Chapter 24: Electric Potential

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Chapter 24 Electric Potential
Introduction Why will we be introducing the
electrostatic potential (V) and electric
potential energy (U)?
Because they are scalars!! not vectors like F and
E.
24-2 Electric potential energy Coulombs law
looks like the gravitation law. Hence, it is
conservative. What does it mean for a force or
field to be conservative?
The work done by the force is independent of path!
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We can assign a potential energy (U) for the
electric force! DU -W where W is the work done
by the electric force field.
If we take U8 0 then, U - W8
The potential energy of a charge at a point is
equal to the negative of the work done by the
field in bringing the charge from infinity to
that point.
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24-3 Electric Potential (V) is the potential
energy per unit charge. V U/q DV DU/q
-W/q V - W8/q
Note that the work you apply to a charge is the
negative of the work that the field applies on
the charge (when there is no change in kinetic
energy). See the work energy theorem!
DV Wapplied/q or Wapplied q DV
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The units of J/C is defined to be the volt (V).
For electric fields we can use N/C or V/m.
What is the electron-volt?
How much energy is an eV?
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24-4 Equipotential surfaces (EP) Adjacent points
that have the same electric potential form and
equipotential surface. W 0 in going from one
point to another on the same EP (regardless of
path).
What is EP for a spherical charge distribution?
What is EP for a uniform field?
E is always perpendicular to EP. (why?)
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24-5 Calculating the potential from the field
What happens to the potential when we move a
distance d along the electric field?
Solve check point 3
What is the direction of the field? In which
process to we (the agent) do most work?
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24-6 Potential due to a Point Charge
With the understanding that V8 0

• Notice that
• V is a scalar (much easier to handle than a
  vector!)
• V can be positive or negative!

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24-7 Potential due to a Group of Point Charges
Solve check point 4
Rank the arrangements according to the net
electric potential produced at point P due by the
protons.
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24-8 Potential due to an Electric Dipole
What is r? What is p? What is q?
What is the induced dipole moment, and what
relation does it has with polarization?
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24-10 Calculating the field from the potential
Solve check point 6
What is the direction of the electric field in
the three situations? Which has the greatest
magnitude?
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24.11 Electric Potential Energy of a System of
Point Charges
The electric potential energy of a system of
fixed point charges is equal to the work that
must be done by an external agent to assemble the
system, bringing each charge from an infinite
distance.
For a single charge U 0
For two charges (q1q2) a distance r apart U k
q1 q2/ r (can you prove it?)
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For three charges (q1, q2 and q3) U k q1
q2/r12 k q1 q3/r13 k q2 q3/r23
What about for four charges?
Solve sample problem 25-36

1. What is the electric potential energy of two
   electrons separated by 2 nm.
2. If the separation increases, does the potential
   energy increase or decrease?

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24.12 Potential of a Charged Isolated Conductor
The electric potential of a charged conductor is
constant. Can you explain why?!
What happens if a conductor is placed in an
electric field?
1- DV 0 in and on the conductor 2- The free
charges in the conductor will redistribute itself
in order to create an (internal) electric field
that cancels the external (applied) field. Also,
the field at the surface is perpendicular to the
surface.
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How can you shield an instrument (or a person!!)
from an electric field?
Place it/ him inside a conductor.
So, if you see lighting, jump into the car!!
Physics can save your life (Allah Willing) ?