A cell can supply current of 0.8A and 0.4A through a 2Ω and a 3Ω resistors respectively. Calculate the internal resistance of the cell.

  • A 0.2Ω
  • B 0.4Ω
  • C 1.0Ω
  • D 3.0Ω
  • E 9.0Ω
waec 1988physics

The correct answer is C. 1.0Ω

Assume the internal resistor = R

Current with 2 Ohm Resistor = 0.80A

Voltage = (R + 2) x 0.80 = V ; That is 0.8R + 1.6 = V ———————[[1]

Current with 3 Ohm Resistor = 0.40 A

Voltage = (R+3) x 0.40 = V ; That is 0.4R + 1.2 = V ————————[[2]

Subtracting 1 from 2 gives

--> (0.8 - 0.4) R 

     ( 1.6 - 1.2 ) 

     ( V - V ) 

Therefore 

0.4R  + 0.4 = 0

R = 0.4 /0.4 = 1 Ohms

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