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JAMB Physics 1980 Question 46

Question 46

Current Electricity

A cell has an open circuit voltage V1 and a closed circuit voltage V2 when a current I is flowing through it. Therefore, the internal resistance of the cell is

  1. A.V2/I
  2. B.V1/I
  3. C.(V1 + V2)/I
  4. D.(V1 − V2)/I
  5. E.(V2 − V1)/I.
Show the answer and explanation

Answer: D ((V1 − V2)/I)

Explanation

The open-circuit voltage is the e.m.f., V1. With current flowing, V2 = V1 − Ir, so the 'lost volts' V1 − V2 = Ir and r = (V1 − V2)/I.