AP PHYSICS 1 — physicsMedium3 min read

Electric Charge & Force

AP Physics 1 review of electric charge, Coulomb's law, conductors and insulators, and charge conservation.

Tutor AI Team2026-02-26T00:08:45.359Z

# Electric Charge & Force — AP Physics 1

Electricity begins with electric charge and the forces between charges. AP Physics 1 covers the basics: charge properties, Coulomb's law, and the behavior of conductors and insulators. These concepts lay the groundwork for circuits and more advanced electrostatics in AP Physics 2.

Key Concepts

Electric Charge

Two types: positive and negative.
Like charges repel; unlike charges attract.
Charge is quantized: $q = ne$ where $e = 1.6 \times 10^{-19}\ \text{C}$ .
Charge is conserved: total charge in an isolated system remains constant.

Conductors and Insulators

Conductors: electrons move freely (metals). Charge distributes on the surface.
Insulators: electrons are bound in place (rubber, glass).
Charging methods: friction, contact, induction.

Coulomb's Law

$F = k\frac{|q_1||q_2|}{r^2}$ where $k = 8.99 \times 10^9\ \text{N·m}^2/\text{C}^2$ .

Follows an inverse-square law (like gravity).
Coulomb's law gives the magnitude; direction is along the line connecting the charges.

Superposition

The net force on a charge is the vector sum of all individual Coulomb forces from other charges.

Comparing Electrostatic and Gravitational Forces

Both are inverse-square laws.
Coulomb force can be attractive or repulsive; gravity is always attractive.
The electrostatic force is enormously stronger than gravity for fundamental particles.

Worked Example

Problem: Two charges, $q_1 = +3\ \mu\text{C}$ and $q_2 = -5\ \mu\text{C}$ , are separated by $0.2\ \text{m}$ . Find the force between them.

Solution:

$F = k\frac{|q_1||q_2|}{r^2} = (8.99 \times 10^9)\frac{(3 \times 10^{-6})(5 \times 10^{-6})}{(0.2)^2}$ $F = (8.99 \times 10^9)\frac{15 \times 10^{-12}}{0.04} = (8.99 \times 10^9)(3.75 \times 10^{-10})$ $F \approx 3.37\ \text{N}$

The force is attractive (opposite charges).

Practice Questions

1. If the distance between two charges is tripled, by what factor does the force change?

$F \propto 1/r^2$ . Factor = $1/3^2 = 1/9$ . The force decreases to one-ninth.

2. Two identical conducting spheres carry $+6\ \mu\text{C}$ and $-2\ \mu\text{C}$ . They are touched and separated. What charge does each carry?

Total charge = $+6 + (-2) = +4\ \mu\text{C}$ . Each gets half: $+2\ \mu\text{C}$ .

3. How many excess electrons are on a $-4.8 \times 10^{-19}\ \text{C}$ object?

$n = q/e = 4.8 \times 10^{-19} / 1.6 \times 10^{-19} = 3$ electrons.

4. Three charges are in a line: $+2\ \mu\text{C}$ at $x = 0$ , $-3\ \mu\text{C}$ at $x = 1\ \text{m}$ , and $+1\ \mu\text{C}$ at $x = 2\ \text{m}$ . What is the direction of the net force on the middle charge?

The $-3\ \mu\text{C}$ charge is attracted toward both positive charges. Force from $+2\ \mu\text{C}$ pulls left (toward $x=0$ ) and force from $+1\ \mu\text{C}$ pulls right (toward $x=2$ ). Since $q_1 > q_3$ at equal distances, the net force is toward the left.

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Summary

Electric charge is conserved and quantized.
Coulomb's law: $F = kq_1q_2/r^2$ — inverse-square law.
Like charges repel; unlike charges attract.
The net force on a charge is the vector sum of all individual forces (superposition).

Tags:

#ap#physics#electricity

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