Page 20 - Physics_Form_2

Physics for Secondary Schools

An electrophorus consists of a circular

Project 1.1 slab of insulating material (polythene) and
a brass disc (conductor) on an insulating
handle. Figure 1.18 (a) shows a picture of
Construct a simple leaf electroscope an electrophorus, and Figure 1.18 (b) is a
using a glass jar, copper wire, straw, schematic diagram of it.
FOR ONLINE READING ONLY
strips of aluminium foil, glue, and a jar

lid. After construction, test different
objects to observe the deflection of Insulating
handle
the leaves and determine whether they
are positively or negatively charged. Brass disc

The electrophorus Polythene slab

It is a simple device used to produce an (a) (b)
unlimited number of electrostatic charges
via the process of electrostatic induction. Figure 1.18: Structure of electrophorus

Mode of action of an electrophorus
An electrophorus works by electrostatic induction. It can be used to generate positive
charges from a single negative charge. The polythene slab is first negatively charged by
rubbing it vigorously with fur. The brass disc is then placed on it, as shown in Figure
1.19 (a). Since the surfaces are only in contact at relatively few points, a positive charge

is induced on the lower surface of the brass disc, and a corresponding negative charge
is produced on its top surface. The top of the brass disc is then touched briefly using a
wire that touches the ground, thereby carrying away the negative charges to the earth,
as shown in Figure 1.19 (b). This is known as earthing.

Insulating handle

Brass disc Electrons

Ebonite or
polythene
slab

(a) Charges induced on the brass disc (b) Brass disc temporarily earthed (c) Brass disc removed

Figure 1.19: Action of the electrophorus

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Student’s Book Form Two

Physics Form 2 Final.indd 14 25/10/2025 10:25