How Does Wireless Charging Work? The Secret Behind Coils and Magnetic Fields
Hi, I’m Ken Kuwako, your friendly science trainer. Every day is an experiment!
Your phone is about to die, and you can’t find your Lightning cable anywhere… Then you spot a wireless charger sitting right there, and you could almost shout, “Saved!”
But when you stop to think about it, it’s kind of strange. There’s no cable connecting anything, so how does electricity get to your phone just by resting it on a pad?
It feels like magic, but it’s actually explained by a law of physics discovered about 200 years ago.
Let’s Peek Inside a Wireless Charger
If you take a wireless charger apart, you’ll find a coil inside.
Your smartphone has a coil inside too. Here’s what it looks like when it’s taken apart.
One coil sends power and the other receives it. Wireless charging works because these two coils face each other. Together with my student teacher, Mr. I, I designed a lesson to uncover the secret behind wireless charging. There were lots of moments where students had to think for themselves, and I think it turned out to be a really fun class.
The Equipment We Used
Here’s what you’ll need.
A tester (multimeter), Pascal wire (we’ll call this the “thick coil”), a bundle of enameled wire (we’ll call this the “thin coil”), and a power supply.
I bought the Pascal wire here:
Fuji Electric Wire Industry VCTF 0.3sq × 6-core cable
This time I used the 6-core version. Cut it into 1 m lengths, then connect the wires at both ends, shifting each connection over by one. You end up with a coil that loops around 6 times, which means you’ve turned 6 m of wire into a single coil.
Even with up to 3 A running through it, it stays cool and safe.
You can learn more about Pascal wire here.
Building the Sender and the Receiver
Connect the thick coil to the DC output of the power supply, and tell students that up to 3 A is fine. This is our model of the charger (the power-sending side).
Next, connect the thin coil to the tester. This is our smartphone (the power-receiving side). Set the tester to measure DC current, down in the microampere (μA) range so it can pick up even tiny currents.
Does Current Only Flow at the Moment You Put It Down?
When you place the thin coil on top of the thick coil, the current doesn’t keep flowing. The needle on the tester twitches only at the instant you set it down, showing a brief burst of current. I showed this to the students as a demonstration.
But if the current stops right after you set the coil down, that’s not much of a charger. So I asked the students, “How can we keep current flowing in the other coil?” They came up with hypotheses and then tested them.
Students Get Creative
The students tried all sorts of approaches. One group placed the thin coil on top of the thick coil carrying 3 A and moved it up and down. Another group moved it side to side.
They also tried rotating it and changing how fast they moved it, and every group confirmed that current keeps flowing as long as the coil is moving. That’s because moving the coil keeps changing the magnetic field passing through it.
Then we shared the results with the whole class.
Here are some of the ideas that came up:
・Move the thin coil closer and farther away.
・Keep turning the dial (switching the current ON and OFF).
・Rotate the thick coil inside the thin coil.
Since flipping the switch on and off over and over might damage the power supply, I had students turn the dial to switch the current off and on instead.
Still, real wireless charging doesn’t work by waving your phone around by hand all day. So how can we keep the magnetic field changing without moving the coils at all?
That’s our next big question.
What If We Switch to AC Power?
When we shared ideas as a class, someone suggested using alternating current. Let’s try it! With AC, the direction of the current flips back and forth dozens of times every second. Every time the direction changes, the magnetic field changes too, so the field keeps changing even if the coils stay perfectly still.
The voltage is 2 V. When we bring the coils close together, current really does keep flowing. And when we switch the tester to AC current mode, we can see that quite a lot is flowing.
Even with a little gap between them, the current still flows.
And it keeps flowing even when you put your hand in between! Magnetic fields pass right through our bodies.
“Wow!” I heard the students gasp.
Explaining It with “Magnetic Fields”
To finish, I asked the students to explain how wireless charging works once more. This time I told them to use the words “magnetic field” in their explanation. They scratched their heads and worked hard to put their thoughts together.
By the way, this idea of electromagnetic induction doesn’t just power wireless phone chargers. It’s also at work in electric toothbrush chargers, induction (IH) cooktops, transit IC cards, and many other things all around us.
Here’s something a little different: I made an LED light up using mutual induction.
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Let’s bring the wonder and fun of science closer to everyday life! This site collects fun science experiments you can do at home, along with tips to make them work, all explained in an easy-to-follow way. Try searching around!
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