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09 May 2012

Just the facts: Electrostatics

By popular request:  Just the facts for Electrostatics, as needed for the AP exam.

Bible Equations and concepts, which are ALWAYS valid no matter what produces an electric field:


Positive charges experience a force in the direction of the electric field
Negative charges experience a force opposite the electric field

Positive charges are forced from high to low voltage
Negative charges are forced from low to high voltage

Once an electric force is calculated, put that force on a free body diagram and use newton's second law.
Once a potential energy is calculated, use it in the full expression of the work-energy theorem.

Equation used to find the UNIFORM electric field produced by parallel plates (this should only be used when the electric field is uniform, or close to uniform):

E = ΔV/Δx*

* This equation is written simply as E = V/d on the AP equation sheet.  My friend Wayne Mullins has convinced me to write it with the deltas, emphasizing that an electric field requires a potential difference to exist.  I switched notation this year, and I'm pleased with the results.

Equations used to find the electric field or potential produced by a charge (these are ONLY VALID WHEN A CHARGE PRODUCES THE ELECTRIC FIELD OR POTENTIAL!!!!!!!):

E = kQ/d2, pointing toward a negative charge and away from a positive charge. E is a vector, so don’t plug in negative signs to this equation.   Draw vectors to determine the resultant electric field due charges.

V = kQ/d; negative charges produce negative potentials, positive charges produce positive potentials.  V is a scalar, so do plug in negative signs.  Just add the potentials due to several charges to determine the net potential.  

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