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NEET Physics · Current Electricity

Cells: EMF, internal resistance and combinations

Taught in NCERT Class 12 Physics, Chapter 3: Current Electricity.

2 verified questions: Easy 0 · Medium 1 · NEET level 1

Sample questions

Sample 1Medium

A circuit shows a cell of emf 2 V and internal resistance 0.5 Ω connected to an external resistor of 3.5 Ω, with an ideal voltmeter across the cell's terminals. What does the voltmeter read?

  1. A

    2.00 V

  2. B

    0.25 V

  3. C

    1.75 V

  4. D

    1.50 V

Show the answer
The answer is C.

Current I = ε/(R + r) = 2/4 = 0.5 A. Terminal voltage V = ε − Ir = 2 − 0.5 × 0.5 = 1.75 V.

  • A.

    2 V is the emf, read only when no current flows.

  • B.

    0.25 V is the drop across the internal resistance, not the terminal voltage.

  • D.

    1.50 V takes the internal drop as 0.5 V, i.e. uses I = 1 A.

NCERT: Class 12 Physics, Chapter 3

Sample 2NEET level

A cell of emf 2.0 V and internal resistance 0.5 Ω is connected across an external resistor of 3.5 Ω. The potential difference across the terminals of the cell is

  1. A

    0.25 V

  2. B

    2.0 V

  3. C

    2.25 V

  4. D

    1.75 V

Show the answer
The answer is D.

The current is I = ε/(R + r) = 2.0/(3.5 + 0.5) = 0.5 A, so the terminal voltage is V = ε − Ir = 2.0 − 0.25 = 1.75 V (equal to IR = 0.5 × 3.5). The cell loses Ir across its own internal resistance.

  • A.

    0.25 V is the drop Ir inside the cell, not the terminal voltage.

  • B.

    2.0 V is the emf, the terminal voltage only when no current flows.

  • C.

    Adds Ir to the emf, as for a cell being charged, not one supplying current.

NCERT: Class 12 Physics, Chapter 3 (Current Electricity), Cells, emf, internal resistance

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