AP Physics 2 Unit 2 Foundations: Charge, Conservation, and the Electric Force

Electric charge

A property of matter that causes electric interactions; objects with net charge exert and experience forces on other charged objects without contact.

Net charge

The overall charge of an object due to an imbalance between protons and electrons; determines whether the object is electrically positive, negative, or neutral.

Proton

A particle in the atomic nucleus with charge +e.

Electron

A particle with charge −e; typically the mobile charge carrier in solids during charging.

Elementary charge (e)

The magnitude of the charge of a proton or electron; e = 1.60 × 10^−19 C.

Coulomb (C)

The SI unit of electric charge; a large unit compared to the charge of individual particles.

Microcoulomb (µC)

A common lab-scale unit of charge equal to 10^−6 C.

Two types of charge (positive and negative)

The two kinds of electric charge; like charges repel and opposite charges attract.

Quantization of charge

The fact that net charge occurs in integer multiples of the elementary charge: q = n e (n is an integer).

Conservation of electric charge

In an isolated system, total charge remains constant: qtotal,initial = qtotal,final; charge can be transferred but not created in ordinary electrostatics.

Conductor

A material in which charges (usually electrons) move freely, allowing charge to redistribute easily (metals are typical examples).

Insulator

A material in which charges do not move freely; charge tends to remain localized where it was placed (e.g., plastic).

Polarization

A slight shift of charge within atoms/molecules that creates regions of partial positive and negative charge even when the object’s net charge is zero; explains attraction of neutral objects to charged ones.

Charging by friction (triboelectric effect)

Charging by rubbing two materials, transferring electrons from one to the other; one becomes negative (gains electrons) and the other positive (loses electrons) while total system charge is unchanged.

Charging by conduction (contact)

Charging by touching a charged object to a conductor, allowing electrons to flow and charge to redistribute so both objects can end with net charge.

Charge sharing (identical conducting spheres)

When two identical conducting spheres touch and separate, charge redistributes equally: q_final on each = (q1 + q2)/2.

Charging by induction

Charging a conductor without contact by bringing a charged object near to polarize it, grounding to allow electron flow, removing the ground first, then removing the external charged object; the conductor ends with charge opposite the rod.

Grounding

Connecting an object to Earth so electrons can flow to/from a huge charge reservoir; important in induction because charge can cross the object’s boundary.

Coulomb’s law

The magnitude of the electric force between two point charges separated by distance r: F = k|q1q2|/r^2, directed along the line connecting them.

Coulomb’s constant (k)

The constant in Coulomb’s law; k = 8.99 × 10^9 N·m^2/C^2 (approximately for vacuum/air).

Permittivity of free space (ε0)

A constant related to electric interactions in vacuum: ε0 = 8.85 × 10^−12 C^2/(N·m^2), with k = 1/(4π ε0).

Inverse-square law

A relationship where a force varies as 1/r^2; in Coulomb’s law, doubling the distance makes the force one-fourth as large.

Principle of superposition (electric forces)

The net electric force on a charge is the vector sum of the individual forces from all other charges: F_net = F1 + F2 + … (including directions).

Newton’s third law (electric force pairs)

For two interacting charges, the forces are equal in magnitude and opposite in direction: F⃗12 = −F⃗21 (accelerations may differ due to mass).

Point charge approximation

Modeling an object as a point charge when its size is much smaller than the separation distance (or for forces outside a charged sphere), so Coulomb’s law can be applied using center-to-center distance.