Electrostatic Forces
Electric Charge


1.
2.
3.
All matter is made up of atoms
Atoms contain
Protons (+)
Neutrons (0)
Electrons (-)
Law of Electric Charges

The law of electric charges states
that like charges repel, and
opposite charges attract.

Protons are positively charged and
electrons are negatively charged,
so they are attracted to each other.

Without this attraction, electrons
would not be held in atoms.
Law of Electric Charges
Electric Force
The force between the
charged objects is an
electric force.
The size of the electric force depends on 2 things:
1.
2.
The amount of charge (the greater the charge,
the greater the force)
The distance between charges (the further the
distance, the less the force)
Electric Field
An electric field is the region around
a charged object where electric
forces can be exerted on another
charged object.
(Repelled or attracted)

Coulomb’s Law – Gives the electric
force between two point charges.
q1q2
F k 2
r
Inverse Square
Law
k = Coulomb’s Constant = 9.0x109
Nm2/C2
q1 = charge on mass 1
q2 = charge on mass 2
r = the distance between the two
charges
The electric force is much stronger
than the gravitational force.
Newton’s Law of Universal Gravitation
m1m2
Fg  G 2
r
M = mass of objects
R = radius between objects
Example 2
Two 40 gram masses each with a charge of 3μC
are placed 50cm apart. Compare the
gravitational force between the two masses to the
electric force between the two masses. (Ignore
the force of the earth on the two masses)
3μC
40g
3μC
40g
50c
m
Charged Objects
Atoms do not have a charge
because the number of electrons
and protons cancel each other out.
Ex.
3 protons (+) & 3 electrons (-) = 0

Charged Objects
How do objects get charged?
 They either gain or lose electrons.
 Why not protons?
Ex.
3 protons (+) & 5 electrons (-) =
7 protons (+) & 2 electrons (-) =
How Can You Charge Objects?

There are 3 ways objects can be charged:
1.
2.
3.
Friction
Conduction
Induction
**In each of these, only the electrons move. The
protons stay in the nucleus**
Friction

Charging by friction occurs when
electrons are “wiped” from one object
onto another.
Ex.
If you use a cloth to rub a plastic ruler,
electrons move from the cloth to the ruler.
The ruler gains electrons and the cloth
loses electrons.
Conduction

Charging by conduction happens when
electrons move from one object to another
through direct contact (touching).
Ex. Suppose you touch an uncharged piece of
metal with a positively charged glass rod.
Electrons from the metal will move to the
glass rod. The metal loses electrons and
becomes positively charged.
Induction

Charging by induction happens when charges in
an uncharged object are rearranged without
direct contact with a charged object.
Ex.
If you charge up a balloon through friction
and place the balloon near pieces of paper,
the charges of the paper will be rearranged
and the paper will be attracted to the balloon.
Conservation of Charge


When you charge something by any
method, no charges are created or
destroyed.
The numbers of electrons and protons
stay the same. Electrons simply move
from one atom to another, which makes
areas that have different charges.
Conductors and
Insulators




An electrical conductor is a material in
which charges can move easily.
Most metals are good conductors because
some of their electrons are free to move.
Conductors are used to make wires. For
example, a lamp cord has metal wire and
metal prongs.
Copper, aluminum, and mercury are good
conductors.
Conductors and
Insulators



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An electrical insulator is a material in
which charges cannot move easily.
Insulators do not conduct charges very well
because their electrons cannot flow freely.
The electrons are tightly held in the atoms of
the insulator.
The insulating material in a lamp cord stops
charges from leaving the wire and protects
you from electric shock.
Plastic, rubber, glass, wood, and air are
good insulators.

Static Electricity



Static electricity
is the electric
charge at rest on
an object.
When something
is static, it is not
moving.
The charges of
static electricity do
not move away
from the object
that they are in.
So, the object
keeps its charge.
Ex. Clothes taken
out of a dryer
Electric Discharge
The loss of static electricity as charges
move off an object is called electric
discharge.
Sometimes,
electric
Sometimes,
discharge
electric
happens
discharge
quickly.
happens
slowly.
Ex. wearing

Ex: static on
clothes
rubber-soled
shoes on
carpet,
lightning
How Lightning Forms
Lightning
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Lightning usually strikes the highest point in a
charged area because that point provides the
shortest path for the charges to reach the ground.
Anything that sticks up or out in an area can
provide a path for lightning.
A lightning rod is a pointed rod connected to the
ground by a wire.
Objects, such as a lightning rod, that are joined to Earth by a
conductor, such as a wire, are “grounded.” Any object that is
grounded provides a path for electric charges to move to Earth.
Because Earth is so large, it can give up or absorb charges without
being damaged.
When lightning strikes a lightning rod, the electric charges are carried
safely to Earth through the rod’s wire. By directing the charge to
Earth, the rods prevent lightning from damaging buildings.