An electric heater supplies heat to a system at 100 W. The system performs mechanical work at a rate of 75 J·s⁻¹. At what rate does its internal energy increase?
Options

Question

An electric heater supplies heat to a system at 100 W. The system performs mechanical work at a rate of 75 J·s⁻¹. At what rate does its internal energy increase?

G1 Physics · Accepted Answer

Step 1: Apply the first law of thermodynamics: dQ = dU + dW, where dW is work performed by the system. Step 2: Divide by dt: dU/dt = dQ/dt − dW/dt. Step 3: Substitute heat input 100 J·s⁻¹ and work output 75 J·s⁻¹. Step 4: dU/dt = 100 − 75 = 25 J·s⁻¹ = 25 W. Final answer: 25 W (Option B).