March 3, 2026

AP Physics 2: Electric Charge & Force

« Back to AP Physics Guide / AP Physics 2 – Unit 10: Electric Force

Conceptual illustration of electrostatic forces between positive and negative point charges with field lines.

Unit 10 transitions us from the thermodynamics of gases into the world of Electrostatics. This unit is highly conceptual, requiring you to visualize invisible forces and “fields” that exist in the space surrounding a charge.

1. Conservation of Charge & Charging Processes

Charge is never created or destroyed; it is only transferred via electrons. You must understand the three primary ways an object becomes charged:

  • Friction: Rubbing two insulators together to “strip” electrons.
  • Conduction: Touching a neutral conductor with a charged object.
  • Induction: Charging an object without touching by using a nearby charge to polarize it and then “grounding” it.

2. Coulomb’s Law (The Force)

Just like Newton’s Law of Gravitation, the electric force (F_e) follows an inverse-square law. It depends on the magnitude of the two charges and the distance between them.

    \[F_e = k \dfrac{|q_1 q_2|}{r^2}\]

Electric force is attractive for opposite charges and repulsive for like charges.

3. Electric Fields

An Electric Field (E) is a map of the force a “test charge” would feel at any point in space. Field lines always point away from positive charges and toward negative charges.

Diagram showing electric field lines for a dipole: curving from the positive charge to the negative charge.

Visualizing the Field: The density of the lines indicates the strength of the field. Closer lines = Stronger field.

4. Quick AP Practice

📚 Unit 10 Practice Problems

1. If the distance between two point charges is tripled (3r), what happens to the magnitude of the electric force?

Check Answer Since F \propto 1/r^2, tripling the distance means the force becomes (1/3)^2 = \mathbf{1/9} of the original force.

2. A positive test charge is placed in a uniform electric field pointing to the right. In which direction does the force on the charge act?

Check Answer Force on a positive charge acts in the same direction as the field (to the right). A negative charge would feel a force to the left.