Once objects are charged, they push and pull on each other without touching. The rules are short, and they explain lightning, clingy socks from the dryer and the way atoms hold together.
Key idea
Like charges repel; unlike charges attract. The force is stronger when the charges are bigger and when they are closer. Like gravity, it weakens with the square of the distance: twice as far, one quarter of the force.
Why it works
A charged object can even attract a neutral one. Bring a negative balloon near a wall: it pushes the wall's electrons slightly away, leaving the near side of the wall a little positive. That near positive side is closer than the farther negative side, so the attraction wins, and the balloon sticks.
Worked example: Attract or repel?
Two balloons are both rubbed on hair, so both are negative. What happens when you hang them side by side?
- Both have the same kind of charge.
- Like charges repel, so the balloons swing apart.
Answer: They push apart
Worked example: Change the distance
Two charged beads push on each other with 0.8 N. They are moved to half the distance apart. What is the force now?
- Distance ÷ 2 means force × $2^2 = 4$.
- $0.8 \times 4 = 3.2$ N.
Answer: 3.2 N
Common mistake
Thinking a charged balloon can't attract a neutral wall, since only opposite charges attract. The wall's charges shift so that the near side becomes slightly opposite in charge to the balloon, and nearer charges pull harder. Check: a charged comb picks up neutral bits of paper the same way.
Lightning is the same kind of attraction on a huge scale: charges separate inside a storm cloud until the pull across the air is strong enough to send a giant spark.