Physics 2 · Unit 3: Electric Force, Field, and Potential · 14 min read · Updated 2026-05-11
Charge and Electric Force — AP Physics 2
AP Physics 2 · Unit 3: Electric Force, Field, and Potential · 14 min read
1. Fundamental Properties of Charge★★☆☆☆⏱ 3 min
Charge is a fundamental intrinsic property of matter that causes it to experience force in an electromagnetic field. There are two types: positive and negative. Like charges repel, opposite charges attract. The SI unit of charge is the coulomb (C), and the elementary charge $e = 1.6 \times 10^{-19}\ \text{C}$ (the magnitude of charge on one proton or electron).
Exam tip: When asked for the number of excess electrons, remember positive net charge means a deficit, which corresponds to a negative number of excess electrons. Always check question wording.
2. Charging by Conduction and Induction★★☆☆☆⏱ 3 min
AP Physics 2 regularly tests conceptual understanding of the two most common charging processes. Conductors allow free electrons to move through the material, while insulators bind electrons to their atoms, a key distinction for understanding charging.
**Conduction (contact):** Requires physical contact between a charged and neutral object. Both objects end up with the same sign of net charge after charge transfer.
**Induction:** Charging without physical contact, relying on polarization and a ground connection to remove excess charge of one sign. The charged object ends up with the opposite sign of net charge to the original charged object.
Exam tip: AP multiple-choice almost always tests the sign difference between conduction and induction. The shortcut: contact = same sign, no-contact induction = opposite sign.
3. Coulomb's Law and Superposition of Electric Force★★★☆☆⏱ 4 min
Coulomb's law describes the magnitude of the electrostatic force between two stationary point charges. When multiple charges act on a single charge, the principle of superposition applies: forces add as vectors.
For systems of more than two charges, the net force on any charge is the vector sum of the individual forces exerted by each other charge. This requires decomposing all forces into $x$ and $y$ components, adding components, then finding the magnitude and direction of the net force.
Exam tip: Always calculate magnitudes first with Coulomb's law, then assign direction based on charge signs, instead of plugging negative signs into the magnitude formula. This avoids common sign errors in vector addition.
4. AP-Style Worked Practice Problems★★★☆☆⏱ 4 min
Common Pitfalls
Why: Students confuse induction with conduction, since both start with a charged object brought near a neutral object.
Why: For uniform spherical charge distributions, we treat them as point charges at the center, and students confuse this with problems involving sphere radii.
Why: Students mix up the rearrangement of $q = ne$, especially when working with small scientific notation exponents.
Why: Superposition of force is often misinterpreted as 'add the numbers', so students forget force is a vector quantity.
Why: Students get so focused on different charge sizes that they forget Coulomb's law is symmetric and follows Newton's third law.