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Question:

The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10Ω is?

0.8Ω

0.5Ω

0.2Ω

1.0Ω

Solution:

I = E/(R+r)
0.2 × (10+r) = 2.1
10 + r = 2.1 / 0.2
10 + r = 10.5
r = 10.5 - 10
r = 0.5Ω