Question

If dQ is the heat given to  thermodynamic system and dU is its change in internal energy and dW is the work done on the system, then the first law of thermodynamics concludes that:
1. dQ = dW
2. dQ = dU - dW
3. dQ = dW + dU
4. dQ = dU

Answer 1

Correct Answer - Option 2 : dQ = dU - dW

The correct answer is option 3) i.e. dQ = dU - dW

CONCEPT:

• The first law of thermodynamics: It states that energy can neither be created nor destroyed, but can only be converted from one form to another. 
  • The heat energy supplied to a thermodynamic system increases its internal energy and the remaining heat energy is converted into the work done by the system.
  • If dQ is the heat given to the thermodynamic system and dU is its change in internal energy and dW is the work done by the system, then first of law of thermodynamics gives

⇒ dQ = dW + dU

​EXPLANATION:

Q → +ve (when the heat is transferred to the system)

Q → -ve (when the heat is taken out of the system)

W → +ve (when the work is done by the system)

W → -ve (when the work is done on the system)

• From the first law of thermodynamics,0

⇒ dQ = dU - dW