Question

A steam piston system is heated by 72 Btu of heat. 7 Btu of heat is lost to the environment. The piston does 12 Btu of work as it rises. What is the change in the energy of the water steam in the system?

Answer 1

ΔU = 53 BTU

Step-by-step explanation:

In thermodynamics we consider heat entering a system as positive and heat leaving as negative.

Also, work performed by the system is positive and work applied to the system is negative.

The total heat applied to the system is:

Q = 72 - 7 = 65 BTU

The work is:

L = 12 BTU

The first law of thermodynamics states that:

Q = L + ΔU

Where ΔU is the change in internal energy.

Rearranging:

ΔU = Q - L

ΔU = 65 - 12 = 53 BTU