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Circuit and electromagnetism basics

A 10 Ω and a 20 Ω resistor are connected in parallel, and a total current of 3 A flows into the combination. What current flows through the 20 Ω resistor?

  1. A1.5 A
  2. B2 A
  3. C1 A
Show answer and explanation

Answer: C. 1 A

The parallel resistance is 10 × 20/(10 + 20) = 20/3 ≈ 6.67 Ω, so the voltage across both is 3 A × 20/3 Ω = 20 V. The 20 Ω resistor carries 20 V/20 Ω = 1 A, and the 10 Ω resistor carries 20 V/10 Ω = 2 A. The current-divider formula I × R(other)/(R1 + R2) = 3 × 10/30 = 1 A gives the same result. More current flows through the smaller resistance.

  • A. 1.5 A is an equal split, which holds only for equal resistors.
  • B. 2 A is the current in the 10 Ω resistor.
  • C. Correct. 3 × 10/(10 + 20) = 1 A.
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