If you double the distance between two charges, how does the force change?

F ∝ 1/r²: doubling r reduces F by a factor of 4.

Two +2nC charges are 10cm apart. What is the force between them?

F = 8.99×10⁹ × (2×10⁻⁹)² / (0.1)².

Compare Coulomb's force and gravity between two electrons. Which dominates?

You can work it out this way: calculate F_E / F_G using the electron's charge and mass.

Pro 🔒~20 min

Coulomb's Law

Measure the electric force between charged particles

How it works

Coulomb's Law describes the electrostatic force between two point charges. The force is proportional to the product of the charges and inversely proportional to the square of the distance between them. Like charges repel; unlike charges attract. The law is the electric analogue of Newton's Law of Universal Gravitation, and both follow the inverse-square relationship.

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Step-by-step

Drag the charges to change their separation.
Adjust charge magnitudes and signs.
The force arrows scale with the force magnitude.
Toggle the force vs. distance graph to verify the inverse-square law.

Key formulas

$F = k\frac{|q_1 q_2|}{r^2}$ Coulomb's Law (k = 8.99×10⁹ N·m²/C²)
$E = \frac{F}{q} = k\frac{q}{r^2}$ Electric field from point charge

Frequently asked questions

If you double the distance between two charges, how does the force change?: F ∝ 1/r²: doubling r reduces F by a factor of 4.
Two +2nC charges are 10cm apart. What is the force between them?: F = 8.99×10⁹ × (2×10⁻⁹)² / (0.1)².
Compare Coulomb's force and gravity between two electrons. Which dominates?: You can work it out this way: calculate F_E / F_G using the electron's charge and mass.