Electric Force
Claimed by Azan Khan — Fall 2025
Introduction: The electric force is one of the four fundamental interactions of nature. It describes how charged objects push or pull on each other. This page explains the physical meaning of electric force, how to calculate it using Coulomb’s Law, and how the force behaves in real-world situations. The goal is to give students an intuitive and mathematical understanding of the concept as used in Physics 2.
Key Concepts
- Like charges repel and opposite charges attract.
- The electric force acts along the line connecting the two charges.
- The magnitude of the force depends on the size of the charges and the distance between them.
- The force decreases with the square of the distance (inverse-square law).
Coulomb’s Law
The electric force between two point charges is:
$$ F = k \frac{|q_1 q_2|}{r^2} $$
where:
- \( F \) = electric force (Newtons)
- \( k = 8.99\times10^9\, \text{N·m}^2/\text{C}^2 \) (Coulomb’s constant)
- \( q_1, q_2 \) = charges
- \( r \) = distance between the charges
Vector Form of the Electric Force
Electric force has direction. The vector equation is:
$$ \vec{F}_{12} = k \frac{q_1 q_2}{r^2} \hat{r}_{12} $$
where \( \hat{r}_{12} \) is a unit vector pointing from charge 1 to charge 2.
Common Misconceptions
- The electric force is NOT zero just because the net charge is zero.
- The force is not "shared" between charges — each charge experiences its own force.
- Coulomb’s Law applies only to point charges or spherically symmetric charge distributions.
Real-World Examples
- Static electricity on clothing is caused by attraction between oppositely charged areas.
- Lightning forms when electric forces overcome air resistance.
- Electric forces guide the motion of electrons inside circuits.
Interactive Simulation
Below is a GlowScript model showing the electric force between two charges.
<iframe src="https://trinket.io/embed/glowscript/31d0f9ad9e" width="100%" height="500"></iframe>
Practice Problems
Problem 1. Two charges +3 μC and -2 μC are 0.40 m apart. Find the magnitude of the electric force.
Solution: $$ F = k \frac{|q_1 q_2|}{r^2} = (8.99×10^9)\frac{(3×10^{-6})(2×10^{-6})}{(0.40)^2} = 0.34\,\text{N} $$
Problem 2. Two electrons are 1 nm apart. What force do they exert on each other?
Solution: $$ F = k \frac{e^2}{r^2} = (8.99×10^9)\frac{(1.6×10^{-19})^2}{(1×10^{-9})^2} = 2.3×10^{-10}\,\text{N} $$
Sources
- OpenStax University Physics (Public Domain)
- HyperPhysics (Public Domain)
- Wikimedia Commons (Public Domain Images)