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Electric Fields
Fields: Action at a distance
• Remember in the lab, a charged
balloon was able to attract small
pieces of paper without touching
them?
Non-contact forces:
• The electric force (and the gravitational
force) are non-contact forces. (Can you
name some contact forces?)
• Gravity can attract objects without
touching them.
• The electric force can attract or repel
objects without touching them.
Electric Field
• Two charged balloons
• How can one balloon reach across space and push
a second balloon away?
• The best explanation to this question involves the
concept of electric field.
The Electric Field
• There is something different about the space surrounding a
charged object.
• Any other charged object that is in that space feels the effect of
the charge.
• A charged object creates an electric field - an alteration of the
space in the region that surrounds it.
• Other charges in that field would feel the alteration of the
space.
• Whether another charged object enters that space or not, the
electric field exists.
Electric Field
An electric field is defined as being present
in any region where a charged object
experiences an electric force. This is a
fancy way of saying that the only way we
can tell if a field exists is to place a test
charge at that spot and see if it feels a force.
(In other words, it takes one to know one.)
A stinky analogy….
• Anyone who has ever been near a skunk that has
sprayed his foul-smelling scent has experienced a
stinky field. There is something about the space
surrounding a skunk that exerts an influence upon
people who enter that space.
• When you get close enough, your detector (i.e., your
nose) begins to detect the presence of a stinky field.
As you move closer and closer to the skunk, the
stinky field becomes more and more intense.
Our analogy…
• In the same manner, an electric charge creates an electric
field - it has altered the nature of the space surrounding it. And
if another charge gets near enough, that charge will sense the
effect.
• An electric field is sensed by a detector charge in the same
way that a nose senses the stinky field.
• The strength of the stinky field is dependent on the distance
from the skunk and the amount of spray.
• Similarly, the strength of the electric field is dependent on the
amount of charge that creates the field and the distance from
the charge.
The Concept of a Field
A field is defined as a property of space in which an
object experiences a force.
m
P.
Above earth, we say there is a
gravitational field at point P.
F
A mass m experiences a
downward force at that point.
The direction of the field is determined by the force.
The Concept of a Field
A field is defined as a property of space in which an
object experiences a force.
P .
Important note: The field exists,
whether or not another mass is
present.
The Electric Field
1. An area of influence
around a charged object.
2. The magnitude of the field is
proportional to the amount of
charge creating the field, and
on the distance from the
charge.
3. Electric field is a vector
quantity.
4. The magnitude of E is
given by the formula:
+q
P +.
F
E
r
+ ++
+
++Q++
Electric Field
F
N
E  ; Units
q
C
Calculating the strength of an
electric field:
From Coulumb’s Law, the force on a charge q when it’s
near another charge Q that is creating a field is:
kQq
F 2
r
The electric field E is therefore:
F kQq r
E 
q
q
2
kQ
E 2
r
Check your understanding
The electric field strength in a certain
region of space is 15 Newtons/Coulumb.
What is the force experienced by a 5.0 C
charge placed at that point in the field?
Like gravity and electric force,
field strength is calculated
with an inverse square
kQ
formula:
E
r
• Electric field strength is location dependent, and its
magnitude decreases as the distance from a location to the
source increases.
• If separation distance increases by a factor of 2, the electric
field strength decreases by a factor of 4 (22). If the
separation distance increases by a factor of 3, the electric
field strength decreases by a factor of 9 (32).
2
Check your understanding:
Charge Q acts as a point charge to create an electric
field. Its strength, measured a distance of 30 cm
away, is 40 N/C. What is the magnitude of the
electric field strength that you would expect to be
measured at a distance of ...
a. 60 cm away?
b. 15 cm away?
c. 3 cm away?
The Direction of Electric Field
The direction of an electric field at any point is the
same as the direction that a positive charge would
move IF it were placed at that point.
Example What is the electric field at point P,
a distance of 3.0 m from a negative charge
of –8.0 x 10-9 C?
E=?
.
r
3m
-Q
First, find the magnitude:
P
9 Nm 2
C2
-9
)(8 x 10 C)
kQ (9 x 10
E 2 
r
(3 m) 2
E = 8.00 N/C
The direction is the same as the force on a positive
charge if it were placed at the point P: toward –Q.
E = 8.00 N, toward -Q
Sample Problem
• What is the electric field strength 0.30 m
away from Van de Graff generator with a
charge of 1.2 x 10-6 C ?
• What is the force on a 0.1 x 10-6 C test
charge at this point from .30 m away from
the generator ?
Electric Field Lines
Electric Field Lines are imaginary lines drawn in
such a way that their direction at any point is the
same as the direction of the field at that point.
BASED ON A POSTITIVE TEST CHARGE IN THE FIELD.
++
+
+
++Q++
-- -Q ---
Field lines go away from positive charges and
toward negative charges.
Electric Field Lines
It is very important that you notice that the
map lines have a direction to them. The
direction represents the motion of the
positive test charge when placed at different
points around the field. You should also
notice that field lines never cross each
other. The closer the field lines are to each
other, the greater the field intensity or field
strength.
Check your understanding:
True or False:
Every electric charge is surrounded by an
electric field.
Check your understanding:
• Draw arrows (NOT charges) on this
diagram to indicate that the charge on
the left is negative, and the charge on the
right is positive: