Physics · Current Electricity · NEET
From I = E/(R + r), current is biggest when R is smallest. The smallest R can be is 0 (a plain wire, a short circuit). But r never disappears — the cell always has internal resistance. So even at R = 0 the current is not infinite; it settles at I_max = E/r. The internal resistance r is the only thing left to limit it.
It means the two terminals of the cell are joined by a path of nearly zero resistance (like a bare wire directly across the cell). There is almost no external resistance R to slow the current, so the cell pushes out its maximum current.
Terminal voltage V = E - I·r. At short circuit I = E/r, so I·r = E, giving V = E - E = 0. All the EMF is used up inside the cell across its own internal resistance. No voltage is left for the outside, which makes sense because the outside resistance is zero.
No. For n identical cells in series, both the total EMF and the total internal resistance grow together: net EMF = nE, net internal resistance = nr. So I = nE/(nr) = E/r. The n cancels — short-circuit current stays E/r, independent of n. This exact idea is a NEET 2018 question.
Because I_max = E/r can be very large when r is small (good cells have tiny r). This huge current makes power I²r heat up the cell fast, which can damage or burst it. That is why NCERT notes the allowed current is kept far below E/r.
A battery of n identical cells (each internal resistance r) in series is short-circuited and the current I is measured. Which graph best shows I versus n?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
I_max = E/r, where E is the EMF of the cell and r is its internal resistance. It is obtained from I = E/(R + r) by putting the external resistance R = 0.
When the external resistance is zero, that is, during a short circuit. Then only the internal resistance r remains to limit the current.
No. During a short circuit the current is maximum (E/r) but the terminal voltage is zero, because all the EMF drops across the internal resistance r.
Rearrange I_max = E/r to get r = E/I_max. If you know the EMF and the measured short-circuit current, you can find the internal resistance.
Because both the total EMF (nE) and the total internal resistance (nr) increase by the same factor n, so I = nE/nr = E/r stays fixed.
