Question

(a) Six lead-acid type of secondary cells each of emf 2.0 V and internal resistance $0.015\Omega$ are joined in series to provide a supply to a resistance of $8.5\Omega$ . What are the current drawn from the supply and its terminal voltage?

(b) A secondary cell after long use has an emf of 1.9 V and a large internal resistance of $380\Omega$ . What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?

Answer 1

(a)

Total resistance in circuit = $R=6\times 0.015+8.5=8.59\Omega$

Total EMF, $E=12V$

So current in the circuit $I=E/R=1.4A$

Terminal voltage, $V=1.4\times 8.5=11.9V$

(b)

The current will be $1.9/380=0.005A$. It is impossible to start a motor because a starter motor requires large current ($\sim$100 A) for a few seconds