Electric Fields and Forces

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Electric Fields and Forces

• AP Physics B

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Electric Force

• The electric force between 2 charged objects is

The electric force on 1 charged object in an
external electric field is F q E
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Superposition Principle
The force of charge Q3 due to the other two
charges can be obtained using Coulombs Law for
each pair of charges separately and then adding
the forces from Q1 and Q2.
-


Q1
Q2
Q3
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Electric Fields

• Always in the direction that a POSITIVE test
  charge would move

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The Rules

• Some important facts
• Begin on positive charges (or infinity)
• End at negative charges (or infinity)
• Must not cross each other
• Density of lines proportional to strength of field

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An Electric Point Charge

• The E field due to 1 charge Q is given by

 
r
Q
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Electric Field of a Conductor

• Field inside a conductor is zero

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Voltage and Potential energy
The amount of energy per charge is voltage
(potential difference)
In constant Electric field
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Electric potential of point charges
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Parallel Plates
-Voltage Provided by battery -Electric field
constant between plates
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Capacitors in Series

• Capacitors in series each charge each other by
  INDUCTION. So they each have the SAME charge. The
  electric potential on the other hand is divided
  up amongst them. In other words, the sum of the
  individual voltages will equal the total voltage
  of the battery or power source.

 
 
 
 
 
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Capacitors in Parallel

• In a parallel configuration, the voltage is the
  same because ALL THREE capacitors touch BOTH ends
  of the battery. As a result, they split up the
  charge amongst them.

 
 
 
 
 
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P VI P I2R P V2/R
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Resistance of a Wire

• Wires have low resistance compared to resistors
  (ideally)

-long wires have more resistance -larger diameter
lowers resistance -resistivity is a property of
the material (depends on temperature) -most
materials increase resistance as temperature
increases -conductors have low resistivity
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Internal Resistance
e IRtotal
Vterminal IRcircuit
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SERIES RULES
 
 
 
 
 
 
 
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PARALLEL RULES