Coulomb's Law
Electric force follows an inverse-square law: F = kq₁q₂/r².
About this topic
Coulomb's law gives the force between two point charges: F = kq₁q₂/r², where k ≈ 9 × 10⁹ N·m²/C². It is attractive if the charges have opposite signs and repulsive if the same, and it follows an inverse-square law just like gravity but is vastly stronger.
Doubling the distance cuts the force to a quarter, while doubling one charge doubles the force. Try an example: two charges of 1 microcoulomb placed 10 cm apart feel a force of 9 × 10⁹ × 10⁻⁶ × 10⁻⁶ / 0.01 = 0.9 N, about the weight of a small apple, from just a speck of charge.
The law explains how atoms hold together, the structure of molecules and why static cling happens. It is also the basis for understanding electric fields and the design of devices such as ink-jet printers and electrostatic precipitators that clean smoke from factory chimneys.
Students often forget to square the distance or to convert centimetres to metres. Remember that the force on each charge is equal and opposite, even when one charge is much larger than the other, as Newton's third law demands.
Key ideas
- Force falls as distance squared
- Product of charges matters
- k ≈ 9 × 10⁹ N·m²/C²
Quick check
Use cases: where Coulomb's Law helps
- Homework: review the key idea behind Coulomb's Law before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Coulomb's Law without notes.
- Visual learning: play with the interactive lab to see how Coulomb's Law changes when you adjust its inputs.
- Teaching a friend: explain Coulomb's Law in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Coulomb's Law?
Electric force follows an inverse-square law: F = kq₁q₂/r².
What will I learn in Coulomb's Law?
Force falls as distance squared. Product of charges matters. k ≈ 9 × 10⁹ N·m²/C².
Is Coulomb's Law part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Coulomb's Law lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Coulomb's Law?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.