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Circuit and electromagnetism basics

When a DC voltage of 10 V is applied across a 100 Ω resistor, how much power does the resistor dissipate?

  1. A0.1 W
  2. B1 W
  3. C1000 W
Show answer and explanation

Answer: B. 1 W

The power in a resistor is P = VI = V²/R = I²R. The current is 10 V/100 Ω = 0.1 A, so P = 10 V × 0.1 A = 1 W (V²/R = 100/100 = 1 W). Power is proportional to the square of the voltage, so doubling the voltage quadruples it. A resistor rated at 1/4 W could be destroyed by the heat, so a part with enough margin above the dissipated power is chosen.

  • A. 0.1 W mistakes the current value (0.1 A) for the power.
  • B. Correct. V²/R = 10²/100 = 1 W.
  • C. 1000 W is voltage times resistance, which is not a power formula.
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