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Electric Fields
What is an electric field???
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It is an area of influence around a charged
object.
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Positively charged objects within that field
will experience an electrical force.
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Electric Field
F
+
Test Charge
F -- - -- --- - - -
Electric Field
Calculating the magnitude of an electric
field

The magnitude is proportional to the
amount of electrical force exerted on the
positively charged object.
Electric Field Strength = electric force/test charge
E = Felectric/q0
SI Unit = N/C
Sample Problem
Mr. Luna pulls a wool sweater over his head, which
charges his body as the sweater rubs against his
cotton shirt. What is the electric field strength at a
location where a 1.60 x 10-19 C piece of lint
experiences a force of 3.2 x 10-9 N as it floats near
Mr. Luna?
F = 3.2 x
10-9
N
q0 = 1.60 x 10-19 C
E = Felectric/q0 = 3.2 x 10-9 N / 1.60 x 10-19 C
E = 2.0 x 1010 N/C
Calculating the strength of an
electric field

The strength of an electric field depends on:
1. the charge produced by the electric field
2. the distance between the test charge and the
electric field
Electric Field Strength = Coulomb constant x field charge/ (distance)²
E = (Kc x q)/r²
Kc = 8.99 x 10 9 N • m² / C²
Sample Problem
A fly accumulates 3.0 x 10-10 C of positive charge as
it flies through the air. What is the magnitude and
direction of the electric field at a location of 2.0 cm
away from the fly?
q = 3.0 x 10-10 C
r = 0.02 m
Kc = 8.99 x 109 N • m² / C²
E = (Kc x q)/r² = (8.99 x 109 N•m²/C²) (3.0 x 10-10 C)
(0.02 m)²
E = 6743 N/C away from the fly
Electric Field Lines

Represent the magnitude and direction of
an electric field with respect to the positive
test charge.
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More arrows = greater magnitude
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Electric Field Lines
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When two point charges of equal
magnitude, but opposite signs interact with
each other, they form an “electric dipole”.
Electric Field Lines

Like charged objects that interact with each
another, have their field lines moving away
from one another.
Electric Field Lines
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When interacting charges are of a different
magnitude, only a fraction of the field lines
from the greater charge will move to the
lesser charge.