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JAMB Physics 2007 Question 18

Question 18

Alternating Current Circuits

A 120 V, 60 W lamp is to be operated on 220 V a.c. supply mains. Calculate the value of non-inductive resistance that would be required to ensure that the lamp is run on correct value.

  1. A.500Ω.
  2. B.300Ω.
  3. C.200Ω.
  4. D.100Ω.
Show the answer and explanation

Answer: C (200Ω.)

Explanation

Lamp current I = P/V = 60 ÷ 120 = 0.5 A. The resistor in series must take the other 220 − 120 = 100 V, so R = 100 ÷ 0.5 = 200 Ω.