The first law of thermodynamics is a statement of the law of conservation of energy applied to thermodynamic systems.
The heat supplied to a system is equal to the sum of the increase in internal energy of the system and the work done by the system.
Q = ΔU + W
where,
Q = heat supplied to the system
ΔU = change in internal energy
W = work done by the system
Temperature constant → Internal energy change is zero.
ΔU = 0 ⇒ Q = W
Volume constant → Work done is zero.
W = 0 ⇒ Q = ΔU
No heat exchange.
Q = 0 ⇒ ΔU = −W
Pressure constant.
Q1. First law of thermodynamics is based on: A) Newton’s law B) Law of gravitation C) Law of conservation of energy D) Hooke’s law Answer: C Q2. Mathematical form of first law is: A) Q = W − ΔU B) Q = ΔU + W C) ΔU = Q + W D) W = Q + ΔU Answer: B Q3. In an isothermal process, the change in internal energy is: A) Maximum B) Minimum C) Zero D) Negative Answer: C Q4. In an isochoric process, work done is: A) Maximum B) Zero C) Negative D) Infinite Answer: B Q5. In an adiabatic process, heat exchanged is: A) Maximum B) Minimum C) Zero D) Infinite Answer: C Q6. If 500 J of heat is supplied to a system and 200 J of work is done by the system, find the change in internal energy. A) 300 J B) 700 J C) −300 J D) −700 J Answer: A Q7. If work is done on the system, W is: A) Positive B) Negative C) Zero D) Constant Answer: B Q8. In which process Q = ΔU? A) Isothermal B) Adiabatic C) Isobaric D) Isochoric Answer: D
Heat supplied to a system is equal to increase in internal energy plus work done by the system.
Q = ΔU + W
Numericals are frequently asked using Q = ΔU + W with correct sign convention.
