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Charges push and pull

Electricity and magnetism · level 3, lesson 2 of 6

A question to start

Hang two balloons side by side on strings and rub them both on your hair. They swing apart. Why?

The idea

Charges push and pull on each other, a little like magnets.

  • Like charges push apart: two negatives, or two positives.
  • Unlike charges pull together: a negative and a positive.
  • The closer they are, the stronger the push or pull.

Two balloons rubbed the same way both become negative, so they push apart. A balloon pulls on your positive hair, so your hair lifts towards it.

A charged balloon can also pull on things with no charge, like a wall or bits of paper. Its negative charge pushes the electrons in them slightly away, leaving their near side a little positive, and that side is pulled.

−−− ++ like charges push apartunlike charges pull

Worked examples

1. Two balloons. Both rubbed on hair, both negative: they push apart, and their strings hang in a V.

2. Hair on end. After rubbing, each strand of hair is positive. The strands push each other apart, so the hair stands out.

3. A bent stream. Hold a charged comb near a thin stream of tap water. The stream bends towards the comb.

Try it

  1. Do two negative charges pull together or push apart?
  2. A positive rod is held near a negative balloon. What happens?
  3. Why does hair stand on end after you pull off a woolly hat?
  4. Why does the push get weaker as two charged balloons move apart?
Answers
  1. Push apart.
  2. They pull together.
  3. The strands all get the same charge, so they push each other apart.
  4. The push between charges gets weaker with distance.

Common mistake

"Only charged things can be pulled." A charged balloon also pulls on a wall or paper with no charge, by shifting their electrons a little.

Where it leads