Ch 18 & 19 Handouts
There are two kind of charge, positive and negative and electric charge is measure in
Coulomb (C) in SI units. The charge carried by an electron is e- = -1.6 x 10-19 C and that
carried by the proton is e+ = + 1.6 x 10-19 C. This is the smallest charge that exists in
nature. To generate 1 C of charge we require 1 / 1.6 x 10-19 = 6.25 x 1018 electrons. This
is an enormous amount of charge and lightning bolts typically carry 5 C to a maximum of
few hundred C of charge.
Law of conservation of charge: Charge can neither be created nor destroyed, but only
transferred from one object to another.
Conductors have free electrons that can conduct current. Metals are good conductors.
Insulators have electrons in covalent bonds or have electrons that are tightly bound and
cannot conduct current. Plastics, wool, rubber, wood are examples of insulators.
Coulomb’s Law: The force of attraction or repulsion between two charges q1 and q2
separated by a distance d, is given by F = k.q1.q2 / d2, where k = 9 x 109 N.m2 /C2 is
the electrostatic constant. The reason electrical forces are much more powerful than
gravitational forces is due to the fact that the electrostatic constant is around 1020 greater
than the gravitational constant.
Any charge creates an electric field around it, and another charge which comes under
the influence of this field experiences a force which could be attractive or repulsive,
depending on whether they are unlike or like charges. The electric field is a vector,
measured in N / C. E = F / q and is the force experienced by the test charge q divided
by the charge itself. The electric field at a distance d from a point charge Q would be
E = k.Q / d2. If the test charge q is placed in this field it experiences a force F = E.q
Electric field lines are imaginary lines that are always drawn pointing away from a
positive charge and towards a negative charge. The direction of the line at any given
point shows the direction of the force on a small test positive charge when placed in that
field. The greater the number of electric field lines the greater the electric field strength.
If a conductor is charged, the charges try to get away from each other as far as possible
due to electrostatic repulsion and they distribute themselves evenly on the surface of the
conductor in such a way that the electric field inside the conductor is always zero. That is
why people are safe if they are in a car during lightning storm due to the metal body.
The electric potential energy, measured in Joule is equal to the work done to move
the charge from one position to another of higher potential. The electron volt (eV) is a
unit of energy and is the energy gained by an electron which moves through a potential
difference of 1 V. 1 eV = 1.6 x 10-19 J.
The electric potential V is the electric potential energy per unit charge and is a scalar
field. It is measured in J / C or Volt. 1 V = 1 J / C. The electric potential is also equal to
V = E.d. So Electric field can also be expressed as E = V / d or in Volts / meter (V/m).
The electric potential at infinity is always considered to be zero. To move a charge from
A to B, the work done per charge must be
W
= VB - VA.
q
If a point charge Q is placed, the electric potential due to the point charge at a distance
d from it would be V = k.Q / d. An equipotential surface is one in which the electric
potential is the same everywhere. No work is done by the electric force in moving a
charge along an equipotential surface.
A capacitor is an energy storage device and in its simplest form consists of two metal
plates connected to the positive and negative terminals. It usually has a insulating
material called a dielectric placed in between. The charge stored on a capacitor
Q = C.V, where C is the capacitance of the capacitor measured in Farad (F). The
capacitance C = .0.A / d. where 0 = 8.85 x 10-12 C2 / N.m2 is the permittivity of free
space, A is the area of the metal plates d is the distance between them, and  is the
dielectric constant. The dielectric constant of any material is the ratio of its permittivity to
that of free space,  =  / 0 ; it therefore has no units. ( = 1 for air or vacuum). The
energy stored by a capacitor is equal to E.P.E = ½ C.V2.