How do electric toothbrushes charge through plastic?

How do electric toothbrushes charge through plastic?

A tricky question with a clever answer!

Last year, we challenged you to ask Planet Science a question. Year 6 at the Willows Primary School in Staffordshire rose to the challenge. They asked us "How does the electricity travel through the plastic coating of an electric tooth brush and the plastic coating of a charger, to re-charge my electric toothbrush. Surely, there need to be metal contacts? There aren't and it charges? How?"

That's a great question. We set off to investigate.

Electric toothbrushes

What's in your bathroom? Lots of water! So you don't want any exposed metal on your charger. Water connecting with the metal could stop the charger working and it could be dangerous.

How do electric toothbrushes charge if there are no metal contacts?

This sort of charging is called induction.

An electrical current produces a magnetic field. You can tell there is a magnetic field near the wire because it moves a compass needle. Turning the current on and off makes the magnetic field near the wire appear and disappear. Have a look at this video:

Did you see in the last circuit that the battery was turned round? This caused the current to flow in the opposite direction. Current flowing in the opposite direction changes the direction of the magnetic field. Did you see the compass needle swing round in the opposite direction?

The current in the electricity supply to your house flips first one way and then the other - it is alternating current (AC). The current alternates - changes direction - around 50 times per second. This means that the small magnetic field around the wires changes direction too.

Your electric toothbrush and charger both have a coil of wire inside them.

Electric toothbrush

Diagram adapted from

The coil of wire inside the charger is powered from a plug socket and creates a magnetic field, just like the wires in the video. The magnetic field keeps changing direction as the current changes direction.

How does the magnetic field produced by the charger coil charge the battery in the toothbrush?

A moving magnetic field creates a current in a wire. The changing magnetic field from the coil of wire in the charger generates a current in the coil of wire in the toothbrush. The electric current from the coil of wire in the electric toothbrush charges the battery.



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