Chapter 20 MCQs

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 polythene rod is rubbed with a dry cloth and becomes negatively charged.

What has happened?

A protons have moved from the cloth to the rod
B electrons have moved from the cloth to the rod
C electrons have moved from the rod to the cloth
D neutrons have moved from the cloth to the rod

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2

A glass rod is rubbed with silk and becomes positively charged.

Which statement is correct?

A the rod has gained electrons
B the rod has lost electrons
C the rod has gained protons
D the rod has lost neutrons

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3

Two identical insulated metal spheres are charged.

Sphere X has charge +8Q.
Sphere Y has charge −2Q.

They are touched together and then separated.

What is the final charge on each sphere?

A +3Q
B +5Q
C +6Q
D +10Q

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4

Two identical conducting spheres are initially charged +12 μC and −4 μC. They touch each other and are then separated.

What is the final charge on each sphere?

A +4 μC
B +8 μC
C +16 μC
D −4 μC

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5

A negatively charged rod is brought near a small uncharged conducting sphere on an insulating stand.

What happens inside the sphere?

A electrons move towards the rod
B electrons move away from the rod
C protons move away from the rod
D positive charge flows from the rod into the sphere

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6

A negatively charged rod is brought near a neutral metal sphere. The sphere is attracted to the rod.

Why can attraction occur even though the sphere has no overall charge?

A the sphere becomes permanently negative
B opposite charge is induced nearer the rod
C the rod loses all its charge
D the sphere has no electrons

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7

A positively charged rod attracts a small object.

Which conclusion is definitely correct?

A the object is negatively charged
B the object is positively charged
C the object is either negatively charged or neutral and polarised
D the object must be a conductor

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8

A charged rod repels a small suspended object.

What can definitely be concluded?

A the object is uncharged
B the object has the same type of charge as the rod
C the object has the opposite type of charge to the rod
D the object is made of plastic only

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9

Which statement about electric charge is correct?

A positive charge means excess electrons
B negative charge means deficiency of electrons
C charging by rubbing transfers electrons
D protons move easily through solids during charging

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10

A student says, “An object becomes positively charged because it gains positive charge.”

Which correction is best?

A it becomes positively charged because it loses electrons
B it becomes positively charged because it gains neutrons
C it becomes positively charged because protons flow into it
D it becomes positively charged because its mass becomes zero

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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

A neutral object has 4.0 × 10¹² electrons removed from it.

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

What is the charge left on the object?

A −6.4 × 10⁻⁷ C
B +6.4 × 10⁻⁷ C
C −2.5 × 10³¹ C
D +2.5 × 10³¹ C

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12

An object has charge −3.2 × 10⁻¹⁶ C.

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

How many excess electrons does it have?

A 2.0 × 10⁻³
B 500
C 2000
D 5.0 × 10¹⁶

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13

A current of 0.20 A flows for 45 s.

How much charge passes a point in the circuit?

A 0.0044 C
B 9.0 C
C 45.2 C
D 225 C

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14

A charge of 18 C passes through a lamp in 2.0 minutes.

What is the current?

A 0.15 A
B 9.0 A
C 36 A
D 2160 A

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15

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

How much charge passes through the component?

A 0.00016 C
B 0.10 C
C 100 C
D 6250 C

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16

A charge of 2.4 × 10⁻³ C passes through a component in 0.60 s.

What is the current?

A 1.4 × 10⁻³ A
B 4.0 × 10⁻³ A
C 0.25 A
D 250 A

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17

A plastic rod is rubbed and gains 8.0 × 10⁻⁹ C of negative charge.

Approximately how many electrons has it gained?

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

A 5.0 × 10⁻¹¹
B 1.3 × 10⁻²⁷
C 5.0 × 10¹⁰
D 1.3 × 10²⁸

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18

An object loses 2.5 × 10¹¹ electrons.

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

What is its final charge?

A −4.0 × 10⁻⁸ C
B +4.0 × 10⁻⁸ C
C −1.6 × 10⁻¹⁹ C
D +2.5 × 10¹¹ C

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19

A metal sphere has charge −6.0 μC. It is connected to Earth for a moment and then disconnected.

What is its final charge?

A −6.0 μC
B +6.0 μC
C 0 C
D −12 μC

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20

A positively charged metal sphere is earthed.

What happens during earthing?

A electrons flow from Earth to the sphere
B electrons flow from the sphere to Earth
C protons flow from Earth to the sphere
D neutrons flow from the sphere to Earth

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21

A negatively charged metal sphere is earthed.

What happens during earthing?

A electrons flow from Earth to the sphere
B electrons flow from the sphere to Earth
C protons flow from the sphere to Earth
D positive ions flow through the wire into the sphere

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22

A metal object is charged by induction using a negatively charged rod.

The rod is brought near the object. The object is earthed while the rod remains nearby. The earth connection is removed first, then the rod is removed.

What is the final charge on the object?

A negative
B positive
C zero
D alternately positive and negative

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23

A metal object is charged by induction using a positively charged rod.

The object is earthed while the rod is nearby, then the earth connection is removed before the rod is removed.

What is the final charge on the object?

A positive
B negative
C zero
D same as the rod only if they touch

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24

During charging by induction, why must the earth connection be removed before the charged rod is removed?

A to trap the induced charge on the conductor
B to allow the conductor to become an insulator
C to make protons leave the conductor
D to remove all electrons from the conductor

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25

A charged rod is brought near the cap of a positively charged gold-leaf electroscope. The leaf divergence decreases.

What is the charge on the rod?

A positive
B negative
C neutral only
D impossible, because the leaf cannot move

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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 negatively charged rod is brought near the cap of a negatively charged gold-leaf electroscope.

What happens to the leaf?

A it falls completely every time
B it diverges more
C it becomes positively charged
D it is unaffected because the rod does not touch

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27

A positively charged rod is brought near the cap of a negatively charged electroscope.

What happens to the leaf?

A divergence increases
B divergence decreases
C the leaf becomes more negative
D the electroscope becomes uncharged immediately

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28

A charged rod is brought near a neutral electroscope without touching it. The leaf rises.

Why?

A charge is induced and redistributed in the electroscope
B protons leave the electroscope through the air
C the leaf becomes heavier
D the rod must have touched the cap secretly

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29

A negatively charged rod touches the cap of a neutral electroscope and is then removed.

What is the final charge on the electroscope?

A positive
B negative
C zero
D positive on cap and negative on leaf only, with zero total charge

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30

A gold-leaf electroscope is charged positively. A student touches the metal cap with a finger.

What happens?

A electrons flow from Earth through the student to the electroscope, discharging it
B protons flow from the electroscope to Earth
C the electroscope becomes more positive
D the leaf rises further permanently

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31

Which statement about conductors is correct?

A they allow electrons to move through them easily
B they contain no electrons
C they can never be charged
D they always repel charged objects

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32

Which statement about insulators is correct?

A electrons move freely through the whole material
B charge placed on them tends to stay where it is put
C they contain only protons
D they cannot be charged by rubbing

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33

A metal rod is held in the hand and rubbed with a cloth. It does not keep much charge.

Why?

A charge leaks through the rod and body to Earth
B metal has no electrons to transfer
C the rod becomes radioactive
D rubbing cannot charge any object

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34

A plastic rod is rubbed while held in the hand. It keeps charge better than a metal rod.

Why?

A plastic is an insulator, so charge does not easily flow away
B plastic has no atoms
C plastic is always positively charged
D plastic conducts charge faster than metal

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35

A charged object is placed near a stream of water from a tap. The stream bends towards the object.

Why?

A water is polarised, causing attraction
B water has no molecules
C the charged object produces gravity
D the water becomes a permanent magnet

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36

A negatively charged balloon sticks to a neutral wall.

Which explanation is best?

A the wall becomes overall negatively charged
B positive charge is induced nearer the balloon in the wall
C protons move from the balloon to the wall
D the balloon loses all its charge immediately

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37

A dust particle near a charged plate becomes attracted to the plate.

Which process most likely causes the attraction if the dust particle is initially neutral?

A electrostatic induction/polarisation
B gravitational attraction only
C nuclear fission
D magnetic induction only

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38

Electric field lines around an isolated positive point charge:

A point radially outwards
B point radially inwards
C form circles around the charge
D do not exist near the charge

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39

Electric field lines around an isolated negative point charge:

A point radially outwards
B point radially inwards
C form square patterns
D point north to south

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40

The direction of an electric field at a point is defined as the direction of force on:

A a negative test charge
B a positive test charge
C a neutron
D an uncharged atom

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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

Two parallel metal plates are connected to a high-voltage supply. One plate is positive and the other is negative.

Which statement about the electric field between the plates is correct, away from the edges?

A it is nearly uniform and directed from positive plate to negative plate
B it is nearly uniform and directed from negative plate to positive plate
C it is circular around the positive plate
D it is zero because the plates are parallel

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42

A small positive charge is placed between two oppositely charged parallel plates.

Which direction is the force on the charge?

A towards the positive plate
B towards the negative plate
C perpendicular to the electric field
D no force acts unless the charge is moving

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43

A small negative charge is placed between two oppositely charged parallel plates.

Which direction is the force on the charge?

A towards the positive plate
B towards the negative plate
C in the direction of the electric field
D no force acts because negative charges are not affected

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44

Electric field lines are closer together near a charged object.

What does this show?

A the electric field is stronger there
B the electric field is weaker there
C the charge there must be zero
D field lines are crossing there

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45

Which statement about electric field lines is correct?

A they cross whenever fields are strong
B they show the direction of force on a positive charge
C they show the path followed by electrons in all cases
D they point from negative to positive charges

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46

A charged oil drop is held stationary between two horizontal charged plates.

Which statement must be correct?

A electric force is equal in size to weight and opposite in direction
B electric force is zero
C weight is zero
D the drop has no charge

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47

A negatively charged oil drop is stationary between two plates. The upper plate is positive and the lower plate is negative.

Which statement is correct?

A electric force is upwards and balances weight
B electric force is downwards and balances weight
C electric field is upwards
D the drop must accelerate upwards

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48

A positively charged particle is placed between two plates. The electric field strength is 4000 N/C. The charge is 2.0 × 10⁻⁶ C.

What is the electric force on the particle?

A 0.00050 N
B 0.0080 N
C 2000 N
D 8.0 × 10⁹ N

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49

An electric field exerts a force of 0.030 N on a charge of 6.0 μC.

What is the electric field strength?

A 1.8 × 10⁻⁷ N/C
B 5.0 × 10³ N/C
C 5.0 × 10⁶ N/C
D 1.8 × 10⁵ N/C

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50

Which statement about electrostatic hazards and uses is correct?

A aircraft are refuelled with earthing to prevent charge build-up and sparks
B electrostatic painting works because paint droplets are uncharged
C lightning conductors work by insulating buildings from Earth
D electrostatic precipitators remove smoke particles by making them neutral only

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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 20 Answer Key

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

1. B

• The rod becomes negatively charged.

• Negative charge means gain of electrons.

• Electrons move from the cloth to the polythene rod.

• Protons and neutrons do not move during ordinary charging by rubbing.

2. B

• The glass rod becomes positively charged.

• Positive charge means it has lost electrons.

• It does not gain protons.

• In solids, protons stay fixed inside nuclei.

3. A

• Total charge = +8Q + (−2Q)

• Total charge = +6Q

• The spheres are identical, so charge is shared equally.

• Final charge on each = +6Q / 2 = +3Q

4. A

• Total charge = +12 μC + (−4 μC)

• Total charge = +8 μC

• Identical spheres share charge equally.

• Final charge on each = +8 μC / 2 = +4 μC

5. B

• A negatively charged rod repels electrons in the conducting sphere.

• Electrons move away from the rod.

• The near side becomes induced positive.

• The far side becomes induced negative.

6. B

• The neutral sphere is polarised.

• The side nearer the negative rod becomes positive.

• Opposite charges are closer together than like charges, so attraction occurs.

• The sphere is still neutral overall.

7. C

• A positively charged rod can attract:

  • a negatively charged object

  • a neutral object that becomes polarised

• Attraction alone does not prove the object is negatively charged.

• Repulsion is the stronger proof.

8. B

• Repulsion only occurs between like charges.

• So the object must have the same type of charge as the rod.

• A neutral object would be attracted, not repelled.

9. C

• Charging by rubbing transfers electrons.

• Negative charge = excess electrons.

• Positive charge = deficiency/loss of electrons.

• Protons do not move through ordinary solids.

10. A

• An object becomes positively charged when it loses electrons.

• It does not gain positive charge in the form of moving protons.

• This is one of the most common electrostatics traps.

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11. B

• Electrons removed means the object is left positive.

• Charge = number of electrons × charge on one electron

• Q = 4.0 × 10¹² × 1.6 × 10⁻¹⁹

• Q = 6.4 × 10⁻⁷ C

• Final charge = +6.4 × 10⁻⁷ C

12. C

• Number of excess electrons = charge / charge on one electron

• n = 3.2 × 10⁻¹⁶ / 1.6 × 10⁻¹⁹

• n = 2.0 × 10³

• n = 2000 electrons

13. B

• Charge = current × time

• Q = It

• Q = 0.20 × 45

• Q = 9.0 C

14. A

• Time = 2.0 minutes = 120 s

• Current = charge / time

• I = 18 / 120

• I = 0.15 A

15. B

• Current = 4.0 mA = 0.0040 A

• Charge = current × time

• Q = 0.0040 × 25

• Q = 0.10 C

16. B

• Current = charge / time

• I = 2.4 × 10⁻³ / 0.60

• I = 4.0 × 10⁻³ A

• Answer = 4.0 × 10⁻³ A

17. C

• Number of electrons = charge / charge per electron

• n = 8.0 × 10⁻⁹ / 1.6 × 10⁻¹⁹

• n = 5.0 × 10¹⁰

• The rod gained 5.0 × 10¹⁰ electrons.

18. B

• Losing electrons leaves the object positively charged.

• Q = 2.5 × 10¹¹ × 1.6 × 10⁻¹⁹

• Q = 4.0 × 10⁻⁸ C

• Final charge = +4.0 × 10⁻⁸ C

19. C

• A negatively charged metal sphere has excess electrons.

• When earthed, excess electrons flow to Earth.

• The sphere becomes neutral.

• Final charge = 0 C

20. A

• A positively charged sphere has lost electrons.

• When earthed, electrons flow from Earth to the sphere.

• This neutralises the positive charge.

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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21. B

• A negatively charged sphere has excess electrons.

• Earthing provides a path for electrons to leave.

• Electrons flow from the sphere to Earth.

• The sphere becomes neutral.

22. B

• A negatively charged rod repels electrons in the metal object.

• When the object is earthed, electrons flow from the object to Earth.

• The earth connection is removed first, trapping a deficiency of electrons.

• The object is left positively charged.

23. B

• A positively charged rod attracts electrons towards the object.

• While earthed, electrons flow from Earth into the object.

• The earth connection is removed first, trapping excess electrons.

• The object is left negatively charged.

24. A

• The earth connection must be removed first to trap the induced charge.

• If the rod is removed first while still earthed, charge can flow back and the object may become neutral.

• Induction is all about the order. Remove earth first, then remove rod.

25. B

• The electroscope is positively charged.

• A negative rod brought near the cap repels electrons down towards the leaf.

• This reduces the positive charge on the leaf.

• Leaf divergence decreases.

• So the rod is negative.

26. B

• The electroscope is negatively charged.

• A negative rod near the cap repels electrons down to the leaf.

• The leaf becomes more negative.

• Repulsion increases, so the leaf diverges more.

27. B

• The electroscope is negatively charged.

• A positive rod near the cap attracts electrons upwards.

• Fewer electrons remain on the leaf.

• Leaf divergence decreases.

28. A

• A charged rod near a neutral electroscope causes charge redistribution.

• The leaf and stem can acquire the same type of induced charge.

• Same charges repel, so the leaf rises.

• The rod does not need to touch.

29. B

• A negatively charged rod touches a neutral electroscope.

• Electrons transfer to the electroscope.

• The electroscope becomes negatively charged.

• The leaf remains raised after the rod is removed.

30. A

• A positively charged electroscope has a shortage of electrons.

• Touching the cap earths it through the student.

• Electrons flow from Earth through the student into the electroscope.

• The electroscope discharges and the leaf falls.

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31. A

• Conductors allow electrons to move through them easily.

• Metals are good conductors because they have free electrons.

• Conductors can be charged, but charge may spread or leak away if earthed.

32. B

• In insulators, electrons do not move freely through the whole material.

• Charge placed on an insulator tends to stay where it is put.

• This is why plastic rods can keep charge after rubbing.

33. A

• A metal rod is a conductor.

• If held in the hand, charge can flow through the rod and body to Earth.

• So it does not keep much charge.

• To charge a metal rod, it should be held with an insulating handle.

34. A

• Plastic is an insulator.

• Charge cannot easily flow through it or through the hand to Earth.

• So it keeps charge better than a metal rod held in the hand.

35. A

• Water molecules can become polarised.

• The side nearer the charged object becomes oppositely charged.

• This causes attraction.

• The stream bends towards the charged object.

36. B

• A negatively charged balloon repels electrons in the wall slightly.

• This leaves positive charge induced nearer the balloon.

• Opposite charges attract.

• The wall remains neutral overall.

37. A

• A neutral dust particle can be attracted by a charged plate due to electrostatic induction/polarisation.

• Charges shift slightly inside the dust particle.

• The opposite induced charge is closer to the plate, causing attraction.

38. A

• Electric field lines around a positive point charge point radially outwards.

• This shows the direction a positive test charge would be pushed.

39. B

• Electric field lines around a negative point charge point radially inwards.

• A positive test charge would be attracted towards the negative charge.

40. B

• Electric field direction is defined as the direction of force on a positive test charge.

• A negative charge feels force in the opposite direction to the field.

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

• Between parallel plates, away from the edges, the electric field is nearly uniform.

• Electric field direction is from positive to negative.

• So the field is directed from the positive plate to the negative plate.

42. B

• A positive charge experiences force in the direction of the electric field.

• The field points from positive plate to negative plate.

• Therefore the positive charge is forced towards the negative plate.

43. A

• A negative charge experiences force opposite to the electric field.

• The electric field points towards the negative plate.

• So the negative charge is forced towards the positive plate.

44. A

• Closer electric field lines mean stronger electric field.

• Wider spacing means weaker field.

• Field lines do not cross.

45. B

• Electric field lines show the direction of force on a positive charge.

• They point away from positive charges and towards negative charges.

• They do not necessarily show the actual path of electrons.

46. A

• The oil drop is stationary.

• Resultant force must be zero.

• Weight acts downwards.

• Electric force must be equal in size and opposite in direction.

• So electric force balances weight.

47. A

• Upper plate is positive, lower plate is negative.

• Electric field direction is downwards.

• A negatively charged drop feels force opposite to the field.

• Therefore electric force is upwards.

• Since the drop is stationary, this upward electric force balances its weight.

48. B

• Electric force = charge × electric field strength

• F = qE

• F = 2.0 × 10⁻⁶ × 4000

• F = 8.0 × 10⁻³ N

• F = 0.0080 N

49. B

• Electric field strength = force / charge

• E = F / q

• q = 6.0 μC = 6.0 × 10⁻⁶ C

• E = 0.030 / 6.0 × 10⁻⁶

• E = 5.0 × 10³ N/C

50. A

• During aircraft refuelling, charge can build up due to friction and fuel flow.

• If charge builds up, sparks may occur.

• Earthing allows charge to flow safely away.

• This reduces the risk of fire or explosion.

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
Negative charge	object has gained electrons
Positive charge	object has lost electrons
Protons in solids	do not move during charging
Charging by rubbing	transfer of electrons
Identical spheres touching	total charge shared equally
Attraction	not proof of charge type
Repulsion	proves same type of charge
Neutral conductor near charge	becomes polarised
Negative rod near conductor	electrons move away
Positive rod near conductor	electrons move towards rod
Charge calculation	Q = It
Electron number	n = Q/e
Earthing negative object	electrons flow to Earth
Earthing positive object	electrons flow from Earth
Induction with negative rod	object ends positive
Induction with positive rod	object ends negative
Induction order	remove earth first, then rod
Conductors	electrons move freely
Insulators	charge stays localised
Electroscope leaf rises	like charges repel
Positive electroscope + negative rod	leaf divergence decreases
Negative electroscope + negative rod	leaf divergence increases
Field around positive charge	radially outwards
Field around negative charge	radially inwards
Electric field direction	force on positive test charge
Parallel plate field	uniform, positive to negative
Positive charge in field	force along field
Negative charge in field	force opposite field
Electric field strength	E = F/q
Electrostatic hazards	earthing prevents sparks