Current is moving charge¶
Electricity and magnetism · level 3, lesson 4 of 6
A question to start¶
A charged balloon cannot light a bulb, but a small battery can light one for hours. What is the difference?
The idea¶
Static electricity is charge that has built up and stays put. A current is charge that keeps moving.
In a metal wire, some electrons are free to move. A battery pushes them round the circuit, all the time, so a steady current flows. That steady flow can keep a bulb lit.
A charged balloon has a small amount of charge. If it all ran through a bulb, it would be gone in a tiny fraction of a second, far too quick to see.
| Static electricity | Current |
|---|---|
| charge builds up and stays put | charge keeps flowing |
| on insulators, like a balloon | through conductors, like a wire |
| lets go in one quick spark | runs steadily, as long as the circuit is complete |
Worked examples¶
1. All at once. When you close a switch, the bulb lights at once. The wire is already full of electrons, and they all start moving together.
2. A balloon on a bulb. Touch a charged balloon to a bulb's contacts: nothing to see. Its charge is tiny next to a battery's steady flow.
3. Lightning. Lightning is a huge current, but it lasts only a moment, because the cloud's charge is used up.
Try it¶
- What is the difference between static electricity and a current?
- Why can a battery keep a bulb lit, but a balloon cannot?
- What moves in a metal wire when a current flows?
- Is lightning static electricity or a current?
Answers
- Static is charge that stays put; a current is charge that keeps moving.
- A battery keeps pushing charge round; a balloon has only a little, which would be gone at once.
- Electrons.
- Both: static charge builds up in the cloud, then flows as a huge, brief current.
Common mistake¶
"Electrons rush round the circuit at the speed of light." Each electron drifts only slowly. The bulb lights at once because the whole wire is full of electrons that all start moving together.