Correct Answer - Option 2 : 20 kJ decrease
Concept:
The first law of thermodynamics is a restatement of the law of conservation of energy. It states that energy cannot be created or destroyed in an isolated system; energy can only be transferred or changed from one form to another.
When heat energy is supplied to a gas, two things may occur:
- The internal energy of the gas may change
- The gas may do some external work by expanding
According to the first law of Thermodynamics:
ΔQ = ΔW + ΔU ...(i)
i.e., When a process is executed by a system, the change in stored energy of the system is numerically equal to the net heat interaction minus the network interaction during the process:
Here, ΔQ = heat exchange, ΔW = work done due to expansion, ΔU = internal energy of the system.
Sign convention:
ΔQ = Positive (Heat supplies to the system)
ΔQ = Negative (Heat rejected from the system)
ΔW = Positive (Work done by the system)
ΔW = Negative (Work done on the system).
Calculation:
Given:
ΔQ = -40 kJ, ΔW = -20 kJ
Using Equation (i)
ΔQ = ΔW + ΔU
-40 = -20 + ΔU
ΔU = -20 kJ
20 kJ Decrease in internal energy.