= PA dx.
\(\therefore\) \(\triangle\) W = P. \(\triangle\) V. [\(\triangle\)V is the change in volume of the gas]
So the first law of thermodynamics takes the form 
\(\triangle\)Q = \(\triangle\)U + P \(\triangle\)V

Consider a gas containing in a cylinder of area A and provided with frictionless movable piston.
P - pressure of the gas.
Force exerted by the gas on the piston.
F =P\(\times\)A.
The gas expands and pushes the piston through a small distance dx.
Work done by the gas dW = Fdx = PAdx = P.dv.
dv = Adx \(\rightarrow\) Change in volume of the gas
The total work done by the gas when its volume increases from V₁ to V₂ will be 
\(w=\int { dw } =\int _{ { V }_{ 1 } }^{ { v }_{ 2 } }{ pdv } \)

Consider a fixed mass of a gas in a cylinder provided with a movable piston.
The internal energy can be increased in two ways.
(i) Place the cylinder over a hot body. Heat energy will flow from the hotter body to the gas due to temperature difference. This increases the Internal Energy.
(ii) Push the piston down by keeping some weight on it. Work is done on the gas. This also increases internal energy.

(i) In regulating the temperature of human body
(ii) In maintaining comfortable room temperature in cold countries
(iii) In the formation of trade winds
(iv) Land and sea breezes.

The condition of heat depends on
(i) Area of cross-section of the body
(ii) Temperature difference between the faces of the body
(iii) Time of heat flow through the body
(iv) Thickness of the body.
(v) Nature of the material of the body.