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

I=εR+ror  IR+Ir=εHere,  R=10Ω, r=?, ε=2.1V,I=0.2A ∴  0.2×10+0.2×r=2.1 2+0.2r=2.1 0.2r=0.1  or  r=12=0.5Ω