Electricity and
Magnetism
EAM 601S
Part 1
E. Emmanuel
2/21/2020
1
Electrostatics

Definition: Electrostatic can be define as the study of charges at rest. It
is produce as friction between two materials. It can also be known as
static electricity. This can be generated by rubbing an ebonite rod with
fur or glass rod with silk.

There are two types of charges, namely:
i.
Positive charge
ii.
Negative Charge
Positive charge is obtained when a glass rod is rubbed with silk or
cellulose acetate is rubbed with silk. Negative charge is obtained when an
ebonite rod is rubbed with fur or polythene is rubbed with fur.
E. Emmanuel
2/21/2020
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Gold leaf Electroscope
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

An electroscope is an instrument used for the detection and
testing of an of electric charges.
It consist of a flat brass disc or cap and brass stem or rod with a
gold-leaf attached to the end of the rod.
A metal case is normally connected to the end of the earth (or
earthed) to prevent accumulation of charges due to external
influence.
The flat brass disc is one terminal and the earth metal case is
the other terminal of instrument.
This leaf is the sensitive part of the instrument Whose position
shows the presence of charge in the disc and rod.
E. Emmanuel
2/21/2020
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Gold leaf Electroscope
Brass disc or cap
B
r
a
s
s
d
i s
c
o
r
c
a
p
Insulating plug
Metal case
Brass rod
Brass Rod extension
Gold leaf
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Charging an Electroscope
There are two method of charging an electroscope
 Frictional method
 Electrostatic induction
Frictional method:
This method is achieved by sliding a charge body on the brass disc of the
instrument. The charge passes from charged body to the disc and stem of
the instrument down to the bottom. The leaf is observed to open when
this happens and is repelled from metal as both carry similar charges.
The degree of divergence of the leaf indicates the amount of charge
acquired by the instrument. Note that the charge on the instrument is
the same as that of the charged body.
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2/21/2020
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
Electrostatic induction : This method involve charging a neutral body by
placing a charge body near it without any constant between them.
Below are the following step involved in charging an electroscope by
induction:
i.
ii.
iii.
iv.
E. Emmanuel
A negative charge rod is brought near the cap of the electroscope.
Some free electrons in the metal cap are repelled to the leaf which
now diverges leaving some positive charge on the cap.
The cap is touch with a finger and electrons are repelled towards the
earth through the hand. The leaf closes as a result.
The finger is remove and the net positive charge is left on the cap.
Finally the rod is removed leaving the net positive charge on the cap.
This positive charge is now distributed between the cap an leaf,
causing the leaf to diverge due to repulsion. This can similarly be
explained of a positively charge rod.
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Methods of Producing Charges

The following are the three method of producing charges:
i.
Friction
ii.
Contact
iii.
Electrostatic induction.
Frictional method: This involved creating friction between the surfaces
concerned. Example :
a.
Rubbing an ebonite rod with fur, a negative charge is produce.
b.
A positive charge is acquired when a glass rod is rubbed with a silk.
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Contact Method: if we bring two uncharged conductors, C and D on
insulating stands in contact, and if a positively charged glass rod is brought
near to C as shown in fig 1.1.

D can be observed to be positively charged on separation from C while
the rod is still near C and C becomes negatively charged.

C is charge by induction but D received its charge from C by contact.
When the rod is eventually removed, the negatively charge spreads on C
and the positively charge spreads on D.
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+
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+
(a) stage 1
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+
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++C
---D
(b) stage 2
C
D
+++
----
(c) final stage
Insulating Stand
Fig 1.1 production of charges by contact and induction
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
Method of Electrostatic induction: Consider an uncharged conducting body, CD
mounted on an insulating stand as shown in fig 1.1. if a positively charged glass
rod is brought near the end C, it is observed that the end C becomes
negatively charged while the extreme end D becomes positively charged. The
charges produced are called induced charges and that on the glass rod is called
an inducing charge.

Similarly, if the negatively charged ebonite rod is brought near the end C, that
end will be positively charged while the end D will be negatively charged. This
happened because the electrons in the rod repel those at C and force them to
the extreme D. Now C having lost electrons becomes positively charge. The
end D having received electrons becomes negatively charged. When the
charging rod is removed, the charged CD will disappear to make the body
permanently charged. For instance positively charged ebonite rod is brought
near C and the end D is touched with a finger thus earthen the conductor, the
negative charged electrons leave away to the earth through the human body
leaving only positive charges on the conductor.
E. Emmanuel
2/21/2020
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Distribution of Charges on a Conductor

If the distribution of charges over the surface conductor is
examined, charges are not uniformly distributed except on
spherical surface. In general, Charges Concentrate at places
where the surface is sharply curved. Thus, the surface charge
density or charge per unit area is very large at sharp points with
very small areas. This is the case with the pear-sharped
conductor. It is found that, the charges reside only on the
outside surface of a conductor and not inside. Therefore,
Surface density,
σ = QA
Where Q is Charge, A is Area.
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+
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+ ++
++
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+
Fig 1.2 Distribution of charges on a conductor
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