Charge: the tiny pull behind all electricity

Rub a balloon on your hair, then hold it near your head. Your hair lifts up to meet it. Put the balloon on a wall and it sticks there, with nothing but air and a little rubber to hold it. What is going on?

The answer is the same thing that lights every bulb and charges every phone. It is called electric charge, and it is where all of electricity begins.

Two kinds of charge

Everything is made of atoms, and every atom has two kinds of tiny parts that matter here:

• Protons, in the middle, carry positive (+) charge.
• Electrons, whizzing around the outside, carry negative (−) charge.

Most of the time an atom has the same number of each, so the pluses and minuses cancel and the object feels neutral. You can't tell there is any charge there at all.

The one rule

Charges follow a rule so simple you can say it in a breath:

• Opposites attract. A plus and a minus pull toward each other.
• Likes repel. Two pluses push apart. Two minuses push apart too.

Try it

Choose a plus or a minus for each charge, then slide them apart and together.

Try these:

1. Make the two charges attract. Which two buttons did you press?
2. Make them repel. Is there more than one way?
3. Set the distance to 10 cm, then to 20 cm. What happens to the force when the distance doubles?

The force gets four times smaller when the distance doubles. Scientists call this an inverse-square rule: the force depends on 1 ÷ (distance × distance). It is the same shape of rule as gravity, but charges can push and pull, while gravity only pulls.

So why does the balloon stick?

Protons are locked inside the middle of atoms and don't move. Electrons can move. When you rub a balloon on your hair, a few electrons hop from your hair onto the rubber. Now the balloon has extra minus charge, and your hair has lost some, so it has more plus than minus.

Your hair is positive, the balloon is negative, and opposites attract. Each strand lifts toward the balloon. Strands that are all positive also repel each other, which is why your hair stands up and spreads out.

On the wall, the balloon's minus charge pushes the wall's own electrons away a little, leaving the wall surface slightly positive right next to the balloon. Opposites attract again, just enough to hold up a light balloon.

Conductors and insulators

Some materials let electrons move freely, and some don't.

• In a conductor, such as copper, iron or any metal, some outer electrons are loose and drift between the atoms. Charge can flow through a conductor easily. Wires are made of metal for this reason.
• In an insulator, such as rubber, plastic, dry wood or glass, the electrons stay put. Charge can't flow through easily. Wires are wrapped in plastic for this reason too.

That is why the balloon keeps its charge: rubber is an insulator, so the extra electrons have nowhere to go. Touch a metal doorknob and they escape in a flash, sometimes with a small spark. You have felt that spark as a little shock.

From sparks to circuits

A spark is charge moving for a moment. Lightning is the same thing on a gigantic scale. A battery does something cleverer. It keeps pushing charge steadily, so electrons keep flowing through a wire instead of jumping once. That steady flow is what we call electricity, and it is the topic of the next lesson.