Question

Six secondary cells each of emf 3V and internal resistance 0.3Ω are joined in series with a 10Ω resistor. The potential drop across the external resistor is .

• A. 16V
• B. 17.2V
• C. 18V
• D. 15.2V

Answer 1

The correct option is D 15.2V

We have 6 cells in the same direction. So their EMF’s and internal resistances get added up.
So find the total EMF and R of the combination.
E=6 $\times$ 3 = 18V
R=6 $\times$ 0.3 = 1.8 $\Omega$
R’ = 10 $\Omega$
Since all these are in series, we just have to find current.
$I=\frac{E}{R+R^{\prime }}$
I=1.52A
So potential drop across 10 $\Delta$ is
V=IR’ = 15.2V