Plan for Today (AP Physics 1)
• Lecture/Notes on Electric Potential
Introduction to Potential Difference
• Break into small groups and answer the
questions on potential difference
Intro to Potential Difference – Big
Ideas
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Force is constant – F = Eq
Work function of F * x
Change in potential energy is positive
PE = F * d
Change in PE = -qEd
– Negative because q negative, d positve (in
direction of E)
Intro to Potential Difference – Big
Ideas
• So electron going with field
• Increases PE – need to push in that direction
• Proton going with E is negative (wants to go in
that direction)
– Like gravity
Intro to Potential Difference - Cont
• Force decreases as you go out (1/R2)
• Work reduces as you go out since force
decreases
– Non-linear
• PE = k q1q2/r
– Like gravity
• Difference between uniform and point charge
electric fields
New Term
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Potential difference
Energy stored in an electron
V = PE/q
Units: J/C = Volts
Why do we use this term?
• Potential difference – potential to cause
change
• If I hold a bowling ball over someone, if I drop
it, it will cause some problems
• If I hold a tennis ball, if I drop it, it’s not a big
deal
• But if I drop 1 million tennis balls, student not
so good
Vandergraph and Wall socket
• Same analogy
• Vandergraph is 1500 V – hurts
• Wall socket 110 V – could kill
• What’s the difference?
– The number of electrons giving energy
Example
• 3 V battery sends one Coulomb of charge to
fan in one second
• How much energy is given to the fan?
• V = PE/q
• PE = v * q = 3 * 1 = 3 J
• If we use a 6V battery with 1 C of charge, what
is the change in potential energy?
• PE = 6 J in 1 second
• What happens to the fan (with 6 V battery)?
• Would spin faster
• Is this your experience?
Charges in a Uniform Electric Field
• First, how can I get a uniform electric field?
– Use two parallel plates
– Attach to battery
– Uniform within the plates
– Starts to bend around the edge
Example
• If point A has E = 300 N/C, what is it at B, C,
and D?
• Answer: same because the electric field is
uniform
Example
• Work done = -
Work required to move electron from
left plate to right plate?
• Work = F * x = (Eq) *d
• Work = -PE = qEd (ends up positive)
– q is negative – electron
– E is negative – against electric field
– d is negative – against electric field
• For a proton
– W = -qEd
• Charge positive
• Same direction so work is negative
• To think about this, compare to gravity
– If charge is going the way it wants to, like a ball
dropping to earth – Work negative
Example
• An electron is placed between 2 parallel plates
3 cm apart and attached to a 10 V battery.
• If the electron starts at the negative plate,
what is the velocity at the positive plate?
Example
• An electron is placed between 2 parallel plates
3 cm apart and attached to a 10 V battery.
• If the electron starts at the negative plate,
what is the velocity at the positive plate?