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Static electricity
This process leaves a net positive charge the charge is transferred through the
on the brass disc after separation, as seen wire to the earth, and the leaf of the
in Figure 1.19 (c). The electric force electroscope then collapses back. If the
between the brass disc and the polythene electroscope is brought near a charged
base is fairly strong, so some mechanical object without touching it, the leaf also
work has to be done to overcome it. diverges from the plate. This is because
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The top of the disc can be repeatedly charges on the metal cap with the same
charged in the same way. In principle, an charge as the object are repelled to the
leaf. This is charging by induction.
unlimited amount of induced charge can
be obtained from a single charge, but the Charging the electroscope by contact
insulated disc slowly discharges to the
surroundings. An electrophorus can be Charging by contact takes place when
used to charge an electroscope through contact is made between a neutral
contact and induction. electroscope and a charged object. If a
positive or negatively charged object
is brought into contact with the brass
Task 1.1
cap of the neutral electroscope, the
Construct an electrophorus using leaf diverges. The neutral electroscope
materials such as a metal tray, a foam becomes charged when contacted by
or rubber disk, and an insulating the charged object. An electroscope that
handle. After assembling the becomes charged by contact always gets
electrophorus, use it to demonstrate the same type of charge as the object used
the charging process by induction. to charge it.
Charging and discharging a leaf Activity 1.7
electroscope Aim: To charge a leaf
A leaf electroscope can be charged electroscope by contact
or discharged either by contact or by
induction. If a charged object touches the Materials: charged ebonite rod,
metal cap, the metal leaf diverges from electroscope, silk cloth,
the plate. This is because the same charge glass rod
has been conducted through the metal Procedure
cap and the metal rod to the metal plate 1. Place the electroscope on a table and
and the leaf. This makes them repel each discharge it by earthing.
other, and thus, the leaf diverges from the
plate. This is charging by contact. 2. Bring the charged ebonite rod in
contact with the brass cap of the
If you touch the brass cap with a electroscope as shown in Figure
conducting wire that touches the ground, 1.20.
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Physics Form 2 Final.indd 15 25/10/2025 10:25
