Find heat given to gas to take it from A to B. (Given: $C_v = 21$ S.I. units, $P_2 = 30$ Pa, $P_1 = 21.7$ Pa, $R = 8.3$ S.I. units, $n = 10$ moles) 
Step 1: Identify the nature of work done
From the P–V diagram, the process A → B occurs at constant volume.
Hence, work done during the process is zero.
W = 0
Step 2: Apply first law of thermodynamics
According to the first law:
Q = ΔU + W
Since W = 0 for an isochoric process:
Q = ΔU
Step 3: Expression for change in internal energy
For an ideal gas, change in internal energy is given by:
ΔU = (Cv / R) × (P2 − P1)V
Step 4: Substitute given values
Cv / R = 21 / 8.3
P2 − P1 = 30 − 21.7
Q = (21 / 8.3) × (30 − 21.7)
Step 5: Final calculation
Q = 21 J
Final Answer:
The heat supplied to the gas during the process is
Q = 21 J