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Electric Fields
Electric Fields
• Electric fields are similar
to gravitational fields.
• The only difference is that
two objects with mass will
always attract each other.
• Charges can either repel
or attract when held
some distance apart.
Here are the gravitational field lines
around the Earth. Notice how they
point toward the center and are
perpendicular at the surface of the
Earth.
Charges produce similar fields…
-
+
• A charged object
creates an electric field
- an alteration of the
space in the region
which surrounds it.
• Other charges in that
field would feel the
unusual alteration of the
space.
Electric field between two
opposite charges…
Electric field between two like
charges…
5 Rules for Electric Fields:
1. Electric field lines always go from positive to negative.
This is the path that a
positive “test charge”
would follow.
5 Rules for Electric Fields:
2. Electric field lines always enter and leave the charge
perpendicularly.
5 Rules for Electric Fields:
3. Electric field lines never cross.
This is just like contour lines.
5 Rules for Electric Fields:
4. Where the electric field lines are closer the electric field
is stronger.
Again remember when the
contour lines were closer
together the slope was
steeper.
5 Rules for Electric Fields:
5. Make sure the electric field lines are in contact with
charged objects.
Charged Parallel Plates
+
+
+
+
+
+
+
-
What would the electric field look
like around these charges?
-
-
What would the electric field look
like around these charges?
+
+
-
Analyzing the path of a test charge:
A test charge is a positively charged object that is used to
test the electric field around other charged objects.
+
-
+
+
+
Electric Field Strength (E)
• This is also
similar to the
gravitational
field strength.
• The
gravitational
field strength of
the Earth is 10
m/s2.
g = w = Fg = GmM
2
r
m m
m
g = GM
r2
E = FE = kqQ
r2
q
q
E = kQ
r2
What is the magnitude of the electric
field strength at a point in a field
where a positive test charge of 8.00 x
10-2 C experiences a 2 N force?
+
E = FE
q
E=2N
8.0 x 10-2 C
E=2N
8.0 x 10-2 C
E = 25 N/C
Electric Potential DifferenceVOLTAGE
Let’s first revisit gravitational
potential energy…
What is gravitational potential energy dependent on?
PE = mgh
Work (energy) is required
to lift these rocks against
the force of gravity.
m
2m
2h
h
Work is also required to move charges in an electric field.
If the direction of an electric field is such that it opposes the
motion of a charged particle, work must be done to move
the particle in that direction.
The potential difference between
two points in an electric field is
the work done per unit charge.
V=W
q
1 J/C = 1 volt
W = Fd
+ + +
+
+
++
+ + +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
For each situation below determine if work is
done on the test charge to move it from point A
to point B.
B
+
A
-
B
-
A
+
NO work is done.
The + charge is moving
with nature; work is not
required when it moves
with the electric field.
YES work is done.
The + charge is moving
against nature; work is
required when it moves
against the electric field.
If there is no difference in an electric field than
no work needs to be done and there is no
voltage.
metal spheres
50e
100e
10e
Both have the same
charge- no potential
difference- no voltage
Here there is a potential
difference- work is done
to move a charge of 50e
Here there less of a potential
difference- work is done
to move a charge of 5e
50e
0
0
THINK OF
VOLTAGE AS
ELECTRIC
PRESSURE.
If 8 Joules of work are required to move 2
Coulombs of charge through a 3-ohm
resistor, what is the potential difference
across the resistor?
V=W
q
V = 8J
2C
V = 4V
Units- Joules vs. eV
How much work is done to move an elementary
charge (+/- e) against an electric field through a
potential difference of 1 volt?
V=W
q
qV = Wq
q
W = Vq
W = (1V)(1.6 x 10-19 C)
= 1.6 x 10-19 J
1.6 x 10-19 J = 1 eV (electronvolt)
Electronvolts are like inches and Joules are like miles.
It’s like saying 1.894 x 10-4 miles = 1 foot
How much energy in eV is needed to move
one electron through a potential difference of
1.0 x 102V?
W = energy
W = Vq
W = (1.0 x 102V)(-1.6 x 10-19C)
W = 1.6 x 10-19J = 1 eV
1 eV = 1.6 x 10-17 J
How many electronvolts are in 320
x 10-19J of energy?
1 eV = 1.6 x 10-19 J
200 eV