A difference in electrical potential between the upper atmosphere and the ground can cause electrical discharge (motion of charge).

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Electric Potential
A difference in electrical potential between the
upper atmosphere and the ground can cause
electrical discharge (motion of charge).
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Ch 25 Electric Potential
So far, weve discussed electric force and
fields. Now, we associate a potential energy
function with electric force. This is identical
to what we did with gravity last semester.
gravity
electricity
?
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Ch 25.1 Electric Potential and Potential
Difference

• Place a test charge, q0, into an E-field. The
  charge will experience a force

• This force is a conservative force.
• Pretend an external agent does work to move the
  charge through the E-field.
• The work done by the external agent equals at
  least the negative of the work done by the
  E-field.

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Do Now (9/19/13)

• What does the word voltage mean to you?
• Where have you seen the word before?
• What is the formula for work? (think back to last
  year!)

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Ch 25.1 Electric Potential and Potential
Difference
Potential difference due to work being done on a
particle

• This physical quantity only depends on the
  electric field.
• Potential Difference the change in potential
  energy per unit charge between two points in an
  electric field.
• Units Volts, V J/C
• A change in electric potential energy can only
  occur if a test charge actually moves through the
  E-field.

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Ch 25.1 Electric Potential and Potential
Difference

• Units of the potential difference are Volts
• 1 J of work must be done to move 1 C of charge
  through a potential difference of 1 V.

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Ch 25.1 Electric Potential and Potential
Difference

• We now redefine the units of the electric field
  in terms of volts.

E-field units in terms of volts per meter
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Voltage

• What is the formula for work?
• Simplify

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Example

• What is the voltage on a proton at rest in an
  E-field of 20 N/C?

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Practice

• Complete AT LEAST one problem from the Potential
  Difference paper
• When you finish, raise your hand to get it
  stamped
• Once you have your stamp you may work on that
  paper, your quiz review, or your notecard

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Ch 25.1 Electric Potential and Potential
Difference

• Another useful unit (in atomic physics) is the
  electron-volt.
• One electron-volt is the energy required to move
  one electron worth of charge through a potential
  difference of 1 volt.
• If a 1 volt potential difference accelerates an
  electron, the electron acquires 1 electron-volt
  worth of kinetic energy.

The electron-volt
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Quick Quiz 25.1

• Points A and B are located in a region where
  there is an electric field.
• How would you describe the potential difference
  between A and B? Is it negative, positive or
  zero?
• Pretend you move a negative charge from A to B.
  How does the potential energy of the system
  change? Is it negative, positive or zero?

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Lets calculate the potential difference between
  A and B separated by a distance d.
• Assume the E-field is uniform, and the path, s,
  between A and B is parallel to the field.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Lets calculate the potential difference between
  A and B separated by a distance d.
• Assume the E-field is uniform, and the
  displacement, s, between A and B is parallel to
  the field.

1
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Ch 25.2 Potential Difference in a Uniform
E-Field

• The negative sign tells you the potential at B is
  lower than the potential at A.
• VB lt VA
• Electric field lines always point in the
  direction of decreasing electric potential.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Now, pretend a charge q0 moves from A to B.
• The change in the charge-field PE is
• If q0 is a positive charge, then ?U is negative.
• When a positive charge moves down field, the
  charge-field system loses potential energy.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Electric fields accelerate charges thats what
  they do.
• What were saying here is that as the E-field
  accelerates a positive charge, the charge-field
  system picks up kinetic energy.
• At the same time, the charge-field system loses
  an equal amount of potential energy.
• Why? Because in an isolated system without
  friction, mechanical energy must always be
  conserved.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• If q0 is negative then ?U is positive as it moves
  from A to B.
• When a negative charge moves down field, the
  charge-field system gains potential energy.
• If a negative charge is released from rest in an
  electric field, it will accelerate against the
  field.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Consider a more general case.
• Assume the E-field is uniform, but the path, s,
  between A and B is not parallel to the field.

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Ch 25.2 Potential Difference in a Uniform
E-Field

• Consider a more general case.
• Assume the E-field is uniform, but the path, s,
  between A and B is not parallel to the field.

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Ch 25.2 Potential Difference in a Uniform
E-Field
If s is perpendicular to E (path C-B), the
electric potential does not change. Any surface
oriented perpendicular to the electric field is
thus called a surface of equipotential, or an
equipotential surface.
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Quick Quiz 25.2
The labeled points are on a series of
equipotential surfaces associated with an
electric field. Rank (from greatest to least)
the work done by the electric field on a positive
charge that moves from A to B, from B to C, from
C to D, and from D to E.
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EG 25.1 E-field between to plates of charge
A battery has a specified potential difference ?V
between its terminals and establishes that
potential difference between conductors attached
to the terminals. This is what batteries do. A
12-V battery is connected between two plates as
shown. The separation distance is d 0.30 cm,
and we assume the E-field between the plates is
uniform. Find the magnitude of the E-field
between the plates.

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EG 25.1 Proton in a Uniform E-field
A proton is released from rest at A in a uniform
E-field of magnitude 8.0 x 104 V/m. The proton
displaces through 0.50 m to point B, in the same
direction as the E-field. Find the speed of the
proton after completing the 0.50 m displacement.