Coulomb's law
The force between two point charges is F = k q1 q2 / r squared, with k = 8.99e9 N m squared per C squared.
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Analogy
Gravity's cousin, but it can push as well as pull.
Example
F = 8.99e9 x 1e-6 x 1e-6 / 0.1^2F = 0.899 N
In practice
Two 1 microcoulomb charges 10 cm apart push each other with about 0.9 N.
Practice
Flashcards
- State Coulomb's law.F = k q1 q2 / r^2.
- How does Coulomb force change when distance doubles?It falls to one quarter.
Questions people ask
How does Coulomb's law compare with gravity?
Both fall off with 1/r squared. Electric force can attract or repel, while gravity only attracts. Electric force is far stronger: between a proton and an electron it is about 10 to the power 39 times bigger than gravity. Gravity wins for planets because overall charge cancels.
How do I add forces from several charges?
The force on a charge is the vector sum of the forces from each other charge, which is the superposition principle. Find each force's size from Coulomb's law, split each into x and y components, then add the components and combine them to get the final size and direction.