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19.1 Potential Energy
The electric force, like gravity, is a conservative force
:
Recall
Conservative Forces
1. The
work done
on an object by a conservative force
depends only on the object’s
initial and final
position
, and not the path taken.
2. The
net work
done by a conservative force in
moving an object around a closed path is
zero
.
Let’s place a positive point charge q in a uniform electric field and let it
move from point A to B (no gravity):
How much work is done by the field in moving the
++
+
+
+
+q
charge from A to B?
*Remember, W = F x d, where F is the component of
the constant
force along the direction of the motion.
A
E
y
o
Here,
,
so
qE
F
=
y
qE
y
y
qE
W
o
f
Δ
=
−
=
)
(
Electrostatic Potential
nergy (EPE)




B
y
f
Energy (EPE)
Analogous to gravity, where
mgh
is the gravitational
potential energy.
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View Full Document Thus,
EPE
W
AB
Δ
=
The work done is equal to the change in
electrostatic potential energy!
Now let’s divide both sides by the charge, q:
EPE
W
AB
Δ
The quantity on the right is the potential energy per unit
q
q
=
charge.
We call this the
Electric Potential, V
:
EPE
The electric potential is
q
V
=
nergy
a scalar!
Units?
[ ] [ ]
V
C
J
=
=
⎥
⎦
⎤
⎢
⎣
⎡
=
⎥
⎦
⎤
⎢
⎣
⎡
Volt
Charge
Energy
Review of
Work
:
1. Work is not a vector, but it can be either positive or negative:
Positive
– Force is in the same
direction as the motion
Negative
– Force is in the opposite
direction as the motion
2. If
positive
work is done on an object, the object
speeds up
.
3. If
negative
work is done on an object, the object
slows down
.
We can talk about the value of the potential at different points in space:
For example, what is the difference in electrostatic potential
19.2 Electric Potential Difference
between two points, A and B, in an electric field???
++
+
+
+
EPE
EPE
V
V
A
B
A
B
−
=
−
W
AB
−
=
+q
A
E
q
q
q
W
B
−
Why is there a




B
So,
q
V
V
V
AB
A
B
=
−
=
Δ
minus sign???
Let’s say the charge we put at point A is positive:
If I release it, which way will it move?
It moves down toward B!
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This note was uploaded on 04/24/2010 for the course PHYS 2002 taught by Professor Blackmon during the Spring '08 term at LSU.
 Spring '08
 BLACKMON
 Physics, Energy, Force, Gravity, Potential Energy, Work

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