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Circuit and electromagnetism basics

Resistors R1 = 2 kΩ and R2 = 1 kΩ are connected in series, in that order from the positive terminal, across a 12 V DC supply (the other end of R2 goes to the negative terminal). What is the voltage across R2? (No current is drawn from the output.)

  1. A4 V
  2. B8 V
  3. C6 V
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Answer: A. 4 V

The same current, I = 12 V/(2 kΩ + 1 kΩ) = 4 mA, flows through both resistors, so the voltage across R2 is 4 mA × 1 kΩ = 4 V. The divider formula V × R2/(R1 + R2) = 12 × 1/3 = 4 V gives the same result. R1 drops 8 V, and the two add up to the 12 V supply (Kirchhoff's voltage law). Connecting a load to the output puts it in parallel with R2 and lowers the voltage.

  • A. Correct. 12 V × 1/(2 + 1) = 4 V.
  • B. 8 V is the voltage across R1—the wrong resistor in the numerator.
  • C. 6 V would be the result if the two resistors were equal.
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