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

__Solution__:∴Heat supplied,

*Q*= 100 J/s

The system performs at a rate of 75 J/s.

∴Work done,

*W*= 75 J/s

From the first law of thermodynamics, we have:

*Q*=

*U*+

*W*

Where,

*U*= Internal energy

∴

*U*= Q –

*W*

= 100 – 75

= 25 J/s

= 25 W

Therefore, the internal energy of the given electric heater increases at a rate of 25 W.

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