Electric Fields and Potentials

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Electric Fields and Potentials
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Electric Force

• Electricity exerts a force similarly to gravity.
• Fe kq1q2
• r2
• where q1 and q2 represent the amount of charge in
  Coulombs (6.24 x 1018), r is in meters and k is
  the electrical constant (9 x 109 Nm2 /C2)
• 1 Coulomb of electrons travels through a 100-W
  lightbulb in about one second

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Electric Fields

• Just like gravity field, charges have a force
  field (E) as well, measured in force per unit
  charge
• E F kQ
• q r2
• where Q is a positive test charge
• Direction of fields away from a positive
  charge, toward a negative charge

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Force Field Lines

• Fields have strength and direction
• Field is determined by the force and direction of
  motion of a positive test charge
• Field is strongest where the force is the
  strongest where the lines are the most
  concentrated

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Electric Shielding

• Electrons repel toward the outside of any
  conducting surface
• Net charge inside is zero
• Electrons flow outward evenly, but pile up on
  sharp corners

Shielding is important in electronic devices such
as televisions and computers
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Faraday Cage

• The Faraday cage is an electrical apparatus
  designed to prevent the passage of
  electromagnetic waves, either containing them in
  or excluding them from its interior space
• It is named for physicist Michael Faraday, who
  built the first one in 1836

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Faraday Cage

• Faraday stated that the charge on a charged
  conductor resided only on its exterior
• To demonstrate this fact he built a room coated
  with metal foil, and allowed high-voltage
  discharges from an electrostatic
  generator to strike the outside
  of the room
• He used an electroscope to
  show that there was no excess
  electric charge on the inside of the
  room's walls.

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Faraday Cage

• A more impressive demonstration of the Faraday
  cage effect is that of an aircraft being struck
  by lightning
• This happens frequently, but does not harm the
  plane or passengers
• The metal body of the aircraft protects the
  interior.
• For the same reason, and if it were not for the
  highly flammable nature of petrol, a car would be
  a very safe place to be in a thunderstorm

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Person in a car hit by artificial lightning. The
lightning strikes the car and jumps to the ground
bypassing the front tire arcing from the axle to
the ground.
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Electrical Potential

• Just like gravitythe potential (possibility) of
  falling to earth, charges have the potential to
  move toward or away from each other

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Electrical Potential

• Force of attraction/repulsion causes the
  potential
• Potential is energy divided by chargesince
  charge is usually small, potential can be
  relatively large5000 volts on a charged balloon
• A larger amount of charge makes larger potential

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Voltage Electrical Potential

• Voltage PE/Q
• PE in Joules and Q in Coulombs
• 100 Volts
• 0.000001-J/0.00000001-C
• 100-J/ 1-C
• 1,000,000-J/10,000-C

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Storing Charges

• Capacitors can store charges on plates which are
  separated as in Franklins Leyden jars

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Storing Charges

• A capacitor is a device that stores electric
  charge
• A capacitor consists of two conductors separated
  by an insulator

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Capacitors and Capacitance
A capacitor in a simple electric circuit.
Charge Q stored
The stored charge Q is proportional to the
potential difference V between the plates. The
capacitance C is the constant of proportionality,
measured in Farads. Farad Coulomb / Volt
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Parallel-Plate Capacitor

• A simple parallel-plate capacitor consists of two
  conducting plates of area A separated by a
  distance d.
• Charge Q is placed on one plate and Q on the
  other plate.
• An electric field E is created between the plates.

Q
-Q
Q
-Q
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Capacitor Applications

• .

• Computer RAM memory and keyboards.
• Electronic flashes for cameras.
• Electric power surge protectors.
• Radios and electronic circuits.
• Power supplies

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Van de Graaf Generator

• This machine is capable of producing very high
  electrostatic potential differences in the order
  of millions of volts
• It works by friction of the belt with the rollers
  and separates charges at combs which take the
  charges to the dome and picks them up from the
  ground at the base

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Van de Graff Generator
http//demoroom.physics.ncsu.edu/movies.html
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Van de Graff Generator
http//demoroom.physics.ncsu.edu/movies.html
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Van de Graff Generator
http//demoroom.physics.ncsu.edu/movies.html