Question 12
Current ElectricityIn the given circuit of Fig. 3 the cell P has an e.m.f. 1.5 V and an unknown internal resistance r while the cell Q has an e.m.f. 2.0 V and an internal resistance 1 ohm. If the ammeter reads 50 mA, 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.
Show the answer and explanation
Answer: D (2.0 ohms)
Explanation
The cells oppose each other, so the net e.m.f. is 2.0 − 1.5 = 0.5 V. Then 0.5 = 0.05 × (7 + 1 + r), so 7 + 1 + r = 10 and r = 2 Ω.