Coulomb's Law and Electric Field Strength
How much do charges push and pull, and how do we measure the invisible field around them?
Big Idea: Charged bodies act on each other even without touching. Coulomb's law tells us exactly how strong that force is, and the electric field strength tells us how forceful the field is at any point in space.
By the end you will be able to:
- state Coulomb's law and use the formula F = k·q1q2/r2,
- explain how the force changes when charge or distance changes,
- define electric field strength E = F/q and give its unit,
- find the field of a single point charge and read field-line pictures,
- solve simple numeric problems with SI units.
1. A quick recap of electric charge
In the previous lesson you met the electric charge. There are two kinds: positive and negative. The rule of interaction is short and famous:
- Like charges repel (two plus charges, or two minus charges, push apart).
- Unlike charges attract (a plus and a minus pull together).
Charge is measured in coulombs (symbol C). One coulomb is a very large charge; in everyday electrostatics we usually deal with tiny amounts such as microcoulombs, where 1 µC = 10−6 C. A charge so small that it fits at a single point, and whose size we can ignore, is called a point charge. Coulomb's law is written for point charges.