### The Electric Field

```Electric Field
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.
 This Van de Graaff Generator
was responsible for creating
a field large enough to ‘fry’
our multi-media device!
Electric Field Vector, E
 Electric Field is designed as follows
 E = F/ qo
 qo , positive test charge
 E is a vector quantity

Direction indicated by small + test charge
 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?
 Hint… Which charge ‘tests’ the field
Electric Field- Diagrams
Electric Field Hockey
Complete pages
Electric Field Lines
Electric Field Lines of two Positive
Charges
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.
 Electric field vectors are tangent to the
curve.
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
Where are you safe during a
thunderstorm?
A) In a car
B) Outdoors
or
Which field is stronger?
A
B
Which field is stronger?
A
B
Electric Field for a Point Charge
Using E=F/qo and Coulomb’s Law prove:
E=k Q
______
r2
where Q is the central charge.
Example 2
 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 3
 If a test charge is moved to a location three
times as far as its original location, how does
the electric field change?
 Inverse-Square Law says… 1/9
Example 4
 Calculate the electric field felt by a positive test
charge located half way between a charge of
+1C and a charge of -3C, that are 2m apart.
 Answer: 3.6 x 1010 N/C (toward the -3C charge)
```