Question

50 pounds of a working substance undergoes a non-flow process. The internal energy at the start of the process is 89 Btu/lbm and at the completion of the process is 150 Btu/lbm. A total of 350 Btu of work is done on the working substance. How much heat was added or removed from each pound of the working substance (Btu/lbm)

Answer 1

Applying the first law of thermodynamics, we know that the heat transferred is equivalent to the work done and the internal energy of the bodies.

In mathematical terms this can be expressed as

$Q = W+\Delta U$

According to the statement, the work is carried out on the substance, therefore, the work will be negative (by convention)

$Q = -350+50lbm(150-89)Btu/lbm$

$Q = 2700Btu$

Therefore 2700Btu were added as heat to the substance.

The value per unit of mass would be

$\dot{Q} = \frac{2700}{50}$

$\dot{Q} = 54Btu/lbm$

Therefore it was added around to 54Btu/lbm