Electrostatics is the study of the effects of stationary
charges on each other in their surroundings.
Charges are created by the transfer of electrons to or from
one body to another. (Protons are NEVER transferred.)
Objects with equal numbers of protons and
electrons are neutral. They have no net charge.
Objects with more electrons than
protons are charged negatively
Objects with less electrons than
protons are charged positively. 2
6.25 x 10 18electrons
Charge on 1 electron = - 1.6 x 10 –19 coulombs
Charge on 1 proton = + 1.6 x 10 –19 coulombs
Fg = gravity force between
m1 and m2 separated by
a distance r
G is the Universal
The weight of an object
is its mass times g’, the
gravity value at location r
Fe = electrostatic force
Between m1 and m2
separated by a distance r
k is Coulomb’s
Coulomb’s Law is an inverse square law
similar to the Law of Gravitation
It is dissimilar in that electrostatic forces can be attraction or
repulsion. Gravity is attraction only.
Electrostatic force is strong, gravity is very weak. 6
Gravity is classified as a weak force because huge
amounts of mass are required to create a reasonably
Note the very small value of the force constant.
Electric forces is classified as a strong force because
small charge quantities can to create large forces.
Note the very large value of the force constant
Electric charges are detected by the presence of an electric field (E).
Recall that a gravity field can be detected by its influence
(attractive force) on a mass (often called a test mass).
Electric fields are detected by their influence (attractive or
repulsive forces) on a charge (often called a test charge).
Electrically charged bodies can be created by physically rubbing
electrons off one object on to another. For example, rubbing a
rubber rod with fur will transfer electrons from the fur to the rod
which becomes negatively charged (it has extra electrons).
Rubbing a glass rod with silk will transfer electrons from the rod to
the silk. The rod is charged positively (it has missing electrons).
- - -
- - -
Leaves of electroscope
Diverge (like charges repel)
An Electroscope is an
instrument used to measure
the presence of an Electric field
(presence of charged bodies).
WHEN OBJECT IS TOUCHED
BY THE CHARGED ROD
ELECTRONS MOVE FROM
THE ROD TO THE SPHERE
EQUILIBRIUM IS REACHED
CHARGED RUBBER ROD
Electrons on sphere move
to the opposite side due to
repulsion of electrons on
the charged rod
E is always out of plus (+) into minus (-)
Recall that a gravity field (g) is measured by dividing the force
acting on it (its weight(w) in Newtons ) by the mass quantity (m)
An electric field (E) is measured by dividing the force
acting on it (in Newtons ) by the charge quantity (q) in
coulombs (C ).
g in N / Kg
E in N / C
Force on a test
This is similar to
the weight of a
in a stronger
E = Force (N) / Charge (Coul) 17
Lines of Flux spread
over a greater area
as distance from
charge increases and
weakensE = k q / r2
(1) E = F/q (by definition)
(2) F = kq1 q2 / r2 (Coulomb’s Law)
(3) E = kq1 q2 / r2 q2 ( by substitution)
E = kq / r2
q = point charge in coulombs
k = coulomb’s constant
r= distance from charge in
E = electric field strength
at that point (N / C)
Electrical Potential is defined as the work required to move a
charge over a distance in an electric field.
Electrical potential is measured in volts.
One volt equals one joule (work) divided by charge (coulombs)
In a uniform electric field (a field between two parallel charged
metal plates) , potential (V) equals work (W) divided by charge (q).
Therefore, since work equal force times distance and force equals
the electric field strength (N/C) times charge in coulombs (C) ,
potential equals electric field times distance the charge moves.
V = W / q = (F x d) / q = (F / q) x d = E x d
V (volts) = E ( N/C) x d (m) 20