The Force Between Charges
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- The electric force between charges follows an inverse-square law, like gravity.
- Like charges repel and unlike charges attract, and the force grows with each charge.
- The electric force is vastly stronger than gravity, but usually cancels out.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
An inverse-square law
Two charges push or pull on each other: like charges repel, unlike charges attract. How strong is that force? It depends on the size of each charge, so doubling one charge doubles the force, and doubling both makes it four times as strong. It also depends on the distance between them, and here comes the surprise. Move the charges twice as far apart and the force is not halved but quartered. Three times as far, and it drops to a ninth. This is called an inverse-square law, and gravity follows exactly the same pattern.
Enormously strong, and usually hidden
The electric force is astonishingly strong. Take a proton and an electron in an atom. Gravity pulls them together, and so does the electric force, but the electric force is about 10³⁹ times larger. So why does gravity seem to rule the universe? Because charge comes in two kinds. Almost all matter contains as much positive charge as negative, so the pushes and pulls cancel out to something close to zero. Mass has only one kind, so gravity never cancels. A feeble force that always adds up can beat a huge one that almost always cancels.
Curriculum links: NGSS HS-PS2-4