Energy and Electric Potential
The work performed moving a charged particle in an electric field can result
in the particle’s gaining potential, or kinetic energy, or both.
Work is needed to move
an object against the force of
gravity and against electric
force.
In both cases the potential
energy of the object is
increased.
Potential Difference (∆V): The work done per unit charge.
Work done (W)
∆V = ------------------------Charge (q)
S.I. unit of potential difference is Volt. 1Volt = 1 Joule / Coulomb.
Electric potential is smaller when two
unlike charges are closer together and
larger when two like charges are closer
together.
The Electric Potential in a Uniform Field
A uniform electric force and field can be
made by placing two large, flat, conducting
plates parallel to each other.
One is charged positively and the other is
charged negatively.
The electric field between the plates is constant, except at the edges of the
plates, and its direction is from the positive to the negative plate.
Work done (W)
Fd/q‫( = ׳‬F/q‫)׳‬d.
∆V = ------------------------Charge (q)
For a point charge, the potential V = K Q / r.
Electric Potential Difference in a Uniform Field, ΔV = Ed
The electric potential increases in the direction opposite the electric
field direction. That is, the electric potential is higher near the
positively charged plate.
The units of E and d are (N/C)(m).
This is equivalent to one J/C, which is the definition of 1 V.
Equipotential: Whenever the electric potential difference between
two or more positions is zero, those positions are said to be at
equipotential.
Mllikan’s Oil drop Experiment
Millikan’s experiment showed that
Charge is quantized. It means that
An object can have only a charge
with magnitude that integral
multiple of the charge of an electron.
Q = + ne
n =0,1,2,3…..
e = 1.6 x 10 –19C.
Sharing of charge : Charge transfer takes place from the point where
the higher potential to the lower potential when they are in contact.
In irregular shapes charges will be closest together at sharp points.
Lightning rod is pointed so that the electric field will be strong near
the end of the rod.
As a result of the rod’s sharply pointed shape, charges in the clouds
spark to the rod, rather than to a chimney or other high point on a
house or other building.
From the rod, a conductor takes the charges safely to the ground.
Capacitor: Device used to store the charges.
It is made up of two conductors of equal and opposite charges
separated by an insulator.
For a given shape and size of an object, the ratio of charge stored to
electric potential difference is a constant called the capacitance, C.
Capacitance is the ratio of charge on one plate to potential difference.
C = q / ∆V.
It is measured by the unit called Farad.
1 Farad is 1 Coulomb per 1 volt.
1Micro farad (1μF) = 10-6F.
1 Pico farad (1pF) = 10-12F.
Capacitance depends only on the construction of the capacitor, not on
the charge q.
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N.Krishnan