Level 1: One Torch Cell in Backwards: What Voltage Is Left?
Add each cell's internal resistance.
Four cells, each of EMF 1.5 V and internal resistance 0.5 Ω, are connected in series with one cell reversed. They drive a current through an external 2 Ω resistor. The current is:
The EMFs add with signs: three cells forward and one reversed give 3 V. The internal resistances do not cancel, because all four cells are still in the path of the current.
Leaving out the reversed cell's internal resistance gives 3/3.5 ≈ 0.86 A, and ignoring the reversal gives 1.13 A or 1.5 A. A reversed cell subtracts its EMF but still adds its resistance.
Every Level 2 starts from a quick question, Level 1, and changes one thing. Seeing what that change does is the skill JEE Main tests. Try it here, free and with no signup, then read the worked answer.
