Question

32 cells, each of emf 3v, are connected in series and kept in a box. externally, the combination shows an emf of 84 V. the number of cells reversed in the combination is
A. 0
B. 2
C. 4
D. 8

Answer 1

Hint: The internal resistance acts to decrease the emf of the battery detected by the voltmeter. So it will show a reading less than the emf of the battery. Kirchhoff’s rule can be used for the loop. So, by that we will get the current in the circuit. Also since cell are reversed they will have negative impact on balance

Complete answer:
Every cell possesses an internal resistance which acts to reduce the actual emf the charges of the cell produce. This internal resistance acts as though a resistor is connected in series to the ideal cell. To calculate the voltage in the voltmeter and current in the ammeter, we apply Kirchhoff’s rule to the loop to find the current.

Since a box of 32 cells are connected in series and the output is 84V and suppose n number of cells are reversed. Then, we can write equation like
$3 \times 32 - 2n \times 3 = 84$
$6n = 96 - 84$
$ \Rightarrow n = 2$

So, the correct answer is 2 cells are reversed. Thus, option B is the right answer.

Note: Electromotive force is defined as the electric potential produced by either electrochemical cell or by changing the magnetic field near it. EMF is the acronym used for electromotive force. A generator is used for the conversion of energy from one form to another. One terminal becomes positively charged while the other becomes negatively charged. Therefore, an EMF is a work done on a unit electric charge.