Example 01
If the positively charged rod below moved
near the neutral metal sphere and was then
removed, which diagram correctly shows the
resulting charge distribution on the metal
sphere?
Answer
The answer is C; this is because when a
positively charged rod is brought nearby a
metal sphere, negative charges will be
induced at the end of the sphere and the
positive charge will repel to the other side
and neutralize the charges hence the sphere
remains neutral (uncharged).
Example 02
A negatively charged metal rod is brought
near side P of a neutral metal sphere PS.
Which diagram in the figure below correctly
shows the resulting charge distribution on
the metal sphere?
Answer
The answer is A; this is because a negatively
charged rod will induce positive charge on
the end of the sphere near it while the
negative charge will repel to the other side
of the sphere. Hence the sphere will have
positive charges near end P with equal
negative charges on end S.
Example 03
The figure below shows two vertical plates
P and Q having positive and negative
charges respectively. When a small
conducting sphere S, with positive charge, is
suspended between P and Q, it swings to Q
and then keep oscillating from Q to P and
back. This is because
A. P has a greater charge than Q B. S is
always repelled from P and attracted by Q
C. S is always attracted by P and repelled by
Q D. Q has greater charge than P E. S
receives a similar charge from P and Q each
time it touches the plates.
Answer
The answer is B; S and P have like charges
hence S repels to Q. When S touches Q the
charges are neutralized hence S remains
neutral as a result P induces the negative
charge on the side of the sphere near P
which results to attraction towards P, since
the induced charges are virtual, when it
reach at P will repel towards Q and hence
keeps oscillating from Q to P.
Example 04
The figure below shows two polythene
spheres P and Q suspended using nylon
threads.
Explain and draw a diagram to show how
the two spheres would behave if:
(a) Only P is rubbed with a cloth
Answer
When P is rubbed with a cloth, it
develops a negative charge. This
charge induces the positive charge on
the end of the neutral sphere Q nearby
P while the negative charge repels to
the far end of Q leaving the positive
charge near P, this causes unlike
charges to attract each other.
(b) Both P and Q are rubbed with a cloth.
Answer
When both P and Q are rubbed with
cloth, they both develop negative
charges by gaining electrons from the
cloth. They both repel from each other
since like charges repel.
(c) What would happen if Q is replaced
with a rubbed Perspex R and
suspended near a rubbed sphere P.?
Solution
Sphere P and R would be attracted
towards each other. This is because P
made of polythene develops a negative
charge while Perspex develops a net
positive charge when both are rubbed
with cloth. The two oppositely charged
sphere attract each other since unlike
charges attract.
Example 05
Explain why small pieces of paper are first
attracted towards a glass rod rubbed with
silk but they immediately move away?
Answer
When a glass rod is rubbed with silk
becomes positively charged when brought
near the paper negative charges are induced
on the piece of papers, attraction occurs
since unlike charges attract.
However, the pieces of paper soon gain
positive charges as well. Since like charges
repel each other, immediately the pieces of
papers move away.
Example 06
Explain why a rubber balloon if rubbed will
often stick to the wall where it has been
rubbed.
Answer
The balloon when rubbed acquires a
negative charge. It induces positive charge
on the wall and become attracted since
unlike charges attract each other.
Example 07
(a) Explain in terms of electrons transfer,
why a polythene rod becomes charged
when rubbed with a duster.
Answer
When a polythene rod is rubbed with a
duster gains electrons from the duster
and becomes negatively charged. The
duster loses electrons and becomes
positively charged.
(b) Why do you find it more difficult to
charge it on a damp day than on a dry
day?
Answer
Damp air is better conductor of
electricity than dry air and charge leaks
away.
(c) State how a positively charged and a
negatively charged object are
distinguished?
Answer
A positively charged body has a
deficiency of electrons but a negatively
charged body has excess electrons.
Example 08
A negatively charged polythene rod is held
near to end A of an uncharged metal rod
suspended by a cotton thread as shown
below.
(a) Why is the metal rod suspended by
cotton thread?
Answer
Cotton thread, being an insulator,
prevents charge leaking away.
(b) State and explain the charges that will
be induced at the end A and B of the
metal rod?
Answer
The end A becomes positively charged
and the end B negatively charged.
(c) State and explain how the rod can be
given a positive charge. What is this
method of charging called?
Answer
By touching the end B with a finger.
The electrons flow down to the Earth
and then remove the finger; the positive
charges will spread all over the metal
rod as shown below.
This method is called electrification by
induction.
(d) How will you make sure that a metal
rod is given a positive charge?
Answer
By bringing a positively charged
acetate rod near the metal rod. The rod
will be repelled, showing that is
positively charged.
Example 09
Two metal spheres A and B are placed on
insulating stands. A rubbed acetate rod is
brought near sphere A as shown.
(a) What charge (if any) would you expect
to find on sphere A? Explain why?
Answer
Negative charge. Because a rubbed
acetate has positive charge, it will
induce the negative charge on sphere A
at the nearby it and repells the positive
charge which moves at the far end of
the rod B as shown below
(b) What charge if any would you expect to
find on sphere B?
Answer
Positive charge
(c) While the rod is in position, B is moved
to the right away from A. What type of
charge would you expect to find on the
spheres?
Answer
A will have negative charge while B
will have positive charge.
This is because if B is moved away, the
two types of charges will separate by
electrostatic induction.
(d) Does the acetate rod lose any of its
charge when the spheres A and B are
being charged? Can it be now used to
charge another metal sphere? Explain
your answer.
Answer
No, it does not lose any of its charge
since the process of charging was by
induction.
Yes, it can be still used to charge
another metal sphere as no charge is
lost from the rod during the charging
process (induction).
Example 10
Two small balls coated with metallic paint
are suspended by insulating threads as
shown in the diagram.
(a) Describe what you would observe if
both balls were given a positive charge.
