alleryStart learning →Why is a plain voltmeter unable to give the emf of a cell exactly?
AIt reads only direct current, never the emf
BIts scale cannot be marked finely enough for it
CIt needs a current to work, so a drop appears
DIt has to be calibrated against a standard cell
Answer & Solution
Correct answer: C. It needs a current to work, so a drop appears
1. Emf is the potential difference of a cell when no current flows.
2. A standard voltmeter has to draw some current in order to give a reading.
3. That current produces an internal drop Ir, so what appears is terminal voltage, not emf.
4. The emf could be recovered from V = emf minus Ir only if r were known precisely, and usually it is not.
5. A null method sidesteps the problem entirely by driving the current in the detector to zero.
_Source: OpenStax College Physics (CC BY 4.0), Ch 21 "Circuits and DC Instruments", section 21.5 Null Measurements_
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