Internal Resistance of a Source

Internal Resistance of a Source MDCAT MCQs with answers

12 past paper MCQs on Internal Resistance of a Source, from the Current Electricity unit of MDCAT Physics. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

Read the notes first Practise the unit as a quiz

1.The terminal voltage $V_t$ of a battery is greater than the emf of the battery when the: (MDCAT 2020)

  1. battery is charging
  2. battery is discharging
  3. battery is connected with a resistance
  4. battery is connected with a voltmeter
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Correct answer: (a) battery is charging

While charging, the current is driven into the battery, so $V_t=\varepsilon+Ir$ is above the emf.

2.A low voltage supply of e.m.f. 20 V and internal resistance $1.5\ \Omega$ is used to work a heater of resistance $6.5\ \Omega$ in a fish tank. What is the power delivered to the water in the fish tank? (MDCAT 2022)

  1. 41 W
  2. 50 W
  3. 53 W
  4. 62 W
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Correct answer: (a) 41 W

$I=20/(1.5+6.5)=2.5$ A, so $P=I^2R=(2.5)^2(6.5)\approx41$ W.

3.When current is neither drawn from a source nor given to it, then: (UHS MDCAT 2017)

  1. $E=V_T$
  2. $V_T>E$
  3. $E>V_T$
  4. both $V_T>E$ and $E>V_T$
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Correct answer: (a) $E=V_T$

With no current there is no drop across the internal resistance, so the terminal potential difference equals the emf.

4.Terminal voltage of a cell equals its EMF only when: (UHS MDCAT 2025)

  1. No current flows
  2. Current is maximum
  3. Internal resistance is infinite
  4. Load resistance is zero
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Correct answer: (a) No current flows

With no current there is no drop across the internal resistance, so the terminals read the full emf.

5.Internal resistance reduces the terminal voltage because it: (SIBA MDCAT 2025)

  1. Produces back emf
  2. Causes power loss inside the battery
  3. Increases the current
  4. Decreases the emf
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Correct answer: (b) Causes power loss inside the battery

Some of the emf is spent driving the current through the cell itself, and that part appears as heat inside it.

6.A high internal resistance battery is not suitable for heavy loads due to: (SZABMU MDCAT 2025)

  1. Excess voltage drop
  2. High terminal voltage
  3. Infinite emf
  4. Constant current
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Correct answer: (a) Excess voltage drop

A heavy load draws a large current, and the drop inside the cell then leaves little at the terminals.

7.A low voltage supply with an e.m.f. of 20 V and an internal resistance of 1.5 ohms is used to supply power to a heater of resistance 6.5 ohms in a fish tank. What is the power supplied to the water in the fish tank? (UHS MDCAT 2023)

  1. 41 W
  2. 50 W
  3. 53 W
  4. 62 W
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Correct answer: (a) 41 W

$I=20/8=2.5\,\mathrm{A}$, and $I^2R=6.25\times6.5$ gives about $41\,\mathrm{W}$ in the heater.

8.The terminal potential difference of a battery is greater than its emf when: (PMC MDCAT 2022)

  1. The internal resistance of a battery is infinite
  2. The internal resistance of a battery is zero
  3. The battery is being charged
  4. The battery is discharged
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Correct answer: (c) The battery is being charged

When current is driven into the cell the drop across the internal resistance adds to the emf.

9.Terminal potential difference of a cell: (PMC MDCAT 2022)

  1. Increases with increase in its internal resistance
  2. Decreases with increase in its internal resistance
  3. Its independent of its internal resistance
  4. Varies exponentially with resistance
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Correct answer: (b) Decreases with increase in its internal resistance

The greater the internal resistance the more of the emf is spent inside the cell.

10.Internal resistance of a battery is ____ ohm, if E=l0V, Vt=9V, I= lA. (PMC Practice 2021)

  1. 1
  2. 0.1
  3. 0.01
  4. None of these
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Correct answer: (a) 1

$r=(E-V)/I=(10-9)/1$, that is $1\,\Omega$.

11.EMF becomes equal to the terminal potential difference when the: (PMC Practice 2021)

  1. Circuit is closed
  2. Current is max
  3. Circuit is open
  4. All of these options are correct
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Correct answer: (c) Circuit is open

With no current there is no drop inside the cell, so the terminals read the full emf.

12.When the current is neither drawn from a source nor given to it then: (UHS MDCAT 2017)

  1. E=VT
  2. VT>E
  3. E>VT
  4. Both “VT>E” & “E>VT”
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Correct answer: (a) E=VT

With no current there is no drop across the internal resistance, so the terminal voltage equals the emf.

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