Transcript Electric Potential Energy

Lecture 5:
Potential Energy and
Electric Potential (Ch. 19)
03-02-07
Potential Energy
Remember Gravitational Potential Energy from 121?
Electric Potential Energy
As the charge moves from A to B,
work is done by the electric force
similar to the work done by
gravitational force
Electric Potential Difference
The electric potential V at a given point is the electric potential energy EPE of a small
test charge q0 situated at that point divided by the charge itself:
SI Unit of Electric Potential: joule/coulomb = volt (V)
Examples 1, 2 3
Electric Potential Difference
Neither the potential V nor the potential energy EPE can be determined in an absolute
sense, because only the differences ΔV and ΔEPE are measurable in terms of work WAB.
A positive charge accelerates from a region of higher electric potential toward a
region of lower electric potential.
A negative charge accelerates from a region of lower potential toward a region of
higher potential.
volt is a unit for measuring electric potential difference.
One electron volt is the magnitude of the amount by which the potential energy of
an electron changes when the electron moves through a potential difference of one
volt.
Why does the bulb come on as soon as you turn on the switch?
Electric Potential Difference Created by
Point Charge
In the limit that rB is infinitely large, the term kq/rB becomes zero, and it is customary
to set VB equal to zero.
When two or more charges are present, the potential due to all the charges is
obtained by adding together the individual potentials
Equipotential Surfaces &
Their Relation to Electric Field
An equipotential surface is a surface on which the electric potential
is the same everywhere.
The equipotential surfaces that surround the point charge +q
are spherical. The electric force does no work as a charge
moves on a path that lies on an equipotential surface, such
as the path ABC. However, work is done by the electric force
when a charge moves between two equipotential surfaces,
as along the path AD.
The net electric force does no work as a charge moves on an equipotential surface.
Problem solving insight: the electric field created by any charge or group of charges is
everywhere perpendicular to the associated equipotential surfaces and points in the
direction of decreasing potential.
Equipotential Surfaces &
Their Relation to Electric Field
Equipotential surfaces (in blue) of an electric dipole.
The surfaces are drawn so that at every point they are
perpendicular to the electric field lines (in red) of the
dipole.
The radially directed electric field of a point
charge is perpendicular to the spherical
equipotential surfaces that surround the charge.
The electric field points in the direction of
decreasing potential.
Equipotential Surfaces &
Their Relation to Electric Field
Example 9
Capacitance of a Capacitor
Dielectric Constant
Relation between charge and potential
difference for a capacitor
The magnitude q of the charge on each plate
of a capacitor is directly proportional to the
magnitude V of the potential difference
between the plates:
where C is the capacitance.
SI Unit of Capacitance:
coulomb/volt = farad (F)
Dielectric Constant
This reduction in the electric field is described by the dielectric
constant , which is the ratio of the field magnitude E0 without
the dielectric to the field magnitude E inside the dielectric:
Capacitance of a Parallel Plate Capacitor
Energy stored in a Capacitor
Biomedical Applications of Electrical
Potential Difference
Neuron
Biomedical Applications