What a learner can do afterwards
- Calculates the force between two point charges and reads its direction from the signs
- Finds E as force per coulomb and as kQ over r squared, and gives both units it is quoted in
- Compares the electric and gravitational forces between two protons and comments on the ratio
1 · Read
Point charges push or pull along the line joining them, with like signs repelling and opposite signs attracting. The size follows Coulomb's law: F equals k times each charge over r squared, with k about 9 times 10 to the 9.
An electron and a proton a tiny distance apart in hydrogen pull on each other electrically far harder than they pull by gravity. Each feels an equal and opposite force, just as Earth and the apple pulled equally hard on each other. Two protons show the same pattern: their electric push dwarfs their gravitational pull.
Field strength E is the force on a tiny positive test charge, divided by the size of that charge. For a point charge Q it is k Q over r squared, pointing away from positive Q and toward negative Q. It is quoted in newtons per coulomb, the same as volts per metre. Lines leave positive charges and enter negative ones, and a map of them has no crossings, since one spot cannot have two directions.
Read direction from the signs first, then compute size from the distances.
Charge signs set the direction along the joining line, and the inverse square sets the size.
2 · Watch
Take it off screen
Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.