AP Physics C: Electricity & Magnetism Unit 7 - Electrostatics

Electric Charge

Charge is a fundamental property of matter, measured in Coulombs (C).
Types of charge: Positive (protons) and Negative (electrons). They are equal in magnitude: $e = 1.60 imes 10^{-19} C$ .
Charge transfer occurs by electron movement: Positive charge indicates an electron deficit.

Atomic Structure

Nucleus contains protons (positive) and neutrons (neutral). Electrons are found in orbitals around the nucleus.
Protons and neutrons are held together by the strong force.

Conductors vs. Insulators

Conductors: Materials (e.g., metals) with free-moving electrons facilitating charge transfer.
Insulators: Materials (e.g., rubber, glass) where electrons are tightly bound, preventing charge flow.

Electroscope

Device to detect charge; diverging leaves indicate presence of charge.

Charging Methods

Conduction: Charging by direct contact (e.g., charged rod touching electroscope).
Induction: Charging without contact by bringing a charged object near and grounding the electroscope.

Coulomb's Law

Electrostatic forces between charges: $extbf{F} = k rac{|Q_1 Q_2|}{r^2}$ , where $k = 8.99 imes 10^9 N m^2/C^2$ .
The direction of the force is determined by the signs of the charges.

Electric Fields

Electric fields (E) created by charges: $E = \frac{F}{q}$ and $E = \frac{kQ}{r^2}$ .
Field lines indicate the direction of force on a positive test charge, originating from positive charges and terminating at negative charges.

Gauss’s Law

Net electric flux through a closed surface: $Φ = \frac{Q_{enc}}{ε_0}$ .
Useful in calculating electric fields for symmetrical charge distributions.

Applications of Gauss’s Law

Infinite Sheets: Electric field due to uniform charge density is $E = \frac{σ}{2ε_0}$ .
Parallel Plates: The electric field inside the plates is uniform, outside is zero.
Long Cylinders: Electric field from an infinite line charge characterized as $E = \frac{λ}{2πrε_0}$ .