# 3 Electrostatics 2

## Slideshow: Electrostatics 2, Capacitors 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.
• 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