Q.Five identical cells, each of EMF $2\ \text{V}$ and internal resistance $1\ \Omega$, are connected in parallel. This combination is connected to an external resistor of $1.8\ \Omega$. Find the current in the external resistor.

Grouping of Cells

Cells can be combined to obtain a required EMF or current. In series the EMFs add and so do the internal resistances: for $n$ identical cells of EMF $\varepsilon$ aiding one another, the net EMF is $n\varepsilon$. If some cells are connected the wrong way round, each reversed cell subtracts twice its EMF from the total: with $n$ cells of which $m$ are reversed, the resultant EMF is $(n-2m)\varepsilon$. For example, $12$ cells of $5\,$V with $3$ reversed give $(12-6)\times5 = 30\,$V.

In parallel the EMF stays that of one cell while the internal resistances combine in parallel, allowing larger currents. A mixed (series–parallel) grouping is used to deliver maximum power to a load, which occurs when the total internal resistance equals the external resistance. Analysing a battery of cells therefore means adding EMFs with the correct signs and combining the internal resistances appropriately.

Grouping of cells in series, parallel and mixed combinations is a core part of the CBSE Class 12 Physics Current Electricity chapter, searched as "grouping of cells formula and examples" or "cells in series parallel important questions class 12". Handling reversed cells and finding the resultant EMF is a classic numerical asked in board exams and NEET.