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Static Electricity
Definitions
• Conductor: Allows current to pass
• Insulator: Does not allow current to pass
• Point discharge: Charge concentrates at a point and if
the electric field is high enough it can ionise air causing
movement of charge to and from the point.
• Coulombs law :The force between 2 point charges is
proportional to the charges and inversely proportional to
the square of t he distance between them.
• Electric field strength: The force per unit charge at that
point.
• Potential difference: The work done transferring a charge
from one point to another.
• Volt: The pd between 2 points is 1 volt if 1 Joule of
energy is used to transfer 1Coulomb of charge between
the points.
What is Static Electricity?
Static electricity is a stationary electric
charge that is built up on a material
creating an imbalance between the
positive and negative charges of the
material.
A common example of static electricity is
the slight electric shock we can get when
we touch a car door during dry weather.
The Atom
When an atom
gains electrons it
become negatively
charged and when
it loses electrons it
becomes positively
charged.
To Demonstrate Forces Between
Charges
Charge a polythene rod by friction and hang from a stand as
shown. Charge another polythene rod and bring it near the
suspended rod. They repel.
Repeat using two perspex rods. They too repel.
If one rod in charged positive and the other negative they will
attract.
Like charges repel, unlike attract.
How do objects get charged?
1.By Friction: When two materials are
rubbed together (like a balloon and your
hair), one will lose electrons and one will
gain them. Polythene tends to gain
electrons and perspex tends to lose them.
It is always the
ELECTRONS that leave.
2. Charging by Induction
It is always the
ELECTRONS
that leave.
The Electroscope
Uses of the
electroscope
•Detecting Charge
•Determining whether
a charge is positive or
negative
•Determining whether
an object is an
insulator or conductor.
Insulator
Charging By Induction
To charge positively by induction.
A negatively charged object is
brought near the electroscope.
The negative charges are repelled
to the leaves.
The electroscope is earthed and
negative charges move to ground
leaving an excess of positive charge
on the electroscope.
The charged object is removed and
the electroscope is positively
charged.
The Van De Graaf Generator
This device can be used to build up a high charge.
Charge Distribution
• Total charge resides on the outside of a metal object.
To demonstrate this, an
insulated hollow metal can is
charged by induction. A proof
plane is touched to the inside of
the can and then brought to the
cap of an electroscope. The gold
leaf does not move showing
there is no charge on the inside
of the can
The proof plane is touched to the
outside on the metal can it and
then brought to the cap of an
electroscope. The gold leaf
moves showing there is charge
on the outside of the can.
Coaxial cables, ESB worksuit
Charge Distribution
• Charge concentrates at a point.
To demonstrate this attach a pointed object to
the dome of a Van De Graaf generator. Hold a lit
candle near the point and switch on the
generator. The flame bends away from the point
as it is affected by the charge being repelled
from the point due the build up of charge here.
Holding the candle next to the dome does not
have the came affect as the charge is more
spread here.
Charge Distribution
The point effect can cause
charges to escape from a
point and cause sparks
which are dangerous in
refuelling planes and in flour
mills.
A lightening conductor uses
the point effect to protect a
building.
Uses of Static Electricity
The Electrostatic Precipitator
This is used to clean the emissions from a chimney.
Uses of Static Electricity
The Photocopier
Coulombs Law
The force between 2 point charges is
proportional to the product of the charges
and inversely proportional to the square of
the distance between them.
F
1
q1q 2 
4 d
2
Electric Field
• This is the area around a charge where
a force is experienced by charges.
• The direction of the field is the direction
that a positive charge would move.
Electric Field Intensity (Strength)
• This is the force per unit charge in an electric
field. Unit : Newton per Coulomb N/C
• Formula
F
E
q
E
1
q
4 d
2
To Demonstrate Electric Field
Intensity
EHT
Connect 2 plates to a high voltage supply and place in a dish of oil.
Sprinkle semolina between the plates and the grains align with the
field.