Due Date: 3/15/09, Monday


  • electric charge:
    • a fundamental property of many of the basic constituents of matter.
    • 2 kinds
      • positive (+)
      • negative (-)
      • a neutron has no charge
    • charge of an electron/proton
      • e = -p
      • [e] = [p] = 1.6E-19 C (Coulomb)
      • mass of proton: 1.673 x 10^ -27 kg
      • mass of electron: 9.11 x 10^-31 kg
    • charge is quantized
    • for an object to be charged, it must have gained or lost electrons
    • charge on an object
      • [Q] = N [e]
      • Q = charge on an object in C
      • N = # of electrons that have been transferred
      • e = charge of 1 electron (1.6E-19 C)
    • a charge can be on an object or move through it
  • Conservation of Charge:
    • charge cannot be created/destroyed but transferred
    • a charged object (+ or -) will attract any other object without the same "sign" of charge
    • a charged object will repel any object with the same charge as itself
  • Polarization:
    • the process of separating charges within an object
    • object will still have the same net charge, the charge is just redistributed
    • external image u8l1e1.gif
  • Electrical Conductors:
    • a material that transfers charge (-) around an object more freely (to and from an object)
    • good thermal conductors are generally good electrical conductors
    • examples of good conductors are aluminum, silver, copper and gold
    • external image u8l1d5.gif
  • Insulators:
    • materials that conduct electric charge poorly
    • electrons are more bound within a material and therefore cannot move freely throughout the material
  • electrons merely redistribute themselves around the atoms/molecules nearest the surface of the insulator
    • think about atoms as having a re-distributable deal of negative charge surrounding a small, positive nucleus
    • types of insulators include rubber, many plastics, and wood
  • 3 main methods to transfer/induce a net charge
    1. Friction: the frictional rubbing of 2 materials (insulators)
      external image u8l2a3.gif
    2. Contact: bring a charged object into contact with an uncharged one
    3. Induction: can only charge conductors by induction... 3 steps...
      1. Polarization of conductor
      2. Grounding of the conductor (grounding: process where you allow charge to transfer to a larger area)
      3. Remove the ground, then the charged object (the net charge of the conductor will reside on the surface of that conductor)
        external image u8l2b1.gif
  • Electric Force: k[q1][q2]/r^2
    • k = 9E9 N-m^2/C^2
    • [q] = magnitude of the charge involved (C)
    • r = distance between the centers of the 2 charges (m)
    • *do not plug in the signs of the charges!

  • Electric Field:
    • in words: the ratio of the force experienced by a charge in a region of space, to the magnitude of the charge experiencing the force
    • equation: E = k[q]/r^2
    • a way to picture the effects that electric charges have on the space around them and on one another
    • a charge creates an electric field in the space around it
    • a charge put down a # in an electric field will experience a force
    • an object with mass will experience a force along the gravitational field lines (see picture)

    • there is no electric field inside a car (must be made of metal) because it acts as a faraday cage (as seen in picture)

  • Electric Field Lines
    • Electric field lines are defined by how a (+) charge would experience a force.
    • Positive Charge:
      • the further you go, the weaker the e-field
    • Negative Charge:
      • the closer you get, the stronger the e-field

electric field lines between a positive and negative charge:

  • Rules governing E-Field lines:
    1. Begin on (+) charges, terminate on (-) charges.
    2. Field lines never cross.
    3. E-Field lines always point perpendicular to a conducting surface.
    4. There is no electric field inside a conductor (when the charge is static).
    5. The number of E-Field lines is proportional to the strength of the field
    6. More charge will accumulate on areas of a surface where the radius of curvature is less.

Change in Potential Energy= -Work Done

- when the potential energy is increasing, negative work id being done by that conservative force
-Often we talk about the energy required to assemble charges from infinity
- How much potential energy is required to assemble two charhes into this orientation from infinity?
1. bring charge one from infinity will require no effort because charge is not moving in a field
2. To bring charge two in from infinity, the charge is moving in charge ones field so work is done and thus there will be a change in Potential energy. The Potential energy should be positive if I bring a charge towards where it DOESNT want to be.

Here are some good sites for electric potential energy practice problems:

- Sometimes it is useful to talk about Potential energy per unit charge because it doesnt depend upon a charge at a point.
- The potential energy depends on charge that is in the electric field.. but if you want know energy at a point in space regardless of the charge-- use electric potential
- So electric potential energy is the amount of energy to bring charges together; electric potential is energy per unit charge!
- Electric potential energy is measure in Joules and has the equation
U_{mathrm{E}} = ; W ;.
U_{mathrm{E}} = ; W ;.

- Electric Potential is measure in Volts and has the equation
 Phi(r_1) = ; k_{mathrm{e}} q_1/r_1
Phi(r_1) = ; k_{mathrm{e}} q_1/r_1

Here are some good sites for electric potential practice problems:,7966,803858-,00.html

- Equipotential Surfaces/lines- along a equipotential surface, the electric potential V will be constant
If you were to move a charge along an equipotential surface, the electric potential energy would not change
If you were to move along electric field line, the electric potential energy will change
Equipotential surface must be perpendicular to electric field lines, as seen below.


Here are some videos with helpful lab presentations and tutorials:

And here are some bad ideas: