Static Electricity
Chapter 19
To Show the Forces Between Electric Charges
Rub two polythene rods with a
woollen cloth.
Place one of them in a paper
stirrup hanging from a retort stand.
Bring the second rod near the first
one.
The rods will repel each other,
showing the Force between
Charged Bodies.
To Show the Forces Between Electric Charges
Rub a polythene rod and a
cellulose acetate rod with a
woollen cloth.
Place one of them in a paper
stirrup hanging from a retort stand.
Bring the second rod near the first
one.
The rods will attract each other,
showing the Force between
charged bodies.
Electric Charge
Name the two types of Electric Charge.
Positive Charge and Negative Charge.
Do electric charges exert Attractive or
Repulsive forces on each other?
Both. Like Charges Repel and Unlike
Charges Attract.
What is an Electrical Conductor?
An Electrical Conductor is a substance through
which electric charge can flow.
Name four electrical conductors.
Silver, Copper and Aluminium are metals commonly
used as conductors.
Water, the Human Body and the Ground are also
conductors.
What is an Electrical Insulator?
An Electrical Insulator is a substance through
which electric charge cannot flow.
Name four electrical insulators.
Glass, any Plastic, Rubber and Air are electrical
insulators.
The Structure of the Atom.
All matter is made up of atoms.
Every atom has a central part,
called the Nucleus, which
contains particles called
Protons.
Particles called Electrons orbit
in the space around the nucleus.
Electrons are attracted to the
protons and are repelled by
other electrons. Protons repel
protons.
The forces of attraction and repulsion between
protons and electrons are called Electrostatic
Forces.
Particles that exert electrostatic forces on each
other are said to be Electrically Charged.
Electrons are said to be Negatively Charged.
Protons are said to be Positively Charged.
The amount of positive charge on a proton is the
same size as the amount of negative charge on an
electron.
Atomic Nature of Electric Charge
If a object is
Negatively
Charged it has
Gained Electrons.
If a object is
Positively Charged
it has Lost
Electrons.
It is only the electrons that
actually move when objects
become charged.
Bring a negatively charged rod near a metal conductor. Free electrons in
the metal are repelled by the negative charge on the rod and head to the
opposite side of the metal.
Thus a negative charge appears on one side of the metal and a positive
charge on the other.
Remove the charged rod and the electrons move back again.
The charges produced on the metal are called Induced Charges.
The induced negative charge is the same size as the induced positive
charge.
Metal Conductor
Charged Rod
Charging by Induction
To Charge a Conductor positively by Induction
Bring a negatively charged rod near, but not touching, the conductor.
Keeping the charged rod in place, earth the conductor, by touching it
with your finger.
Remove your finger and then remove the rod. The conductor will be
positively charged.
A Gold-Leaf Electroscope can be used to:
Detect electric charge.
Indicate the approximate size
of an electric charge.
Test if a charge is positive
(+) or negative or (–).
Test if an object is a
conductor or an insulator.
Indicate roughly the size of a
potential difference.
Why does the gold leaf on an electroscope
diverge when a negatively charged object is
brought close to the metal cap?
The negative charge on the
object repels electrons from
the cap down to the leaf and
the end of the metal rod.
The negative charge on the
end of the rod repels the
negative charge on the leaf.
Thus the leaf diverges.
To Charge an Electroscope by Induction
Bring a charged rod near – but not touching – the cap.
Keeping the charged rod in place, earth the cap of the electroscope, by
touching it briefly with your finger.
Remove the rod.
The electroscope will now have a charge opposite to that on the rod.
Where on a conductor does Static
Charge reside?
All Static Charge resides on the Outside Surface
of a conductor.
Distribution of Charge on Conductors
Static charge on a spherical conductor is distributed
uniformly over the sphere.
Static charge on a pear-shaped conductor accumulates
most at the pointed end.
Static Charge on a conductor tends to accumulate
where the conductor is most pointed.
Point Discharge
Point Discharge
There is a very large charge density (charge per unit area) at the point.
This causes a very strong electric field in the region around the point.
Ions in the air are either attracted to or repelled from the point.
Ions with opposite charge to that on the point head towards the point
and neutralise the charge on it.
Ions with the same charge head away from it creating an "electric wind".
It is as if the charge was removed from the point. The loss of charge
from a point by this manner is called Point Discharge or the Point
Effect.
Point Discharge
An “Electric Wind” produced by ions
streaming away from a highly charged point.
State Coulomb’s Law of Force
between electric charges.
Coulomb’s Law states that the force of attraction
or repulsion between two point charges is:
Directly Proportional the product of the charges
and
Inversely Proportional to the square of the
distance between them.
F is the force
Q1 and Q2 are the charges
d is the distance between the charges
ε is the permittivity of the medium
1 Q1 Q2
F 
2
4 d
Coulomb's Law is an Inverse Square Law
This means that:
The force is inversely proportional to the square of the
distance between the charges.
i.e. If the distance between the charges is doubled
the size of the force is 4 times smaller.
If the distance between the charges is made 3 times
bigger the size of the force is 9 times smaller, etc.
What is an Electric Field?
An Electric Field is any region of space where
a static electric charge experiences a force
other than the force of gravity.
An Electric Field is always caused by other
static charges in the vicinity.
What is an Electric Field Line?
(also called a line of force).
An Electric Field Line is a line drawn in an
electric field such that the tangent to it at any point
is along the direction of the force on a positive (+)
charge placed at that point.
Where the electric field is strong the field lines are close
together.
Where the electric field is weak the lines are far apart.
The Electric Field Lines around an isolated
Positive Charge
The Electric Field Lines around an isolated
negative charge
The Electric Field Lines around a positive
charge near a negative charge
The Electric Field Lines around two Positive
Charges
The almost uniform Electric Field between
two Oppositely Charged Parallel Plates
Experiment to demonstrate an Electric
Field Pattern
Use the equipment shown.
Connect a High Voltage
Source to the metal plates
which are in the oil.
The semolina particles line
up in the direction of the
field, showing the electric
field pattern.
What is Electric Field Strength?
The Electric Field Strength (E) at a point is the
force per unit charge at that point.
Electric Field Strength is a Vector Quantity.
What is the SI Unit of Electric Field Strength?
The SI Unit of Electric Field Strength is the
newton per coulomb ( N C-1).
A charge of Q coulombs
experiences a force F newtons
when placed in an electric field of strength E
newtons per coulomb.
Write down a formula relating Q, F and E.
F
E 
Q