Question

An electric heater supplies heat to a system at a rate of 120 W. If system performs work at a rate of 80 J $s^{-1}$, the rate of increase in internal energy is :

• A. 30 J $s^{-1}$
• B. 40 J $s^{-1}$
• C. 50 J $s^{-1}$
• D. 60 J $s^{-1}$

Answer 1

The correct option is B 40 J $s^{-1}$

Q=120 W

W=80 Js${}^{-1}$

From the first law of thermodynamics:

$Q=U+W$

where U=internal energy

$\therefore U=Q-W=120-80W=40J{s}^{-1}$