How does wireless charging recharge a device?

1. Lay a phone on a charging pad and its battery starts to fill. The pad has a cable, but nothing plugs into the phone. So how does the energy get in?
2. Look inside. The pad holds a flat coil of copper wire. Hidden in the back of the phone is another coil, just like it.
3. Electricity running around a coil turns it into a magnet. The pad switches its current back and forth very quickly, so its magnetic field grows, shrinks and flips again and again.
4. That changing field reaches through the phone's coil and pushes electricity around it. The phone tidies this electricity up and feeds it to the battery.
5. The coils must line up. Slide the phone off to one side and most of the field misses its coil, so it charges slowly or not at all. Centered, it charges fast.
6. The pad is careful. It first checks that a real phone is there, so a coin or a key gets no power. When the battery is full, the phone tells the pad to stop.

The pad and the phone are cut through the middle. Tap each part to see its job.

• Power cable. The cable brings electricity from a wall adapter to the pad. Only the pad is plugged in.
• Pad circuit. A little circuit that switches the electricity back and forth very quickly, because only a changing magnet can push electricity in the phone's coil.
• Pad coil. A flat spiral of copper wire. When the flipping current runs around it, it makes a magnetic field that reaches up out of the pad.
• Magnetic shield. A dark sheet behind each coil. It guides the magnetic field toward the other coil and keeps it away from the other parts.
• Phone coil. A thin spiral of wire in the back of the phone. The changing field from the pad pushes electricity around it.
• Charging chip. The electricity from the coil flips back and forth, but a battery needs it to flow one way. This chip straightens it out and controls the charging.
• Battery. The battery stores the energy as it arrives, ready to run the phone later.

Drag the phone over the pad. Watch the two coils, the field in the side view and the charging speed. What happens off-center?

Experiment: The phone is charging in the middle of the pad. What happens if you slide it partly off to one side?

1. Drag the phone until its coil sits right over the pad's coil.
2. Slide it a little to one side and watch the charging speed.
3. Slide it further until the pad gives up.

What we found: The further the coils are from lining up, the less of the pad's field passes through the phone's coil. Charging slows down, more energy is wasted as heat, and at last the pad stops.

Turn on JavaScript to try it yourself.

Three phones, and each hides its coil in a different place. Move each one until it charges at full speed.

Electricity and magnetism are two sides of one thing. When electricity flows around a coil of wire, the coil becomes a magnet. And it works the other way too: when a magnet's field changes near a coil of wire, it pushes electricity around the coil. That second effect is called induction.

A charging pad uses both. Inside the pad is a flat coil of copper wire. The pad sends electricity around it, switching direction many thousands of times every second. So the coil is a magnet whose field keeps growing, shrinking and flipping over.

The phone has its own flat coil, tucked in its back. When the phone lies on the pad, the pad's changing field reaches through the phone's coil and pushes electricity around it. A chip in the phone straightens that electricity out and feeds it to the battery. Energy has crossed the gap with no wire, carried by the magnetic field.

The trick only works up close. The field spreads out and fades fast, so the two coils must be near each other and lined up. A thick case or a phone that is off-center means less field is caught, slower charging and more wasted heat. Electric toothbrushes charge the same way, which keeps their bases sealed against water. The magnet under the table is a good picture, but remember that a steady magnet would do nothing here: the field has to keep changing.

Key words

Coil

Wire wound round and round. A charging pad and a phone each have a flat one.

Magnetic field

The invisible push and pull around a magnet or around a wire carrying electricity.

Induction

A changing magnetic field pushing electricity around a nearby coil.

Alternating current

Electricity that keeps switching direction, back and forth, many times a second.

Coupling

How well the two coils are linked: how much of the pad's field the phone's coil catches.

Efficiency

The share of the energy put in that arrives where it is wanted. The rest is wasted, mostly as heat.

Check your thinking

1. What carries the energy from the pad to the phone?

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   A changing magnetic field. The pad's coil makes a magnetic field that keeps changing, and that pushes electricity around the phone's coil.

2. Where does a phone charge fastest on a pad?

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   With its coil directly over the pad's coil. When the coils line up, most of the pad's field passes through the phone's coil.

3. Why does the pad switch its electricity back and forth so quickly?

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   Only a changing magnetic field pushes electricity in the phone's coil. A steady magnet does nothing to a coil sitting still. The field has to keep changing.

4. You put a very thick case on your phone. What will probably happen on the pad?

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   It charges more slowly, because the coils are further apart. The field fades quickly with distance, so less of it reaches the phone's coil.

5. Why does a charging pad check for a phone before it sends full power?

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   So that a metal object such as a coin is not heated by mistake. A changing field induces currents in any metal. The pad transmits only when a receiver answers, and stops if energy goes missing.