Force Fields

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Force Fields

• Gravitational fields-act on masses-Strength
  Mass / d2-attractive-cannot be shielded-can
  store energy-overall very weak force

• Electrical fields-acts on charges-Strength
  Charge 1 / d2-attractive or repulsive-can be
  shielded-can store energy-generally stronger
  than gravity

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Electric Field

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Definition

• Electric field is the
• strength direction of the forces in space
  surrounding a positive test charge

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Top tips on electric field Field lines go from
to chargesCloser field lines are
strongerfield lines cant cross another!
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Question 1

• Suppose an electric field points south
• Which way would a proton movea. south
• B. north
• C. west
• D. east.

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Question 1 answer

• Suppose an electric field points south
• Which way would a proton moveA. south
• Since electric field shows direction of a
  positive charge from high to low energy

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Question 2

• Suppose an electric field points south
• Which way would an electron movea. south
• B. north
• C. west
• D. east.

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Question 2 answer

• Suppose an electric field points south
• Which way would an electron moveB. north
• Since electron is a negative charge it moves
  opposite direction of field

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Electric field as coulombs law

• F q1 (k q2 /d2)
• rewrite the force equation
• F q1 E
• q1 is the positive test charge
• E is the field created by charge(s) q2

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Question 3

• The electric field in a certain region of space
  is 40 N/C. A. What is the force on a -10 C
  charge placed in the region?B. Is it attractive
  or repulsive?

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Question 3 Answer

• FqE
• F (40 N/C)(-10 C)
• F - 400 N(attractive force since negative
  answer, - - )

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Question 4

• Imagine a metal ball placed in an electric field
• IF an electric field moves charges and conductors
  are full of electrons which can move, where will
  all the electrons go?a. stay on left side
• B. spread out on outer surface
• C. stay in middle
• D. dont move

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Question 4 answer

• Imagine a metal ball placed in an electric field
• IF an electric field moves charges and conductors
  are full of electrons which can move, where will
  all the electrons go?B. spread out on outer
  surface since the charges move away from another
  to lower their energy
• the charges on opposite sides cancel out forces
  so E0 inside

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Electric Shielding- Faraday Cage

• Inside a Conductor, the Electric Field MUST be
  ZERO!

• Any net charge on the inside of a conductor will
  be pushed to the surface distributed to
  equilibrium!

• Electric field E is perpendicular to the outer
  surface,and zero inside any conductor (metal)

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Faraday cage

• Dont try this at home!

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Electric field demos
Pith balls inside the metal screen dont move
since there is no electric field inside a
faraday cage
Metal cage blocks out the electric field from the
radio station or cell phone tower-so no music!
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Common Faraday Cages

• Purpose
• Keep in or out electro-magnetic fields used in
  electronics/telecommunications
• Examples
• Metal foil around wires inside phones(so less
  pick-up of unwanted noise/calls)
• Rebar in concrete or bridges- bad reception
• Metal boxes around electronic trigger switches

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coulomb
Charles Coulomb1750

• How many charges are in 1C? coulomb?
• 1c is about 1 million charges!
• The man, the law

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Joules of energy

• Charges have a type of energy called
• electron potential energy
• High energy is close to
• or is far from
  -

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Question 5

• Imagine moving a positive charge in an electric
  field
• If you move the charge to the left against the
  electric field, what happens to the energy of the
  test charge?a. no change
• B. increases
• C. decreases
• D. not enough info

E
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Question 5

• Imagine moving a positive charge in an electric
  field
• If you move the charge to the left against the
  electric field, what happens to the energy of the
  test charge?B. increases
• since field points from high to low energy

E
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Question 6

• Imagine moving a positive charge against an
  electric field
• If it takes 2000 Joules of work to move the
  charge..?a. how much energy does the charge
  gain?
• B. how much work does the field do to move the
  charge back to the right?

E
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Question 6

• Imagine moving a positive charge against an
  electric field
• If it takes 2000 Joules of work to move the
  charge..?a. 2000 J----positive work needed to
  add energy?
• B. -2000 J--- negative work done by the field,
  (field loses energy while charge gains)

E
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Electric Potential (Voltage)

• ?V ?PE/Q
• voltage energy/Charge
• Volts joules/coulombs

• Analogiesvoltage is like water pressureenergy
  heightwater charge

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Question 7coulombs, joules, voltmatch these!

• Coulomb unit of energy/charge
• Joule unit of charge
• Volt unit of energy

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Question 7coulombs, joules, voltmatch these!

• Coulomb unit of charge
• Joule unit of energy
• Volt unit of energy per charge

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Volts is an energy density

• Voltage is also called potential
• 1 volt 1 joule / 1 coulomb
• Example 12 V battery every coulomb of
  charge has 12 joules of energy

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Example of high voltage, low charge

• Whats the voltage on a balloon if it has
  1 joule of energy (medium) .001 coulombs
  of charge (low)
• voltage energy/charge v 1
  J / .001C 1000 Volts !

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Question 8

• Whats the voltage on a Van de Graaf static
  generator
• 1 joule of energy (medium)
  .00001 coulombs of charge (v.low)
• v ?

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Question 8

• Whats the voltage on a Van de Graaf static
  generator
• 1 joule of energy (medium)
  .00001 coulombs of charge (v.low)
• v 1 j/ .00001c
• 100,000 Volts ouch!