Electrostatics (靜電學) - charges

In primary school, you must have tried to make use of a rubbed plastic ruler to attract small pieces of paper.

In winter, sound of sparks can be heard when putting off woolen sweaters. Sometimes, you feel an electric

shock when touching the metal part of the rail on an escalator. All these can be explained by electrostatics.

1. Charging by friction 2. Induction of charge 3. Earthing 4. Sharing of charge

*English:

‘Insulating’ ‘Insulating’ means it’s an insulator. An insulating rod = The rod is an insulator.

vs An insulated conducting sphere = It is a

‘Insulated’ means it’s isolated.
‘Insulated’ conductor and is isolated from other things.

‘like charge’ ‘like charge’ means both +ve/ both -ve The two spheres carry like charge.

vs ‘the same charge’ means the amount of The two spheres carry the same charge =
‘the same charge’ charge and the sign are the same The amount of charge is also the same.

Electric charge in atom electrons

Matters are made up of atoms. An atom consists of positively charged protons

and neutral neutrons, forming a nucleus with negatively charged electrons
orbiting around them. Normally, an atom has the same number of protons and
electrons. The atom is said to be neutral, having no ________________.

neutrons protons
Electric force between charges
Fby B on A Fby A on B Fby D on C Fby C on D
+ ‒ + +
A B C D

The force between electric charges is called electric/electrostatic force.

 ____________ charges repel. ____________ charges attract.

 Electric force __________________ with the distance. (Closer: _______________ )
 The electric force experienced by A is due to B and vice versa. So, the electric force on A
and that on B are ___________________________, as stated in _________________
law, having ____________ magnitude. It is true even the two objects carry different
amount of charges.
4N 4N
‒ + ++
A B

Electrostatics [CE] E1 - P1
Insulators vs conductors

Insulators Charges cannot flow in an insulator. E.g. _________________________

Conductors Charges can flow in a conductor. E.g.__________________________

 Both insulators and conductors can be charged. The difference is that the charge in insulator
cannot ___________ while those on conductors can.

Charging an insulator by friction

OR

 For some materials, upon rubbing, some ______________ are rubbed away. The object will
carry more positive charges than negative charges. The object is positively charged
afterwards, having a net positive charge.
 On the contrary, for some materials, _______________ are gained upon rubbing. The
object is negatively charged afterwards, having a net negative charge.

 The charging process is due to the transfer of ______________, NOT ___________.

 Charging by friction is more desirable in _______ condition.

 If a ruler is positively charged after rubbing with a cloth, the cloth will be ______________
charged afterwards.

 Some materials (e.g. polythene) are negatively charged upon rubbing while some (e.g. acetate)
are positively charged upon rubbing. (No need to remember.)

P2
Induction of charges

When a charged object is brought near a neutral object without contact, charges will be induced in
the neutral object.

1. When a positively charged object is brought close to a neutral object,

Conductor Insulator

2. Some electrons are drawn to the left. 2. The particles will be orientated in such a way
that the negative ends are towards the left.
+
+ +
+ +
neutral + neutral

3. Negative charge is induced on the left end while positive charge is induced on the right
end of the neutral object.
+
+
+

 After the induction of charge, the neutral object is still on the whole ____________.

Attraction between neutral and charged objects

 Neutral objects is _______________ by charged objects.

When a positively charged ruler is placed near a neutral aluminium foil/paper, negative charge is
induced at the top of the paper/foil while positive charge is induced at the bottom.

Induced 7N
–ve charge 4N

Induced
+ve charge 3N

The positively charged ruler attracts the induced negative charge of the paper with __________
force than repelling the induced positive charge of the paper as the induced negative charge is
___________ to the ruler. Therefore, the net force acting on the paper is attractive.

Remarks:
The paper/foil being attracted is still on the whole ____________ after the induction of charge.

P3
Newton’s 3rd law
Action-and-reaction pair?

Fby paper on ruler = 4 N

7N
Fby ruler on paper = 4 N

3N

 Force on induced +ve charge and that on induced –ve charge

 Net force on the paper by ruler and that on the ruler by paper

Example
1. If a charged ruler is brought near a stream of running water, the
water will be attracted towards the ruler.

2. T/F
(a) If a ball is placed near a negatively charged ruler, the ball is attracted to the ruler. The ball
must be positively charged.
(b) If a balloon is placed near a negatively charged ruler, the balloon is then repelled. The
balloon must be negatively charged.

Q: 02-31

A plastic ruler is placed close to a stream of running water coming from an insulated tap. The stream of

water is attracted towards the ruler as shown above. Which of the following statements are correct?

(1) The ruler carries an electric charge.

(2) The ruler and the running water are attracting each other with forces of equal magnitude.

(3) Both positive and negative charges are induced on the running water.

Q: [06-22]

P4
Q: [90-45- ]

When a negatively charged rod is brought near to an insulated A net positive charge is induced on the sphere

neutral metal sphere, an attractive force exists between them

Q: [04-27]

Q: [01-28-]

Two charged spheres A and B are hanging at a point on a ceiling by two identical nylon threads.

They remain at rest as shown above. Which of the following statements must be correct?

(1) Both spheres carry positive charges.

(2) The force acting on B by A is larger than the force acting on A by B.

(3) The mass of A is larger than B.

Q: [95-29-]

Two insulated uncharged metal spheres X and Y are in contact with each other. A positively charged rod is brought near X as

shown above. Which of the following diagrams correctly shows the distribution of charges on the spheres?

 For induction of charges, two or more conductors in contact/connected are regarded as one
single conductor

P5
Earthing a conductor

 Free charges can be removed from a conductor by connecting the conductor to the
earth/ground via a conducting path such as an electrical wire or human body.
 Earthing takes place by means of electron flow, NOT protons.

The free charges can be removed no matter how long the conductor is touched or how much charge
it carries initially.

- +
- +
- +
electrons electrons

Electrons flow from the ball to the ground. Electrons flow from the ground to the ball.

 For negatively charged conductor, upon earthing, ___________ flowing from the ________
through the conducting wire to the _________.

 For positively charged conductor, upon earthing, ___________ flowing from the _________
through the conducting wire to the _________.

 When earthing takes place, the man will experience electric shock since
there is a flow of charge (current) through the man.

Example 2
(a) Explain why a conductor cannot be charged by friction.
(b) What should be done in order to charge a conductor by friction?

P6
Sharing of charges among conductors

 If two conductors are in contact, sharing of charge occurs.

E.g. If an uncharged metal ball touches a positively charged metal sphere, the positive charges will
be shared among the conductors. Afterwards, both metal balls are positively charged.

Initially charged
conductor Initially uncharged
conductor
+
+
+

___ flows from the ball on the

_______ to that on the ______

+
+ + -
+

___ flows from the ball on the

_______ to that on the ______

 Sharing of charges happens only if both objects in touch are _______________.

 The bigger sphere will share more charge than the smaller one in the end.

(Earthing can be explained by the sharing of charge: The earth is very huge compared to the object
being earthed so in the end the earth obtains all the charges.)

-
-

P7
Van der Graaff generator

Van de Graaff generator is an apparatus in which (negative) charges are

accumulated on its metal dome when operating. The charges are produced by Power
supply
friction (rubbing a rubber belt).

(a) Hairs placed on the dome of van de Graaff generator repel each other since
they obtain the charges from the dome.

(b) When a small neutral metal ball is placed near the dome, what happens?

(c) If Makibao standing on the floor touches the dome, what happens?
(d) If Makibako standing on plastic touches the dome, what happens?
(e) If Makibao, standing on the floor touches her, what happens?

Example 1
Explain why sometimes you get an electric shock when touching the metal part of the rail on an
escalator.

Example 2
Sometimes after walking on carpet, we get an electric shock when we touch a metal door handle.
Explain briefly the phenomenon.

P8
Charging a conductor by induction

Charged rod brought near: induction of charge Earthing the sphere Removal of rod

Induction of charges Earthing Separate the spheres Removal of rod

Induction of charges Earthing Remove the rod Separate the spheres

 While earthing, it doesn’t matter at which point the object is touched.

 The charges held by the charged rod cannot be removed.

Example 1
Two insulated uncharged metal spheres X and Y are places in contact. A negatively-charged rod is brought near X as shown.

X is then touched by a finger momentarily. Then the charged rod is removed. Which of the following describes the

charges on X and Y?

Q: 14-20

P9
Q: [98-29-]

Three insulated uncharged metal spheres X, Y and Z are placed in contact as shown above. A negatively charged rod is

brought near X and sphere Y is then earthed momentarily. If the charged rod is still near X, which of the following

describes the charges on X, Y and Z?

Sphere X Sphere Y Sphere Z

A. positive zero zero

B. positive zero negative

C. positive positive positive

D. zero zero zero

E. zero negative negative

Q: [94-24-]

An insulated charged metal rod is brought near an insulated uncharged metal sphere as

shown above. Which of the following can charge the sphere?

(1) Touching the sphere momentarily with a finger

(2) Touching the sphere momentarily with the rod

(3) Touching the rod momentarily with a finger and then touching the sphere momentarily with the same finger

Q: [95 AL, Q.26] 5*

Two uncharged metal spheres, A and B, supported by insulating stands are placed side by side but not touching each other.

A student places a positively-charged rod near sphere A and he touches sphere B with his finger momentarily. When the

rod is removed afterwards, what are the signs of the charges induced on the spheres?

Sphere A Sphere B Sphere A Sphere B

A. positive neutral D. neutral positive

B. negative negative E. neutral negative

C. negative neutral

Q: [90 AL, Q.27] 5* + _

A light conducting sphere is hanged from a long insulating thread between oppositely charged metal plates connected to

a high voltage supply. If the sphere is given a positive charge, it will

A. move to the positive plate and stick to it.

B. move to the negative plate and stick to it.

C. remain still.

D. oscillate, touching each plate in turn, beginning with the negative plate.

E. oscillate, touching each plate in turn, beginning with the positive plate

P10
 CONDUCTORS INSULATORS

1. By connecting to a power supply

By friction

2. By sharing of charges

Charging

3. By induction

By earthing Cannot be discharged by earthing

By nearby ions in air. E.g. A flame produces ions in air.

Discharging
The object (conductor/insulator)

becomes neutral.

Humid condition: water is conductive which carries away the charge built up on the
object.

P11
Amount of charges [Unit: ]

 The unit of charge is coulombs (C), not 粒.

 An electron carries - 1.6 x 10 -19 C of charge. [1 粒電子有 1.6 x 10-19 C 咁多負 charge]

 1 粒 electron ≠ -1 C of charge.

Q  N  (1.6  1019 ) where Q is the amount of charge

N is the number of electrons
Q N
19

1.6  10 1

Example 1
A ruler gains 30 x 106 electrons upon rubbing. Find the amount of net charge carried by the ruler.

Example 2
For an amount of charge of -2 x 10-6 C, find the number of electrons.

Example 3

Amount of charge Number of electrons

-1 C
5 x 1017
-50 x 10-6 C
7.3 x 1015

P12
Appendix: Electrostatics in daily lives

Applications of electrostatics

Electrostatic precipitator

 It is installed in the chimneys in power stations to filter dust and ash from the exhausts.
Wires connected to high voltage are negatively charged. When dust and ash passes, they are
charged by the wire and attracted to the positively charged metal plate on the side. The
sticked dusts are then removed.

Electrostatic spraying

 Droplets of paint are given charge by the spray gun.

 The droplets repel each other so they spread more uniformly. As the car is oppositely
charged or neutral while painting, the droplets stick to the car strongly.

Photocopying

1. A light-sensitive drum inside a photocopying machine is positively charged.

2. The document is projected onto the drum. The region exposed to light (unwritten) will become
neutral. The region without light (written) is still positively charged.
3. Negatively charged carbon powder is sprayed and sticks to the positively charged region
(written) of the drum.
4. A positively charged sheet of paper passes through the drum, attracting the negatively
charged carbon powder onto it.

P13
Troubles caused by electrostatics

 The accumulation of a large amount of charges may lead to sparks which may ignite
flammable gas/ fuel.
 To prevent this, anti-static methods are employed. Common anti-static techniques
including earthing, discharge by ions or anti-static chemicals.

Petrol tankers (載油車)

 When running on the road, charges produced by friction accumulate on the tanker.
 In order to remove the produced charge, the tanker is connected electrically to the ground via
a metal chain. The charges produced will be removed continuously.

Conducting material on tyres

Metal chain

Aircraft

 Similarly, charges accumulate on an aircraft as charges are produced by friction.

 The tyres are made up of conductive material so that charges can be removed to the ground
while landing.

Operation centres

 Anaesthetic (麻醉劑) contains flammable gas which can be ignited in the presence of sparks.
The charges produced by friction such as rubbing blankets and apparatus have to be removed
continuously.
 All things have to be earthed. Hence, the floor, the gowns (手術衣) of patients, doctors and
nurses and trolley wheels have to be made up of conductive materials.

P14