Page 23 - Physics_Form_2

Static electricity

Procedure
1. Place the electroscope on a table and 5. Remove the electrophorus and
discharge it by touching it with an observe the leaf of the electroscope.
earthed wire. 6. Test for charges on the electroscope
2. Charge the electrophorus by using charged rods of glass and
FOR ONLINE READING ONLY
induction. ebonite.
3. Hold the charged electrophorus close
to the cap of the electroscope. Questions
4. Earth the electroscope momentarily, What charge does the electroscope
as shown in Figure 1.22
acquire?

When the electroscope is charged this way,
it acquires a negative charge. The charged
glass rod, therefore, causes a collapse
Electrons
flow of the leaf, while a charged ebonite rod
causes further leaf divergence. Recall, the
glass is positively charged while ebonite
is negatively charged after being rubbed
Figure 1.22 with silk and cloth, respectively.

Discharging a leaf electroscope
Having charged a leaf electroscope by either contact or induction, the same can be
effectively discharged through induction. If a negatively charged object is brought near
the brass cap of a positively charged electroscope, electrons in the brass cap are repelled
and move down to the leaf, as shown in Figure 1.23 (a). When a positively charged
electroscope is earthed by a wire, the excess electrons flow from the electroscope to
the earth, causing the electroscope leaf to collapse, causing it to discharge. This cancels
the positive charges. With no net charge, the leaf collapses back to the plate, and the
electroscope becomes discharged, as shown in Figure 1.23(b).

(a) Discharging by induction (b) Discharged by induction
Figure 1.23: Electroscope discharged by induction

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Physics Form 2 Final.indd 17 25/10/2025 10:25