Download Electric Potential and Potential Energy

Electric Potential and Potential
Energy
Electric Potential Energy
• Electrical potential energy is the energy contained in a
configuration of charges. Like all potential energies, when it
goes up the configuration is less stable; when it goes down,
the configuration is more stable.
• Electrical potential energy increases when charges are
brought into less favorable configurations (ex:, like-sign
charges getting closer together, or unlike-sign charges
farther apart).
• Electrical potential energy decreases when charges are
brought into more favorable configurations.
• The unit is the Joule.
Different Electric Potentials
• Differences in Electric potential are easy to measure.
• If 2 places have different potential energies, we say
there is a “potential difference” (see pic)
d
E
• We commonly call it “voltage”, and its unit is the Volt.
DV
(V)
Electrical Potential and Potential Energy
• The change in potential energy is directly related to
the change in voltage.
• DU = qDV
– DU: change in electrical potential energy (J)
– q: charge moved (C)
– DV: potential difference (V)
• All charges will spontaneously go to lower potential
energies if they are allowed to move, like all masses
fall to lower gravitational potential energies!
Electrical Potential and Potential Energy
• Since all charges try to decrease potential
energy, and DU = qDV, this means that
spontaneous movement of charges result in
negative DU.
• Positive charges like to DECREASE their
potential (DV < 0)
• Negative charges like to INCREASE their
potential. (DV > 0)
Sample Problem
A 3.0 μC charge is moved through a potential difference of 640 V.
What is its potential energy change?
Electrical Potential in Uniform Electric Fields
• The electric potential is related in a simple way
to a uniform electric field.
• DV = -Ed
– DV: change in electrical potential (V)
– E: Constant electric field strength (N/m or V/m)
– d: distance moved (m)
E
d
DV
Sample Problem
An electric field is parallel to the x-axis. What is its magnitude
and direction if the potential difference between x =1.0 m and
x = 2.5 m is found to be +900 V?
Sample Problem
What is the voltmeter reading between A and B? Between A and C? Assume
that the electric field has a magnitude of 400 N/m.
y(m)
1.0
C
A
B
1.0
2.0
x(m)
Sample Problem
How much work would be done BY THE ELECTRIC FIELD in moving a 2 mC
charge from A to C? From A to B? from B to C?. How much work would be
done my an external force in each case?
y(m)
1.0
C
A
B
1.0
2.0
x(m)