Download The Electric Field

The Electric Field
Some Introductory Notes
The magnitude and strength
of Electric fields and
Gravitational fields vary
inversely with the square of
the distance between them.
More Notes…
• An electric field is very strong.
• A gravitational field is not
strong.
• An electric field is caused by a
charged object affecting its
surroundings.
Son of More Notes…
• An electric field caused by a
point charge exerts a force
on any other charge placed
in its environment.
• A collection of all the forces
makes up an electric field.
Electric Field Lines
• They display the direction and
magnitude of the field.
• They emanate from a positive point
charge.
• They emanate towards a negative
point charge.
More
• They show a picture of the field.
• If they are close the field is strong
strong.
• If they are far apart the field is weak.
• An electric field is real and it can do
work.
Electric Field Intensity
Defintion
Electric Field intensity is the ratio
of the force on a test charge to its
charge.
F
E '
q
What are the Units?
• The units are Newtons per
Coulomb…N/C.
• E is the electric field intensity
• F is the force
• q’ is the test charge
More on E-Field
The electric field intensity can also
be calculated using the point
charge, Q.
KQ
E 2
d
This, of course, means that the
electric field intensity does
not depend on the magnitude
of the test charge.
Electric Potential Difference
• A charge in an electric field
experiences a force.
• If the charge moves in
response to the force, work is
done by the field.
• The system loses energy.
• If the charge is moved against
the field, work is done by an
outside force.
• Energy is stored in the system.
• If work is done as a charge
moves from one point to
another point…the points are
said to differ in electric
potential.
• This is referred to as the
potential difference between
the two points.
The Potential Difference
The potential difference, V,
between any two points in an
electric field is the work done
per unit charge as the charge
is moved between the points.
The Equation
W
V
q
The Units
joule
volt 
coulomb
Sample Problem
• If the potential difference
between two points in a field is
6.0V, what is the work
required to move a charge of
300. mC between the points?
• Voltage, or electric
potential, can be described
as electric pressure.
• It is analogous to water
pressure in a garden hose.
• The voltage of a battery is
the pressure with which
charge travels through a
wire.
• As the battery dies, the
pressure wanes.
Equipotential Lines
• Lines of equal distance from a
point charge constitute an
equipotential line.
• The electric field intensity can
be also designated as change in
potential per unit distance.
volts
E
meter
•Prove that a N/C is
the same as a V/m.
Uniform Electric Field
Definition
• Two parallel conducting plates
a distance, d, apart constitute a
uniform electric field.
• The uniform field emanates
from the positive plate to the
negative plate.
Voltage in a Uniform E-field
• Voltage equals work per unit
charge.
• That is V = Nm/C
• N/C is E – electric field intensity.
• m is distance between the plates.
• Therefore…
V  Ed
Sample Problem
• It requires 2.5 X 10-5 N of force to
move an electron from the
positive to the negative plate in a
uniform electric field. If the plates
are separated by a distance of
0.005 m, what is the electric
potential between the plates?