From P - V diagram, Process 1: P= constant, Volume (V) increases and temperature (T) also increases. Work done W=∫PdV= positive, ∆U=
f
2
nR∆T= positive Therefore heat is absorbed by the gas. ∵∆Q=∆U+W Process -2: Volume (V)= Constant, Pressure (P) decreases. ∴ Temperature (T) decreases. ∵P∝T ∴W=∫PdV=0 ∆T is negative and ∆U=
f
2
nR∆T ∆Q=∆U+W= Negative Heat is rejected by the gas. Process 3: P= Constant, Volume (V) decreases. ∴ Temperature (T) also decreases. W=P∆V= Negative ∆U=
f
2
nR∆T= negative ∴∆Q=W+∆U= negative ∴ Heat is rejected by the gas. Process 4: V= constant, Pressure (P) increases. ∴W=∫PdV=0 PV=nRT⇒ Temperature increases ∴∆U=
f
2
nR∆T is positive ∴∆Q=∆U+W= positive Therefore heat is absorbed by the gas.