Example of current electricity | Learn Physics

Example of current electricity

Example  of current electricity: Twelve cells each having the same e.m.f. are connected in series and are kept in a closed box. Some of the cells are connected in reverse order. This battery is connected in series with an ammeter, an external resistance R and two cells of the same type as in the battery. The current when they aid each other is 3 ampere and is 2 ampere when the two opposite each other. How many cells are connected in reverse order?

Solution: if one cell is connected in reverse order, it will destroy one more cell from the circuit. If n cells are connected in reverse order, then 2n cells will be destroyed. If E is the e.m.f. of each cell, then effective e.m.f. of 12 cells will be = ( 12 – 2n)E

This battery if supports two similar cells inseries and then supplies a current of 3 amperes to resistance

3=[(12 – 2n)E +2E]/R=(14-2n)E/R                              ………………..(i)

If two cells and battery oppose each other, then the current in the external resistance is 2 ampere.

Hence,

2 = [(12-2n)E-2E]/R = (10-2n)E/R                    ……………………….(ii)

Dividing (i) by(ii), we get

3/2=(14-2n)/(10-2n)    or    n=1

Ans: 1 cell