Electric Forces and Fields

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Electric Forces and Fields

• Chapter 20

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Charges and ForcesExperiment 1

• Nothing happens
• The objects are neutral

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Charges and ForcesExperiment 2

• The two rods repel each other
• A rubbed rod is charged
• Long range repulsive force

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Charges and ForcesExperiment 3

• These rods attract each other
• Positive and negative charges

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Charges and ForcesExperiment 4

• When two rods are rubbed more vigorously the
  strength of the forces is greater
• The strength of the
• charges decreases
• as the distance
• between the rods
• increases

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Charges and ForcesExperiment 5

• Rub a plastic rod with wool
• The rod is weakly attracted to the wool
• The rod is repelled by a piece of silk that has
  been used to
• rub glass

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Charges and ForcesExperiment 6

• Charged plastic rod held over paper
• The neutral paper is attracted to the rod
• A charged glass rod attracts the paper
• A neutral rod has no effect on the paper

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Charge Separation

• A charged object can induce a charge separation
  on an uncharged object that can lead to an
  attractive force. http//phet.colorado.edu/simulat
  ions/sims.php?simBalloons_and_Static_Electricity

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Insulators and ConductorsHow does charge move on
different materials?

• Charge a plastic rod by rubbing it with wool
• Touch a neutral metal sphere
• The sphere acquires the charge of the rod

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Insulators and ConductorsHow does charge move on
different materials?

• The metal sphere that is touched by the charged
  plastic rod will pick up small pieces of paper
  (it is charged)
• The other sphere will not (it remains neutral)

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Insulators and ConductorsHow does charge move on
different materials?

• Touch one sphere with a charged plastic rod
• Both spheres will attract small bits of paper
  (they are charged)

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Transfer of Charge

• Charge can be transferred from one object to
  another when the objects touch.
• Removing charge from an object is called
  discharging
• Charge is conserved (it cant be created or
  destroyed

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Transfer of charge

• Conductors material through or along which
  charge easily moves
• Metal
• Insulators materials on or in which charges
  remain immobile
• Glass and plastic
• Both insulators and conductors can be charged.
  They differ in the mobility of the charge

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Charge diagram

• An insulating rod is charged by rubbing
• Charges on the insulator rod dont move

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Charge diagram

• Charges in a conductor are free to move

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Electrostatic Equilibrium

• The charges on an isolated conductor are in
  static equilibrium other than the brief interval
  when the charges are adjusting
• The charges are at rest
• No net force on any charge

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ElectroscopeCharge polarization

• The charges move around but
• Charge is conserved
• Slight separation of
• the positive and
• negative charge in a
• neutral object

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Polarization Force

• The polarization force arises because the charges
  in the metal are separated NOT because the rod
  and metal are oppositely charges

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Fundamental Charge

• Charge is represented by the symbol q
• The SI unit of charge is the coulomb (C)
• Protons and electrons have the same amount of
  charge but opposite signs
• The fundamental or elementary charge (e) is the
  magnitude of the charge of a proton or electron
• e 1.60 X 10-19 C

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Fundamental Charge

• Charge is conserved
• The total amount of charge remains constant

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Coulombs Law

• Electric force increases for objects with more
  charge and decreases as charged objects are moved
  farther apart.

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Coulombs Law Equation

• K is the electrostatic constant
• K 8.99 x109 N m2/C2
• C are units of electric charge called coulombs
• r is the distance the charges are apart

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Direction of forces

• The forces are directed along the line joining
  the two particles
• The forces are repulsive for two like charges
• The forces are attractive for two opposite
  charges
• These forces are an action/reaction pair equal in
  magnitude but opposite in direction.

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Forces are Vectors

• Electric forces are represented by vectors
• Like other forces they can be superimposed
• If multiple charges are acting on charge j, the
  net electric force on charge j is the sum of all
  the individual forces due to each charge.
• Fnet F1onj F 2on j F3onj

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Adding Electric Forces in 1 dimension

• Example 20.1
• A. Two 10nCcharged particles are 2.0 cm apart on
  the x-axis. What is the net force on a
• 1.0 nC charge midway between them?
• Fnet F1on3 F2on3
• Fnet 0
• q1 and q2 exert repulsive forces on q3. Equal in
  magnitude, opposite in direction

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Adding Electric Forces in 1 dimension

• 20.1 b. What is the net force if the charged
  particle on the right is replaced by a -10 nC
  charge?
• Forces are equal in magnitude and direction
• Fnet 2F1on3

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Adding Electric Forces in 1 dimension

• 20.1 B. The magnitude is given by Coulombs Law
• F1on3 9.0 x 10-4 N
• Fnet 2(F1on3)
• 1.8 X 10-3 N to the right

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Adding Electric forces in 2 dimension

• Example 20.2
• Three charges particle with q1 -50nC, q2
  50nC, and q3 30nC are placed as shown. What
  is the net force on charge q3 due to the other 2
  charges

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Adding Electric forces in 2 dimension

• Define a coordinate system with q3 at the origin.
  
• Draw the forces on the charge q3 with direction
  determined by the signs of the charges.

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Adding Electric forces in 2 dimension
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Adding Electric forces in 2 dimension

• Determine the net force on charge q3 using
  Coulombs law
• 2.7 x 10-3 N

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Adding Electric forces in 2 dimension

• Determine the net force on charge q3 using
  Coulombs law
• Magnitude and distance are the same for F2on3
• 2.7 x 10-3 N

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Adding Electric forces in 2 dimension

• Compute values for the componentsusing trig.
• (F1on3)x -1.9x10-3 N
• (F1on3)y -1.9x10-3 N
• (F2on3)x -1.9x10-3 N
• (F2on3)y -1.9x10-3 N

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Adding Electric forces in 2 dimension

• Next add the components of the net force
• F3x F(1on3)x F(2on 3)x
• -1.9 X 10-3 N -1.9 x 10-3 N 3.8 x10-3N
• F3y F(1on3)y F(2on3)y
• 1.9 X 10-3 N -1.9 x 10-3 N 0
• Net force is F3
• 3.8 x10-3N, -x direction

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The Electric Field

• Grass seed in a pan of oil
• When charged spheres
• ( and -) touch the surface,
• the grass seeds line up
• in a regular pattern.
• The pattern suggests that some kind of electric
  influence fills the space around the charges.
• This alteration of space could be the mechanism
  by which the long range Coulombs Law force is
  exerted.

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The Field Model

• The alteration of space around charge is the
  agent that exerts a force on charge B
• This alteration of space is called a field
• The charges make an alteration everywhere in
  space
• Other charges then respond to those alteration at
  their position

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The Field Model
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The Field Model

• The field model applies to many branches of
  science
• Electric field-alteration of space around a
  charge
• Gravitational Field-alteration of space around a
  mass
• Magnetic Field-alteration of space around a magnet

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Field Model

• Source Charges alter the space around them by
  creating an electric field E.
• A separate charge in the electric field then
  experiences a force F exerted by the field

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Electric Field Diagram
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Electric Field Diagram
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Electric Field Diagram