The Physics of Wireless Charging

What if you could charge your phone (or device) without having to worry about the charging cable? Well, you can. This is the idea behind wireless charging. In short, you place your device on some type of pad and then phone gets power without a wire (as long as the phone also supports wireless charging). That’s where they get the term “wireless charging” – you know…because there are no wires.

Magnets and Wires

Let’s start with a very simple demonstration. Here I have a coil of wire connected to a Galvanometer. I could write a whole post on just the Galvanometer, but for now I will just say that it measures electric current. Inside the red coil I am holding a very strong magnet.

If I just hold the magnet inside the coil, nothing happens. However, if I move the magnet either in or out of the coil I get a current.

This is all about changing magnetic flux. Yes, just like a “flux capacitor” even though that isn’t a real thing. You can have flux for all sorts of things. My favorite flux to use as an example is rain flux. This is simply the rate that falling rain hits some area – let’s say it’s a sheet of paper.

There are three things you could change that would also change this “rain flux”. First, you could change how much it rains. If the rain comes down faster of course more water will hit the paper (note – real rain drops aren’t shaped like that). Second, you could change the angle between the paper and the rain. Third, you could change the area of paper. That’s rain flux.

We can do the exact same thing with the magnetic field. Guess what we call this? Yes, it’s called the magnetic flux. This magnetic flux depends on the strength of the magnetic field, the angle between the field and the area and the size of the area.

The curved lines are representations of the magnetic field from the magnet.

Here is the physics part. When you change the magnetic flux, you create an electric field inside the wire. This electric field then makes an electric current and electric currents can recharge your phone. Remember, CHANGE in flux is the important part. Actually, you could just use a spinning magnet and a coil of wire and make as much electricity as you want. In fact, this is exactly what happens with a gasoline powered generator. Oh, it’s also how a nuclear power plant makes electricity (the nuclear reactions just turn water to steam and the steam turns a turbine).

Magnetic Flux Without Magnets

The wireless chargers don’t have magnets in them. If you place a wire with current over a magnetic compass you can see that these currents also make magnetic fields.

If you replace a moving magnet with a wire that has alternating current, you are all set. The changing electric current in one wire makes a changing magnetic field. This changing magnetic field then induces an electric current in another loop. Also, the more loops you have (in both coils of wires) the greater the effect. Here is simplest version of wireless charging.

On the bottom is a huge coil of wire. This wire is then attached to a household style plug. Yes, it’s just a loop of wire with a plug on the end. When you plug this thing into the outlet, electric current runs through the wire. All the outlets in your house have alternating current. This means the current oscillates with a 60 Hz frequency and provides the changing current needed to make a changing magnetic field. On top of this large coil is a smaller coil (in my hand). This coil is just connected to a small lightbulb. When this small lightbulb-coil is near the changing magnetic field, you get an induced current. The current is large enough to light up the lightbulb.

Maybe you prefer a video demonstration. Here you go.

Of course, an actual wireless charger is a little bit smaller – but same idea.

Last question. Previously, I looked at the possibility of charging a smartwatch just by shaking it. Could you power a smartwatch with a wireless charger? Yes, you could. However, the smart watch would have to be right on the charger. It wouldn’t work over a long distance – at least not with this type of wireless charger.