In the given circuit of below the cell P has an e.m.f. 1.5V and an unknown internal resistance r while the cell Q has an e.m.f. 2.0V and an internal resistance of 1ohm. If the ammeter reads 50mA, then r is equal to A. 0.5 ohm B.0.62 ohm C. 1.0 ohm D. 2.0 ohms E. 3.0 ohms.
ANSWER: Option D
EXPLANATION: Since the cells are connected -ve to -ve, the total emf will be 2.0V – 1.5V = 0.5V
emf = I(R + r + r2)
Where; emf is the resultant Volts the two cells gives
I is the current in the circuit
r is the internal resistance of the first cell
r2 is the internal resistance of the second cell
0.5 = 50 × 10⁻³ (7 + r + 1)
0.5 = 50 × 10⁻³ (8 + r)
Now, solve for r to get your answer!
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