# 3 Electrostatics 2

Textbook: Chapters 14.5 through 15

## Objectives:

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:

Equations:

### Common Misconceptions

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

Strategies, notes and limitations

- Which charge is it talking about?
- In E = F/q and ΔU
_{E}= qΔV, q is the charge in the field (aka test charge or charge experiencing the force). - In E = kq/r
^{2}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)

- In E = F/q and ΔU
- 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 F
_{E}especially when you have multiple charges.

- For V you WILL USE the sign of the charge, even with multiple charges.

- Directions can be confusing and the signs on the charges often do not work consistently with the vector directions.
- To Be Continued