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```					   Electric Potential (Chapter 21.2)

When a system has the potential to move, we
potential
say that the system has ___________ energy.
kinetic
In both of the above situations, the _________
energy acquired equals the decrease in
potential
__________ energy.
Work
______ is required to      Work
______ is required to
compress the spring,      push the charge
increasing the                        electric
against the ________
potential energy of
________                   field
_____, increasing the
the system.              potential energy of
________
the system.
If two charges were pushed
to the same location,
twice
________ as much work is
required, and the resulting
twice
potential energy is ________
as much.
Likewise, a group of 5 charges pushed to the
5
same location would have ____ times the PE.
In each of these cases, the total Potential
Energy was different, but the
Electric Potential Energy per Charge isconstant
______.
PE 2 PE 5PE
    
q   2q   5q
This ratio (potential energy per charge) is
Electric _________,
called the __________ Potential or more
Voltage
commonly called __________.
PE W          Units: 1Volt V  
1 Joule
V    
q   q                            1Coulomb
EXAMPLES:
1. If it takes 20 J of work to move 1 Coulomb
of charge into an electric field, by how much
will its voltage (or electric potential) increase?
W 20 J
V        20 V
q   1C
How many Joules of work would be required
to move 3 Coulombs of charge into the same
location in the field?
W      W
V   20   W PE is ___, but electric
60↑J
q      3                  same
potential is _______.
2. A classroom Van de Graaff Generator is
charged up to 100,000 Volts (!!) at its surface.
If 1 microcoulomb of charge (1 x 10-6 C)
eventually builds on the dome and then moves
through the air to a person standing on the
ground, is this a dangerous situation? Why or
why not?
PE                PE
V    100,000V      6
q             1x10 C
PE  0.1 J
NOT Dangerous… if the
charge is small, the actual
energy is small as well!
?
By Historical Convention:
• Neutral Ground is assigned the value 0 Volts
______.
positive
• Positively charged objects have ________
“high”
electric potentials (_____ potential).
negative
• Negatively charged objects have ________
“low”
electric potentials (_____ potential).
So… positively charged objects would move from
high      low
_____ to _____ electric potential (similar to how a
high       low
mass would naturally move from _____ to _____ PE
in a gravitational field.)
low
Negatively charged objects would move from _____
high
to _____ electric potential.
Relationship between
Electric Fields (E) & Electric Potential (V):
W                         F
Recall: V             and         E
q                         q

W  Fd  qV  (qE)d
V
V  Ed            OR:       E
d
Electric Field units, then, can be   N       or V
C          m
Millikan’s Apparatus to measure the
charge of an electron revisited…

Animation of Millikan's Apparatus
at http://highered.mcgraw-
hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%25::100%25::/sites/dl/free/0072512644/117354/02_Millikan
_Oil_Drop.swf::Milikan%20Oil%20Drop
Air is usually an insulator, but will
Example:                  “break down” and conduct if the
electric field, E > 3 x 106 V/m. If
the distance between the center of
a Van de Graaff and the Ground
Sphere is 0.5 m, find the required
electric potential of the Van de
Graaff in order to see a discharge
through the air to ground.
V              V
E   3x10 6V

d          m 0.5 m
V  1.5x10 Volts    6

Note: The “V” in the equation is actually the electric potential difference
between the two points, but since the ground is defined as V = 0 …
Water and Static Electricity
Demo: Bring a negatively charged rod (or
balloon) near a thin stream of water. What
happens to the stream? Attracted to the
charged object!

What happens if a positively charged rod is
brought near the stream? Same thing!
WHY?
polar
Water is a _______ molecule, meaning
neutral
that while it is ________ overall, it’s
negative
slightly _________ on one side and
positive
_________ on the other other.

Oxygen has a higher
electronegativity
________________, so
the oxygen side is
negative
__________.
Water molecules will “flip” to align
+
themselves so that the ___ side will be
turned towards the (-) charged rod or
attract
balloon. Then… opposites _______.
An Application:
Why do we observe less static electricity
(less build-up of charge) when the air is
“humid”?
water vapor
More “humid” air has more ___________
“leaks” off
in the air. Charge ___________ onto
water vapor
______________ molecules in the air.
Water “drains” the static charge!

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