Chapter 21 MCQs

Assume cells, ammeters and voltmeters are ideal unless stated otherwise.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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1

A current of 0.40 A flows through a lamp for 2.5 minutes.

How much charge passes through the lamp?

A 1.0 C
B 6.0 C
C 60 C
D 375 C

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2

A charge of 180 C passes a point in a circuit in 3.0 minutes.

What is the current?

A 0.017 A
B 1.0 A
C 60 A
D 540 A

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3

A component has a current of 25 mA through it for 4.0 s.

How much charge passes through the component?

A 0.00625 C
B 0.10 C
C 6.25 C
D 100 C

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4

A charge of 7.2 × 10⁻³ C passes through a component in 1.8 ms.

What is the current?

A 4.0 × 10⁻⁶ A
B 4.0 × 10⁻³ A
C 4.0 A
D 4000 A

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5

Which statement correctly describes conventional current in a metal wire?

A flow of electrons from positive to negative
B flow of positive ions from negative to positive
C direction from positive terminal to negative terminal
D direction opposite to the electric field

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6

In a metal wire connected to a cell, the charge carriers are:

A protons only
B neutrons only
C free electrons
D positive ions moving through the metal lattice

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7

A current of 2.0 A flows in a metal wire.

Approximately how many electrons pass a point each second?

Charge on one electron = 1.6 × 10⁻¹⁹ C.

A 3.2 × 10⁻¹⁹
B 8.0 × 10⁻²⁰
C 1.25 × 10¹⁹
D 3.2 × 10¹⁹

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8

A lamp transfers 240 J of energy when 20 C of charge passes through it.

What is the potential difference across the lamp?

A 0.083 V
B 12 V
C 220 V
D 4800 V

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9

A component has a potential difference of 6.0 V across it.

How much energy is transferred when 35 C of charge passes through it?

A 5.8 J
B 29 J
C 210 J
D 1260 J

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10

A charge of 15 C passes through a component. The component transfers 90 J of electrical energy to other forms.

What is the potential difference across the component?

A 0.17 V
B 6.0 V
C 75 V
D 1350 V

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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11

Which quantity is defined as energy transferred per unit charge?

A current
B resistance
C potential difference
D power

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12

A resistor has a potential difference of 12 V across it and a current of 0.30 A through it.

What is its resistance?

A 0.025 Ω
B 3.6 Ω
C 40 Ω
D 360 Ω

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13

A 15 Ω resistor has a current of 0.80 A through it.

What is the potential difference across the resistor?

A 0.053 V
B 12 V
C 15.8 V
D 18.75 V

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14

A resistor has resistance 48 Ω and potential difference 12 V across it.

What is the current?

A 0.25 A
B 4.0 A
C 36 A
D 576 A

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15

A wire has resistance 6.0 Ω. A charge of 30 C passes through it in 10 s.

What is the potential difference across the wire?

A 0.50 V
B 18 V
C 60 V
D 180 V

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16

A resistor transfers 720 J of energy when a current of 2.0 A flows through it for 60 s.

What is its resistance?

A 3.0 Ω
B 6.0 Ω
C 12 Ω
D 360 Ω

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17

A circuit contains a 9.0 V battery and a resistor. The current is 0.45 A.

How much energy is transferred by the battery in 20 s?

A 4.05 J
B 81 J
C 180 J
D 400 J

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18

A 24 Ω resistor is connected to a 12 V supply.

What charge passes through the resistor in 2.0 minutes?

A 0.50 C
B 24 C
C 60 C
D 240 C

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19

A student connects a voltmeter incorrectly in series with a resistor.

What is the most likely effect on the circuit current?

A it becomes very large
B it becomes nearly zero
C it is unchanged
D it reverses direction repeatedly

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20

An ammeter is connected incorrectly in parallel across a cell.

What is the main danger?

A the current may become very large because the ammeter has very low resistance
B the current becomes zero because ammeters have infinite resistance
C the cell voltage becomes infinite
D the ammeter measures potential difference correctly

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21

Which row shows the correct connection of meters?

	ammeter	voltmeter
A	parallel	series
B	series	parallel
C	series	series
D	parallel	parallel

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22

Why should an ideal voltmeter have very high resistance?

A so it takes almost no current from the circuit
B so it increases the current through the component
C so it acts like a fuse
D so it reduces the supply voltage to zero

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23

Why should an ideal ammeter have very low resistance?

A so it does not significantly change the current being measured
B so it blocks all current
C so it measures energy directly
D so it behaves like an open switch

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24

A resistor obeys Ohm’s law.

Which graph shows this behaviour?

A current against voltage is a straight line through the origin
B current against voltage is horizontal
C voltage against current curves upwards
D resistance against current is a straight line through the origin

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25

A component has these readings.

potential difference / V	1.0	2.0	3.0	4.0
current / A	0.20	0.40	0.60	0.80

What can be concluded?

A resistance increases with voltage
B resistance decreases with voltage
C resistance is constant at 5.0 Ω
D resistance is constant at 0.20 Ω

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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26

A filament lamp has these readings.

potential difference / V	1.0	2.0	3.0	4.0
current / A	0.50	0.75	0.90	1.00

What happens to the resistance of the filament as voltage increases?

A it decreases because current increases
B it remains constant because voltage increases
C it increases because the filament gets hotter
D it becomes zero at high voltage

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27

A thermistor is placed in a circuit. Its temperature increases.

For an NTC thermistor, what happens to its resistance?

A increases
B decreases
C remains constant
D becomes infinite

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28

An LDR is placed in brighter light.

What happens to its resistance?

A increases
B decreases
C remains constant
D becomes negative

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29

A fixed resistor and an NTC thermistor are connected in series with a battery. A voltmeter is connected across the thermistor.

The temperature of the thermistor increases.

What happens to the voltmeter reading?

A increases
B decreases
C remains unchanged
D becomes equal to the supply voltage

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30

A fixed resistor and an LDR are connected in series with a battery. A voltmeter is connected across the fixed resistor.

The light intensity on the LDR increases.

What happens to the voltmeter reading?

A decreases because total current decreases
B increases because total current increases
C remains unchanged because the resistor is fixed
D becomes zero because the LDR is illuminated

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31

Two resistors of 4.0 Ω and 8.0 Ω are connected in series.

What is their combined resistance?

A 2.7 Ω
B 4.0 Ω
C 8.0 Ω
D 12 Ω

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32

Two resistors of 6.0 Ω and 3.0 Ω are connected in parallel.

What is their combined resistance?

A 2.0 Ω
B 3.0 Ω
C 4.5 Ω
D 9.0 Ω

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33

Three identical 12 Ω resistors are connected in parallel.

What is their combined resistance?

A 4.0 Ω
B 12 Ω
C 24 Ω
D 36 Ω

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34

Two identical resistors are connected in parallel. Their combined resistance is 5.0 Ω.

What is the resistance of each resistor?

A 2.5 Ω
B 5.0 Ω
C 10 Ω
D 20 Ω

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35

A 10 Ω resistor and a 15 Ω resistor are connected in parallel.

What is their combined resistance?

A 6.0 Ω
B 12 Ω
C 25 Ω
D 150 Ω

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36

A 3.0 Ω resistor is connected in series with a parallel combination of 6.0 Ω and 12 Ω.

What is the total resistance?

A 4.0 Ω
B 7.0 Ω
C 9.0 Ω
D 21 Ω

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37

A 12 V battery is connected to a 4.0 Ω resistor and an 8.0 Ω resistor in series.

What is the current in the circuit?

A 0.50 A
B 1.0 A
C 1.5 A
D 3.0 A

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38

A 12 V battery is connected to a 4.0 Ω resistor and an 8.0 Ω resistor in series.

What is the potential difference across the 8.0 Ω resistor?

A 4.0 V
B 6.0 V
C 8.0 V
D 12 V

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39

A 6.0 V battery is connected to two resistors in series. The resistors are 2.0 Ω and 4.0 Ω.

Which row gives the current and the potential difference across the 2.0 Ω resistor?

	current	p.d. across 2.0 Ω
A	1.0 A	2.0 V
B	1.0 A	4.0 V
C	3.0 A	2.0 V
D	3.0 A	6.0 V

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40

Two resistors, 3.0 Ω and 6.0 Ω, are connected in parallel across a 12 V supply.

What is the total current from the supply?

A 2.0 A
B 3.0 A
C 4.0 A
D 6.0 A

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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41

A 2.0 Ω resistor and a 4.0 Ω resistor are connected in parallel across a 12 V supply.

Which row is correct?

	current in 2.0 Ω	current in 4.0 Ω
A	3.0 A	6.0 A
B	6.0 A	3.0 A
C	6.0 A	6.0 A
D	3.0 A	3.0 A

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42

A 5.0 Ω resistor is connected in series with a parallel combination of 10 Ω and 10 Ω. The supply is 20 V.

What is the total current from the supply?

A 1.0 A
B 2.0 A
C 3.0 A
D 4.0 A

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43

A 5.0 Ω resistor is connected in series with a parallel combination of 10 Ω and 10 Ω. The supply is 20 V.

What is the potential difference across one 10 Ω resistor?

A 5.0 V
B 10 V
C 15 V
D 20 V

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44

A 9.0 V battery is connected to three identical resistors in series. A voltmeter is connected across one resistor.

What does the voltmeter read?

A 0 V
B 3.0 V
C 9.0 V
D 27 V

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45

A 9.0 V battery is connected to three identical resistors in parallel.

What is the potential difference across each resistor?

A 3.0 V
B 4.5 V
C 9.0 V
D 27 V

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46

A circuit has two branches in parallel.

Branch X has one 6.0 Ω resistor.
Branch Y has two 6.0 Ω resistors in series.

Which statement is correct?

A current in branch X is twice current in branch Y
B current in branch Y is twice current in branch X
C both branches have equal current
D no current flows in branch Y

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47

A battery of 12 V is connected to a resistor network. The total current from the battery is 3.0 A.

What is the total resistance of the network?

A 0.25 Ω
B 4.0 Ω
C 9.0 Ω
D 36 Ω

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48

A 12 V supply is connected to a 6.0 Ω resistor. A second 6.0 Ω resistor is then connected in parallel with the first.

What happens to the total current from the supply?

A it halves
B it stays the same
C it doubles
D it becomes zero

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49

A 12 V supply is connected to a 6.0 Ω resistor. A second 6.0 Ω resistor is then connected in series with the first.

What happens to the total current from the supply?

A it halves
B it stays the same
C it doubles
D it becomes four times larger

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50

Two lamps are connected in series to a battery. One lamp breaks and becomes an open circuit.

What happens to the other lamp?

A it becomes brighter
B it stays the same brightness
C it goes out
D it receives the full battery voltage and explodes every time

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For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

Chapter 21 Answer Key

Q	Ans	Q	Ans	Q	Ans	Q	Ans	Q	Ans
1	C	11	C	21	B	31	D	41	B
2	B	12	C	22	A	32	A	42	B
3	B	13	B	23	A	33	A	43	B
4	C	14	A	24	A	34	C	44	B
5	C	15	B	25	C	35	A	45	C
6	C	16	A	26	C	36	B	46	A
7	C	17	B	27	B	37	B	47	B
8	B	18	C	28	B	38	C	48	C
9	C	19	B	29	B	39	A	49	A
10	B	20	A	30	B	40	D	50	C

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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Detailed Explanations

1. C

• Time = 2.5 minutes = 150 s.

• Charge = current × time.

• Q = It.

• Q = 0.40 × 150.

• Q = 60 C.

2. B

• Time = 3.0 minutes = 180 s.

• Current = charge / time.

• I = Q / t.

• I = 180 / 180.

• I = 1.0 A.

3. B

• Current = 25 mA = 0.025 A.

• Charge = current × time.

• Q = 0.025 × 4.0.

• Q = 0.10 C.

4. C

• Time = 1.8 ms = 1.8 × 10⁻³ s.

• Current = charge / time.

• I = 7.2 × 10⁻³ / 1.8 × 10⁻³.

• I = 4.0 A.

• The trap is forgetting that both values have powers of 10.

5. C

• Conventional current is defined as the direction of positive charge flow.

• In a circuit, conventional current goes from the positive terminal to the negative terminal.

• Electron flow in metals is opposite to conventional current.

6. C

• In a metal wire, the moving charge carriers are free electrons.

• Positive ions are fixed in the metal lattice and only vibrate.

• Protons do not flow through the wire.

7. C

• Current = charge per second.

• 2.0 A means 2.0 C passes each second.

• Number of electrons = total charge / charge per electron.

• n = 2.0 / 1.6 × 10⁻¹⁹.

• n = 1.25 × 10¹⁹ electrons.

8. B

• Potential difference = energy transferred / charge.

• V = E / Q.

• V = 240 / 20.

• V = 12 V.

9. C

• Energy transferred = charge × potential difference.

• E = QV.

• E = 35 × 6.0.

• E = 210 J.

10. B

• Potential difference = energy / charge.

• V = 90 / 15.

• V = 6.0 V.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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11. C

• Potential difference is energy transferred per unit charge.

• Formula:

  • V = E / Q

• Current is charge per unit time, not energy per charge.

12. C

• Resistance = potential difference / current.

• R = V / I.

• R = 12 / 0.30.

• R = 40 Ω.

13. B

• Potential difference = current × resistance.

• V = IR.

• V = 0.80 × 15.

• V = 12 V.

14. A

• Current = potential difference / resistance.

• I = V / R.

• I = 12 / 48.

• I = 0.25 A.

15. B

• Current = charge / time.

• I = 30 / 10.

• I = 3.0 A.

• Potential difference:

  • V = IR

  • V = 3.0 × 6.0

  • V = 18 V

16. A

• Energy transferred in a resistor:

  • E = I²Rt

• Rearrange:

  • R = E / I²t

• R = 720 / (2.0² × 60).

• R = 720 / 240.

• R = 3.0 Ω.

17. B

• Energy transferred by battery:

  • E = VIt

• E = 9.0 × 0.45 × 20.

• E = 81 J.

18. C

• Current:

  • I = V / R

  • I = 12 / 24 = 0.50 A

• Time = 2.0 minutes = 120 s.

• Charge:

  • Q = It

  • Q = 0.50 × 120

  • Q = 60 C

19. B

• A voltmeter has very high resistance.

• If connected in series, it greatly increases circuit resistance.

• Current becomes nearly zero.

• Proper connection: voltmeter in parallel.

20. A

• An ammeter has very low resistance.

• If connected in parallel across a cell, it provides an almost short circuit.

• The current may become very large and damage the ammeter/cell.

• Proper connection: ammeter in series.

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21. B

• Ammeter measures current, so it must be connected in series.

• Voltmeter measures potential difference across a component, so it must be connected in parallel.

• Correct row: ammeter series, voltmeter parallel.

22. A

• A voltmeter is connected in parallel.

• If it had low resistance, it would take too much current and disturb the circuit.

• A very high resistance means it takes almost no current.

23. A

• An ammeter is connected in series.

• If it had high resistance, it would reduce the current being measured.

• A very low resistance means it does not significantly change the current.

24. A

• For an ohmic resistor:

  • V ∝ I

  • resistance is constant

• A graph of current against voltage is a straight line through the origin.

• Straight line but not through origin? Suspicious. Through origin is the clean ohmic signature.

25. C

• Calculate resistance:

  • R = V / I

• At 1.0 V:

  • R = 1.0 / 0.20 = 5.0 Ω

• At 4.0 V:

  • R = 4.0 / 0.80 = 5.0 Ω

• Resistance is constant at 5.0 Ω.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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26. C

• Resistance = V / I.

• At 1.0 V:

  • R = 1.0 / 0.50 = 2.0 Ω

• At 4.0 V:

  • R = 4.0 / 1.00 = 4.0 Ω

• Resistance increases.

• In a filament lamp, the filament gets hotter, so resistance increases.

27. B

• NTC means negative temperature coefficient.

• As temperature increases, resistance decreases.

• Thermistors are used in temperature sensors.

28. B

• LDR = light-dependent resistor.

• In brighter light, resistance decreases.

• In darkness, resistance increases.

29. B

• The thermistor is in series with a fixed resistor.

• Voltmeter is across the thermistor.

• Temperature increases, so thermistor resistance decreases.

• A smaller share of the supply voltage is across the thermistor.

• Voltmeter reading decreases.

30. B

• LDR resistance decreases when light intensity increases.

• Total circuit resistance decreases.

• Current increases.

• The fixed resistor has constant resistance, so V = IR increases across it.

• Voltmeter reading increases.

31. D

• Series resistances add directly.

• Total resistance = 4.0 + 8.0.

• Total resistance = 12 Ω.

32. A

• For parallel resistors:

  • 1/R = 1/6.0 + 1/3.0

  • 1/R = 1/6 + 2/6 = 3/6 = 1/2

• R = 2.0 Ω.

33. A

• Three identical 12 Ω resistors in parallel.

• Total resistance = R / n.

• Total resistance = 12 / 3.

• Total resistance = 4.0 Ω.

34. C

• Two identical resistors in parallel have combined resistance equal to half one resistor.

• Combined resistance = R / 2.

• 5.0 = R / 2.

• R = 10 Ω.

35. A

• For two resistors in parallel:

  • R = R₁R₂ / (R₁ + R₂)

• R = 10 × 15 / (10 + 15).

• R = 150 / 25.

• R = 6.0 Ω.

36. B

• Parallel part:

  • 6.0 Ω and 12 Ω in parallel

  • R = 6 × 12 / (6 + 12)

  • R = 72 / 18 = 4.0 Ω

• Add series resistor:

  • total R = 3.0 + 4.0

  • total R = 7.0 Ω

37. B

• Series total resistance:

  • R = 4.0 + 8.0 = 12 Ω

• Current:

  • I = V / R

  • I = 12 / 12

  • I = 1.0 A

38. C

• Current in the series circuit = 1.0 A.

• Potential difference across 8.0 Ω resistor:

  • V = IR

  • V = 1.0 × 8.0

  • V = 8.0 V

39. A

• Total resistance = 2.0 + 4.0 = 6.0 Ω.

• Current:

  • I = 6.0 / 6.0 = 1.0 A

• Potential difference across 2.0 Ω:

  • V = IR = 1.0 × 2.0

  • V = 2.0 V

40. D

• In parallel, each resistor gets 12 V.

• Current in 3.0 Ω:

  • I = 12 / 3.0 = 4.0 A

• Current in 6.0 Ω:

  • I = 12 / 6.0 = 2.0 A

• Total current = 4.0 + 2.0.

• Total current = 6.0 A.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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41. B

• In parallel, both resistors have the full 12 V across them.

• Current in 2.0 Ω:

  • I = 12 / 2.0 = 6.0 A

• Current in 4.0 Ω:

  • I = 12 / 4.0 = 3.0 A

• Smaller resistance takes larger current.

42. B

• Parallel combination of 10 Ω and 10 Ω:

  • R = 10 / 2 = 5.0 Ω

• Series total:

  • R = 5.0 + 5.0 = 10 Ω

• Total current:

  • I = 20 / 10

  • I = 2.0 A

43. B

• Total current = 2.0 A.

• Potential difference across the series 5.0 Ω resistor:

  • V = IR = 2.0 × 5.0 = 10 V

• Supply = 20 V.

• Remaining voltage across parallel section = 20 − 10 = 10 V.

• Each 10 Ω resistor in that parallel section has 10 V across it.

44. B

• Three identical resistors in series share the voltage equally.

• Total voltage = 9.0 V.

• Voltage across one resistor = 9.0 / 3.

• Voltmeter reads 3.0 V.

45. C

• In parallel, each branch has the full supply voltage.

• Battery voltage = 9.0 V.

• Potential difference across each resistor = 9.0 V.

46. A

• Branch X resistance = 6.0 Ω.

• Branch Y resistance = 6.0 + 6.0 = 12 Ω.

• Both branches have the same voltage across them.

• Current is inversely proportional to resistance.

• Branch X has half the resistance, so it has twice the current.

47. B

• Total resistance:

  • R = V / I

  • R = 12 / 3.0

  • R = 4.0 Ω

48. C

• One 6.0 Ω resistor across 12 V:

  • I = 12 / 6.0 = 2.0 A

• Two 6.0 Ω resistors in parallel:

  • total R = 3.0 Ω

  • I = 12 / 3.0 = 4.0 A

• Total current doubles.

49. A

• One 6.0 Ω resistor:

  • I = 12 / 6.0 = 2.0 A

• Two 6.0 Ω resistors in series:

  • total R = 12 Ω

  • I = 12 / 12 = 1.0 A

• Total current halves.

50. C

• In a series circuit, there is only one path for current.

• If one lamp breaks, the circuit becomes open.

• No current flows anywhere in the series circuit.

• The other lamp goes out.

For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.

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Common Traps From This Chapter

Trap	Correct Rule
Charge	Q = It
Current	I = Q/t
mA to A	divide by 1000
ms to s	divide by 1000
Conventional current	positive to negative
Electron flow in metal	negative to positive
Potential difference	energy per unit charge
Energy transfer	E = QV
Resistance	R = V/I
Electrical energy	E = VIt or I²Rt
Ammeter	series, very low resistance
Voltmeter	parallel, very high resistance
Voltmeter in series	current becomes nearly zero
Ammeter in parallel	dangerous large current
Ohmic resistor	straight I-V graph through origin
Filament lamp	resistance increases when hot
NTC thermistor	resistance decreases as temperature rises
LDR	resistance decreases as light increases
Series resistors	add resistances
Parallel resistors	total resistance is less than smallest branch
Identical resistors in parallel	Rtotal = R/n
Series circuit current	same everywhere
Parallel circuit voltage	same across each branch
Series voltage sharing	larger resistance gets larger p.d.
Parallel current sharing	smaller resistance gets larger current
Adding resistor in parallel	lowers total resistance, increases total current
Adding resistor in series	raises total resistance, decreases total current
Broken series lamp	whole circuit goes out