Question

Gas contained in a closed system consisting of piston-cylinder arrangement is expanded work done by the gas during expansion is 50 kJ. The decrease in internal energy of the gas during expansion is 30 kJ. Heat transfer during the process is equal to
1. -20 kJ
2. +20 kJ
3. -80 kJ
4. +80 kJ

Answer 1

Correct Answer - Option 2 : +20 kJ

CONCEPT:

• The first law of thermodynamics gives the relationship between heat, work, and internal energy.
• The first law of thermodynamics applies the conservation of energy principle to systems.
• The first law of thermodynamics states that the change in internal energy of a closed system equals the net heat transfer into the system minus the net work done by the system.

In equation form, the first law of thermodynamics is given by:

Q = ΔU + W

where Q is the sum of all heat transfers into and out of the system, W is the net work done by the system and ΔU is the change in internal energy.

Calculation:

Given:

Decrease in Internal Energy, ΔU = - 30 kJ

Work done by the system W = + 50 kJ

By the first law of thermodynamics Q = ΔU + W

Q = - 30 + 50

Q = +20 kJ