Difference between conductors and

insulators

Conductors Insulators

Electrons are free to

move

Electrons are not

free to move

Electrons are loosely

held outside the

nucleus

Electrons are

strongly held outside

the nucleus

Electricity and heat

flows through them

easily

Electricity and heat

cannot flow through

them

DETECTION OF CHARGES

The presence of charges on a body can be

detected by a device known as electroscope.

The common type of electroscope used to

detect the presence of charges is known as

gold leaf electroscope.

The gold leaf electroscope

An electroscope is a device used to detect

the presence of an electric charge and

determine the type of charge present on the

material.

Construction

It consists of a metal (brass) cape mounted

on a brass rod with a leaf made of gold and a

brass plate enclosed in a metal or glass case

to protect the leaf from air current.

The figure below shows an example of an

electroscope.

Mode of action of an electroscope

If a metal cape is touched by a charged

object, the meal leaf diverges from the plate.

This is because the same charges are

conducted to the leaf; this makes them repel

each other hence the leaf diverges from the

plate. If the leaf is touched by a wire that

touches the ground the leaf of the

electroscope collapses back because the

charges are transferred to the earth through

the wire.

If the electroscope is brought near a charged

object without touching it, the leaves also

diverge from the plate. This is because the

charges on the metal cap with the same sign

are repelled to the leaf.

Determining the sign of a charge

The sign of a charge can be determined by

the following ways.

(i) By using the electrophorus

(ii) By using the electroscope

The electrophorus

Electrophorus is a simple electrostatic

generator used to produce unlimited number

of electrostatic charges by electrostatic

induction.

Construction

An electrophorus consists of a circular slab

of insulating material (polythene) together

with a brass disc (conductor) on an

insulating handle.

Mode of action of an electrophorus

An electrophorus works by electrostatic

induction to generate many positive charges

from a single negative charge.

The polythene slab is first negatively

charged by rubbing it with far and the brass

disc is then placed on it.

The positive charge is induced on the lower

surface of the brass disc near the negative

slab while the negative charges repel to the

top surface.

The top of the brass is then touched by a

wire that touches the ground thereby

carrying away the negative charges to the

ground (Earthing).

On separating the brass disc remain with net

positive charges as shown below.

Points to note

The working of electrophorus obeys the

principle of conservation of energy because;

the electric force between the brass disc and

the polythene base is strong so that the

mechanical work is to be done in order to

overcome it.

The top of the disc can be repeatedly

charged in the same way. In principle

unlimited amount of induced positive

charges can be obtained from a single

charge, but the insulated disc slowly

discharges to the surrounding.

Applications of an electrophorus

The electrophorus is used to charge the

electroscope through contact and induction

CHARGING THE ELECTROSCOPE

The electroscope can be charged by contact

or by induction as explained below.

Charging by contact

If a negative charged body is made to touch

the brass cape of the neutral electroscope,

the leaves diverges. This is because it

acquires negative charges which are

conducted through the metal rod to the leaf

and the plate. This makes them repel each

other since like repel. Thus the leaf diverges

from the plate. This is charging by contact.

Example 01

The electroscope is placed on the table and

discharged by earthing as shown below.

The negatively charged ebonite rod is placed

in contact with the brass cape.

(a) Explain what will be observed on the

gold leaf?

Answer

When a negatively charged rod is

brought into contact with the

electroscope, it becomes negatively

charged and the leaf diverges since like

charges repel from each other.

(b) The charged glass rod is then brought

near the cape and the leaf collapse.

Explain why?

Answer

When a positively charged glass rod is

brought near the brass cap, positive

charges on the cape are repelled

towards the brass plate and make the

plate to become positively charged.

This causes the leaf to collapse since

unlike charges attract each other.

electroscope

Answer

Charging by contact

Charging by induction

The electroscope is charged by induction

using charged body placed nearby it.

Example, if a positively charged

electrophorus is placed nearby the

electroscope, the negative charges are

induced on the brass rod and move to the

late and the leaf. The leaves diverge due to

repulsion.

Discharging a leaf electroscope

Consider the positively charged electroscope

below.

If a negatively charged object is now

brought near the brass cap, electrons in the

brass cap are repelled and move down to the

leaf as shown below. This cancels the

positive charges. With no net charge, the

leaf collapses back to the plate.

Applications of electroscope

(i) Testing the sign of charge on a body

(ii) Identifying the insulating properties of

materials

(iii) Detecting the presence of charge on a

body

Testing for the sign of charge on a body

Test for positive sign

To test for positive sign the initial charge on

the electroscope should be negative.

If a body of unknown charge is brought near

a negatively charged electroscope and the

leaf diverges, then a body has negative

charge. If the leaf collapse it is clear that a

body has positive charge.

Test for negative sign