10 Electric Field, Force, and Potential

Slideshow: Electric Field, Force and Potential Slideshow

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. 

  • Test your skills using static charges to produce desired forces with this game called Polarity Shift at UniverseAndMore.

• 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: 

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

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.

Viral connections