A system absorbs 2 Kcals heat and does 500 J of work, then internal energy of the system will be …….

Options

A. 6400 J
B. 5400 J
C. 7900 J
D. 8900 J

Answer

C

Explanation

System absorbs heat
`therefore` dQ=+2000 cal =2000 x 4.2 J
=+8400 J
System does work,
`therefore DeltaW`=+500 J
From first law of thermodynamics,
`therefore DeltaW`=+500 J
dQ=dU+dW
`therefore` dU=dQ-dW
=8400-500
`therefore` dU=7900 J
It is positive so internal energy increases .