A source has emf 12 V and internal resistance 2.0 Ω. What is the maximum power it can deliver to an external resistor?
Correct answer D. 18 W
Explanation
Maximum power occurs when R = r = 2.0 Ω, giving I = 12/4 = 3.0 A. Then P = I²R = 9.0 × 2.0 = 18 W, which equals E²/4r = 144/8.
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