Answer
The balls would repel each other and both
threads would be at an angle to the vertical.
(b) How does the effect depend on the
distance AB?
Answer
If the distance is small, there will be high
repulsive force. The sphere will make
greater angle with the vertical.
(c) Draw a diagram showing all the forces
acting on one of the balls.
ELECTRIC FIELD
Electric field is a region around an electric
charge where an electrostatic force can be
experienced.
Electric field is represented by using field
lines.
An electric field line is a line drawn at any
point in an electric field to show the
direction of an electric force at that point.
Properties of electric field lines:
(i) They move from positive charge to
negative charge.
(ii) They do not cross each other.
(iii) They repel one another side ways.
(iv) They are in a state of tension.
(v) The number of electric field lines at
certain point indicates the strength of
the electric field. If there is strong
magnetic field, the field lines are very
close to each other.
Electric field for a point single charge
For a point single source of charge, the
electric field points outward from a positive
charge but points inwards to a negative
charge as shown below.
Electric field for two unlike charges
Two unlike charges experiences attraction
force, the field lines move out from a
positive charge and enters into a negative
charge.
Case I; two charged spheres
Case II; two charged plates
Case III; A positive charge near a negative
plate
Case IV; A negative charge near a positive
plate
Electric field for two like charges
Two like charges experiences repulsion
force, the field lines move out from a
positive charge and enters for a negative
charge.
Between the two charges, the magnetic field
cancels out and a Neutral point is formed.
Neutral point is the point in electric field
where a net electric force is zero.
Case I: Two positive charges
Where “X” is the neutral point
Case II: Two negative charges
Case III: Two positive plates
Case IV: Two negative plates
Example 01
Sketch the electric field pattern for a
negative point charge placed near a
positively charged plate.
Answer
Example 02
Sketch the electric field pattern for two
negatively point charges when are placed
close to each other.
Answer
Example 03
Draw a diagram and explain how the two-
point charges behave when placed nearby if:
(a) One is positively charged while the
other is negatively charged
Answer
The two-point charges will
experience an attraction force since
unlike charges attract each other.
(b) Both charges are negative
Answer
The two charges will experience a
repulsion force since like charges
repel from each other.
Example 04
What will happen if a positively point
charge is placed nearby a negative plate?
Explain and give diagram to support your
answer.
Answer
A positive charge will be attracted to the
negative plate since unlike charges attract
each other. The forces of electric field are
shown below.
1.20 CONDUCTORS AND
INSULATORS
Conductors:
A conductor is a material that permits
electrons to flow freely from one atom
within the material to another.
The electrons in a conductor are free to
move because they are loosely held outside
the nucleus of an atom. These electrons are
called conduction elections.
Examples of conductors include; are metals
like copper, silver, steel, iron, silver and
gold, carbon in form of graphite.
Insulators
An insulator is a material that does not allow
electrons to flow freely from atom to atom.
Electrons in an insulator are not free to
move because are strongly held outside the
nucleus of an atom
When a charge is induced in an insulator, it
remains at the same position. Therefore,
charges are not evenly distributed in an
insulator.
Example of insulators is Rubber, glass, dry
wood and plastics
Conductivity
Conductivity is the ability of the material to
conduct electricity or heat.
Materials which have free electrons are good
conductors because the electron carries
charges from one point to another
Example 01
A comb is rubbed is rubbed with a piece of
cotton cloth and brought near an iron nail
hanging from a string.
(a) What type of the charge does the comb
acquire? Give reason.
Answer
When a plastic comb is rubbed with
cotton, it becomes negatively charged.
This is because the comb gains
electrons from the cotton cloth.
(b) What charge will be developed on the
glass rod if rubbed with a piece of
woolen cloth?
Answer
When a glass rod is rubbed with a
woolen cloth it becomes positively
charged.
(c) Why does an iron nail suspend by
using string?
Answer
String, being an insulator, prevents
charge leaking away.
(d) Use a diagram to explain what is
observed when the rubbed comb is
brought near the iron nail? Give reason
for your answer.
Answer
Attraction will occur between the
comb and the iron nail.
This is because, placing the comb near
the iron nail the conduction electrons
of the nail near the comb repel and
move to the opposite end of the nail.
This makes the end of the nail near the
comb to be positively charged which
causes attraction since unlike charges
attract each other
