A device consumes 100 W of power when connected to a 120 V source. Calculate its resistance.

Analyze the question: We're given power (P = 100 W) and voltage (V = 120 V), and we need the resistance (R).

Concept: Electrical power, voltage and resistance are related by:

P = V²/R

This comes from combining P = IV with Ohm's law (V = IR). Rearranging for resistance:

R = V²/P

Solution:

1. Write down the values: V = 120 V, P = 100 W
2. Substitute into the formula: R = (120)² / 100
3. Square the voltage: 120² = 14,400
4. Divide: R = 14,400 / 100 = 144 Ω

Check with current: I = P/V = 100/120 ≈ 0.833 A, then R = V/I = 120/0.833 = 144 Ω ✓

Answer: D. 144 Ω

Tip: When a question gives you power and voltage but no current, go straight to R = V²/P. Common mistakes are to divide 120 by 100 (giving 1.2 Ω, option A) or to multiply instead of divide. Always remember to square the voltage. As a sanity check, a 100 W bulb on a 120 V supply should have a resistance of a few hundred ohms at most, not just a few ohms.