Electric charge and electric forces
Chapter
1 1
Chapter
electric charge
The ruler is charged by being rubbed
with a cloth towel. Electrons transfer
from the cloth to the ruler, leaving it
with a net negative charge.
The excess electric charge
on the ruler induces a polarisation
(separation of charge) in scraps
of paper, and thus attracts them.
This phenomenon is called static electricity.
The ancient Greeks knew that a piece of amber would attract small
pieces of leaves or dust after being rubbed with some wool.
electric charge and its conservation
Are all electric charges the same, or is there more than one type?
Charge comes in two types, positive and negative;
like charges repel and opposite charges attract.
The law of conservation
of electric charge:
whenever a certain amount of charge
is produced on one object,
an equal amount of charge of opposite sign
is produced on another.
Electric charge can neither
be created nor destroyed.
atoms and molecules
Only within the past century has it become clear that an
understanding of electricity originates from within
the atom itself. This simplified model of an atom shows
a small, dense, positively charged nucleus containing
a mixture of protons and neutrons surrounded by
negatively charged electrons.
All protons and electrons have exactly the same magnitude of electric charge.
Charge on the electron:
e = −1.602× 10−19
C
In the SI, the unit of electric charge is the Coulomb.
Electric charge is quantised in units of electron
charge. The total charge carried by any object
is a whole multiple of the electron charge.
A polar molecule is neutral
overall, but its charge is not evenly
distributed.
water molecule
insulators and conductors
Conductors
Charge flows freely:
metals
Insulators
Almost no charge flows:
most other materials
From an atomic point of view:
• for insulators, the electrons are bound very tightly to the nuclei;
• for good conductors, some of the electrons are bound very loosely and can move freely within the
material.
The relative magnitude of conductivity between silver (a good conductor)
and rubber (a good insulator) is on the order of 1021.
charging methods
Charging by conduction
If a positively charged metal rod
is brought into contact
with an uncharged metal rod,
the free electrons in the neutral rod
are attracted by the positively charged
rod and some will pass over to it.
Charging by induction
If a positively charged metal rod is brought
close to an uncharged metal rod, but does
not touch it, the free electrons in the neutral
rod do not leave it but move within the metal
rod towards the external positive charge.
electroscope
An electroscope is a device used for detecting charge.
It is made of two metal leaves that are free to move
(often made of gold), which are situated
inside an isolated case.
The electroscope on the left is charged
by induction and the one on the right
by conduction.
A previously charged electroscope
can be used to determine the sign
of a charged object.
Coloumb’s law
The French physicist Charles-Augustin de Coulomb (1736-1806)
investigated electric forces in the 1780s.
He argued that the force that one charged object (q1) exerts on a second tiny
charged object (q2) is directly proportional to the product of their charges
and inversely proportional to the square of the distance separating them (r2).
F k
q1 q2
r2
This equation gives the magnitude of the force.
The direction of the 
force is always along the line joining the two charges.
In SI units, the value of the constant k is:
k = 8.99 x 109 N  m2/C2
Coloumb’s law
The force always acts along the line connecting the charges:
• it is attractive if the charges are of opposite sign;
• it is repulsive if the charges are of the same sign.


The magnitudes of F12 and F21 are equal in agreement with Newton’s third law:
for every action there is an equal and opposite reaction.

