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Electric Fields
Physics
Analogy
The electric field is the space
around an electrical charge
just like
a gravitational field is the space
around a mass.
Electric Field
 Space around a charge.
What is the difference?
Van de Graaff Generator
 Builds up static electric
charges.
Electric Field Vector, E
 E = F/ qo
 qo , positive test charge
 E is a vector quantity
 Unit: N/C
 E is analogous to the gravitational
field, g, where g=F/m
Example 1
 A charge of 3µC is used to test the electric
field of a central charge of 6C that causes a
force of 800N. What is the magnitude of the
electric field?
 Answer: 2.7 x108 N/C
Electric Field Lines
Electric Field Lines of two Positive
Charges
Electric Field Lines
 Lines that indicate the strength and
direction of the electric field.
 The more dense the lines, the stronger the
field.
Example 2
 A positive test charge is placed between an
electron e and a proton p. Which way will the
test charge move?
e-
p+
+
Example 3
 An object with a net charge of 4.80 x 10-6 C
experiences an electrostatic force having a
magnitude of 6.00 x 10-2 N when placed near
a negatively charged metal sphere. What is
the electric field strength at this location?
Electric Field Lines Between Two
Plates
Positive Plate (+)
Accelerates toward
positive plate
+
Accelerates toward
negative plate
Negative Plate (-)
 Electric field lines go away from a positive
plate and go toward the negative plate.
Electric field strength is the same
everywhere between the plates.
Final Thought…
+++++++++++++++++++++++++++++++++
p+
n
e- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 A proton, neutron and electron are located between
two oppositely charged plates, the magnitude of
acceleration will be greatest for the
 Proton
 Neutron
 electron
Example 4
 If an electron is introduced directly between two
oppositely charged, parallel plates, the electron
will




Travel at constant speed toward the positively charged plate.
Travel at constant speed toward the negatively charged
plate.
Accelerate toward the positively charged plate.
Accelerate toward the negatively charged plate.
Conductors and Electric Fields
(under electrostatic conditions)
 “The electric field is zero inside a charged
conductor”.
 “Excess charge on an isolated conductor
resides on the surface”.
 “Excess charge accumulates on sharp points”.
 Electric field lines meet the conductor
perpendicular to the surface of the conductor.
Shielding
 The electric field is zero inside a charged
conductor.
Where are you safe during a
thunderstorm?
A) In a car
B) Outdoors
or
Why can you not get radio
reception in a tunnel or in a steel
bridge?
Which field is stronger?
A
B
Example 5
 A test charge of +3µC is located 5m to the
east of a -4µC charge.
 A) Find the electric force felt by the test
charge.
 B) Find the electric field at that location.
 Answer: 4.32x10-3 N, 1.44 x 103 N/C along
the –x axis.
Example 6
 If a test charge is moved to a location three
times as far as its original location, how does
the electric field change?