12.8 An electric heater supplies heat to a system at a rate of 100W. If the system performs work at a rate of 75 joules per second. At what rate is the internal energy increasing?

It is given in the question that:

Heat supplied, Q = 100 J/s

Work done, W = 75 J/s

From the first law of thermodynamics, we have: 

Q = U + W

U = Q – W

= 100 – 75

= 25 J/s

Therefore, the internal energy of the given electric heater increases at a rate of 25 J/s.