110 J of heat is added to a gaseous system, whose internal energy change is 40 j. then the amount of external work done is

Options

A. 140 J
B. 70 J
C. 110 J
D. 150 J

Answer

B

Explanation

Quantity of heat supplied to system is equal to increase in internal energy and external work.
According to first law of thermodynamics , when some quantity of heat (dQ) is supplied to a system capable of doing external work, then the quanitity of heat absorbed by the system (dQ) is equal to sume of the increases in the internal energy of the system (dU) due to rise in temperature and the external work done by the system (dW) in expansion , i.e . `dQ=dU+dW`
When a gas expands , work done by the gas, dW, is taken as positive , when a gas is compressed , work is done on the gas, dW is taken as negative.
`DeltaW=DeltaQ-DeltaU`
=110-40=70