ELECTROSTATIC

MYP - Lecture Notes

Dr. Belal AlQassem

Mashrek International School
 Static Electricity (Electrostatic)

• There are two types of electric charge, positive and negative.

• Like charges repel each other.
• Unlike charges attract each other.

• Electricity originates in the atom, which consists of a positively charged nucleus

surrounded by negatively charged electrons.

• The nucleus contains protons, which are positively charged, and neutrons, which
have no net electric charge.

• All electrons and all protons have EXACTLY the same magnitude of electric charge;
but their signs are opposite.
• Each electron has a charge -e = -1.6 × 10-19 C.
• Each proton has a charge +e = +1.6 × 10-19 C.

• If a body has the same amount of positive and negative charge, they cancel out, forming
a neutral body.

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Conductors and Insulators

• A material that allows the flow of charge through it (because it has many ‘free’
electrons) is called an electrical conductor.
Examples: All metals such as copper, aluminum, and silver are good electrical
conductors.

• If charge cannot flow through a material (does not have many ‘free’ electrons) it is
called an electrical insulator.
Examples: Materials such as glass, rubber, and wood fall into the category of
electrical insulators.

Methods of Charging

Object can be charged by:

a. Friction (Rubbing)
b. Conduction (Contact)
c. Induction

a. Charging By Friction (Rubbing)

When two insulators are rubbed together it can be charged by friction

• Electrons are transferred from one object to the other.
• Forming a positive charge on one object and a negative charge on the other.
• A positive static charge forms on object which loses electrons
• A negative static charge forms on object which gains electrons
• Which object loses/gains electrons depends on the materials involved

Example 1: when a glass rod is rubbed with silk, as in the Figure below, the silk obtains a
negative charge that is equal in magnitude to the positive charge on the glass rod.

• The glass rod loses some electrons (becomes positively charged)

• The silk gains these electrons (becomes negatively charged)
• No charges created (just transferred from one material into another one),
so charge is conserved.

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 Example 2: when a rubber or plastic rod is rubbed with fur, the
fur obtains a positive charge that is equal in
magnitude to the negative charge on the rubber
rod.

• The fur loses some electrons (becomes

positively charged)
• The rubber gains these electrons (becomes
negatively charged)
• No charges created (just transferred from
one material into another one), so charge is
conserved.

The Law of Conservation of Charge

The net amount of electric charge produced in any process is zero.

or

No net electric charge can be created or destroyed.

Electric Charge is Quantized

Since an object cannot gain or lose a fraction of an electron, the net charge on any object must
be an integral multiple of this charge. Electric charge is thus said to be quantized (Existing
only in discrete amounts: 1e, 2e, 3e, etc.).

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 b. Charging By Conduction (Contact)
• Charging by conduction happens when electrons move from one object to another
through direct contact (touching).

Suppose you touch an uncharged

metal sphere with a negatively charged
Ebonite rod. Electrons from the
ebonite rod will move to the metal
sphere. The metal sphere gains
electrons and becomes negatively
charged.

Gold-leaf electroscope

A gold-leaf electroscope consists of a metal cap on a metal rod at the foot of which is a metal
plate with a leaf of gold foil attached. The rod is held by an insulating plastic plug in a case
with glass sides to protect the leaf from draughts.

The gold-leaf electroscope is a device used to check whether an object is charged or not
(without knowing the sign of the charge). For example, draw a charged polythene strip firmly
across the edge of the cap. The leaf should rise and stay up when the strip is removed. The
electroscope has now become negatively charged by contact with the polythene strip, from
which electrons have been transferred.

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i) Charging By Induction

Charging by induction happens when charges in an uncharged object are rearranged

without direct contact with a charged object.

Grounding Method

-
-
-

+- +- +-

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 ii) Separation Method

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Electric Forces

The electric force between two point charges

• is inversely proportional to the square of the distance
r between them;
• is proportional to the product of their charges.

Notes
The direction of the electric force is always along the line joining the two charges.

• If the charges have the same sign, the force on either charge is directed away from
the other (they repel each other).

• If the charges have opposite sign, the force on one is directed toward the other (they
attract each other).

• The force one charge exerts on the second is equal but opposite to that exerted by the
second on the first, in accord to Newton’s third law.

𝐹⃗12 = −𝐹⃗21

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The Electric Field
• An electric field is said to exist in the region of space around a charged object. When
another charged object enters this electric field, an electric force act on it.
• The direction of the field, denoted by arrows, is the direction of the force on a small
positive charge placed in the field.

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1. a) Suggest how a plastic rod may be given an electrostatic charge.
By rubbing the plastic rod with a cloth. (By Friction)

b) A charged sphere is suspended on an insulating thread.

When a plastic rod with a positive charge is held near the suspended charged
sphere, the sphere moves to the position shown in the Figure below.

-
-
-
(i) State the sign of the charge on the sphere.
Negative

1. (ii) Give the reason for your answer to (b)(i).

Because the positively charged rod attracted the charged sphere, so they have opposite
sign of charges (i.e., the sphere is negatively charged).

c) On the Figure, draw an arrow to show the electrostatic force on the sphere.

d) The positively-charged plastic rod is removed and replaced by a plastic rod with a
negative charge.
Describe the position that the suspended sphere now takes.
The suspended sphere will move away from the negatively charged rod.
It will keep swinging or oscillating.

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2. a) The diagram shows a polythene rod being rubbed with a woolen cloth.

The polythene rod becomes negatively charged.

Explain how this happens.

Electrons from the cloth will be transferred to the polythene rod.

So the rod gains electrons and becomes negatively charged.

The cloth loses electrons and becomes positively charged.

b) A student put the charged polythene rod on to a balance. The rod was separated
from the metal pan of the balance by a thin block of insulating material. The
student then held a second charged polythene rod above, but not touching, the
first rod. The reading on the balance increased.

(i) Explain why the reading on the balance increases.

The polythene rod at the top will repel the polythene rod that’s on the scale
(because they are both negatively charged). As a result, there is a downward force
on the polythene rod so the reading on the scale increases.

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 (ii) The student observed that the nearer the two rods are to each other, the bigger
+ balance reading.
the increase in the
What should the student conclude from this observation?
As the distance decreases, the force increases and vice versa.
So Force is inversely proportional with distance (squared)

The diagram shows a negatively charged plastic rod held near to a thin stream of
3.
water. The water is attracted towards the rod.

- + - -
- + - -
-

Which one of the following statements explains what is happening to the charge in
the water?
Tick () one box.
The positive and the negative charges in the water are attracted to the
rod.

The positive and the negative charges in the water are repelled by the
rod

The negative charge in the water is repelled by the rod and the positive
charge is attracted to the rod.

The negative charge in the water is attracted to the rod and the positive
charge is repelled by the rod.


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