3 Electrostatics 2

Textbook: Chapters 14.5 through 15

Objectives:

  • Charge is a property of matter
    • Demonstrate that charge is conserved in all interactions (this involves defining systems as open or closed)
    • Predict the charge on objects based on conservation of charge.
    • Explain that all objects are made of charged particles & neutral objects have equal amount of each charge
    • Explain how a neutral object can appear to have a charge if the charges are not distributed evenly (polarization)
    • Explain that the smallest possible isolated charge is the fundamental charge (charge of an electron or proton).
  • Charges create electric fields (areas in space affected by their charge)
    • Demonstrate that electric field strength decreases with distance from the source following an inverse square relationship
    • compare and contrast gravitational fields and electric fields
    • Describe and/or solve proportionally for the electric charge resulting from a net electric charge CREATING the field. (can just be vectors)
    • Calculate electric field in a location based on up to 4 point charges. Use vectors and math to solve/represent the answer.
  • Charged plates create uniform electric fields.
    • Draw the uniform electric field between charged parallel plates (including the non-uniform field near the edges)
    • Calculate the magnitude and direction of electric field between 2 charged parallel plates based on q or V.
    • Compare the motion of a charged particle between plates to projectile motion in earth's gravitational field.
  • Electric potential
    • Use potential difference between any 2 points to determine the electric field between the points.
    • Draw isolines (of equal potential) from both electric field and gravitational fields. Explain the behavior of objects/charges along those lines.
    • Predict electric field based on isolines.
  • Charges exert forces on each other (similar to how 2 objects with mass exert forces on each other)
    • Use Free Body Diagrams to depict electrostatic force; solve net force equations.
    • Refute a claim that an object can exert a force on itself (identify 2 objects and the force pair for all forces)
    • Use Coulomb's Law to relate charge amounts to forces exerted on each other.
    • Use Coulomb's Law with 2-4 charges (or more if highly symmetrical)
    • Compare the strength of gravitational force to electrostatic force.
    • Use F=qE to predict the direction and magnitude of the force a charge EXPERIENCES. Also solve for other variables.
  • Charging Objects
    • Discuss charge manipulation when charging by friction, conduction & induction; include discussion of electric fields and develop graphical representations of charge involving conductors and insulators.
    • Plan/carry out a lab investigating charging by induction

Equations:

Common Misconceptions

Electric Fields and Forces

  • A moving charge will always follow a field line as it accelerates.
  • If a charge is not on a field line, it feels no force.
  • Field lines are real.
  • Coulomb's law applies to charge systems consisting of something other than point charges.
  • A charged body has only one type of charge.
  • The electric field and force are the same thing and in the same direction.
  • Field lines can begin/end anywhere.
  • There are a finite number of field lines.
  • Fields don't exist unless there is something to detect them.
  • Forces at a point exist without a charge there.
  • Field lines are paths of a charges motion.
  • The electric force is the same as the gravitational force.
  • Field lines actually radiate from positive to negative charges and convey motion.
  • Field lines exist only in two dimensions.

Elementary Charge

  • Charge is continuous and can occur any amount.
  • An electron is pure negative charge with no mass.
  • Oil drops are electrons.
  • The scientific method is pure and absolute.
  • Scientists always stumble on discoveries.
  • Millikan measured the mass of the electron.

Equipotentials and Fields

  • Voltage flows through a circuit.
  • There is no connection between voltage and electric field.
  • Voltage is energy.
  • Equipotential means equal field or uniform field.
  • High voltage by itself is dangerous.
  • It takes work to move a real charge on an equipotential.
  • Charges move by themselves.
  • Sparks occur when an electric field pulls charges apart.

Capacitors

  • A capacitor and a battery operate on the same principle.
  • A potential difference is only on plates of a capacitor and not in region between.
  • Charge flows through a dielectric, such as glass.
  • Designations of (+) and (-) are absolute.
  • Q = CV is a basic conceptual law.
  • No work is required to charge a capacitor.
  • A capacitor requires two separate pieces.
  • There is a net charge on a capacitor.
  • The capacitance of a capacitor depends on the amount of charge.
  • A positive charged capacitor plate only has positive charges on it.
  • Charges flow through a capacitor.

Strategies, notes and limitations

  • Which charge is it talking about?
    • In E = F/q and ΔUE = qΔV, q is the charge in the field (aka test charge or charge experiencing the force).
    • In E = kq/r2 and V = kq/r, q is the charge causing the field (source charge)
    • Q is always the charge on a single parallel plate, which generally come in matched pairs (although a problem may use a capital q for a literal magnitude of a point charge)
  • Why do some equations have absolute values and others don't?
    • Directions can be confusing and the signs on the charges often do not work consistently with the vector directions.
      • So, strategy: when you see absolute values, drop all signs for the math and figure out the vector directions logically.
      • Use this for E field and FE especially when you have multiple charges.
    • For V you WILL USE the sign of the charge, even with multiple charges.
  • To Be Continued