hoverboard hendo working

Image: Hendo Hoverboards

This hoverboard looks like the real deal – unlike the recent fake hoverboard.  Although I’m not exactly sure how the Hendo Hoverboard works, I have a pretty good guess. Let’s look at electromagnetic repulsion physics that it might use.
If you read the description on the kickstarter page, it says:

“The magic behind the hoverboard lies in its four disc-shaped hover engines. These create a special magnetic field which literally pushes against itself, generating the lift which levitates our board off the ground.”

That “pushes against itself” makes me worried. You can’t make yourself fly by pulling up on your belt, can you?  No, you can’t. But there is a way that this could work.

Changing Magnetic Fields

Check out this simple experiment. Here I have a coil of wire connected to a galvanometer (which measures small electric currents). I also have a magnet. If I just hold this magnet inside the coil, nothing happens. Moving the magnet does something interesting.

Image: Rhett Allain

A changing magnetic field induces a current in a conducting wire. Yes, that’s cool – but it’s also important. This physics principle behind many of the electric generators (but not all). The magnitude of this induced electric current depends on how fast the magnetic field is changing. Move the magnet faster and you get a larger current. Keep the magnet stationary and the magnetic field doesn’t change at all and you have zero current.
But now that there is an electric current in loop of wire, that induced current also makes a magnetic field. It turns out that this induced current makes a magnetic field that is in the opposite direction as the CHANGE in magnetic field due to the magnet. Yes. I know that is confusing. Maybe this diagram will help.

If the magnet was moving to the right, the magnetic field due to the magnet (which I labeled B_m) would still be pointing to left, but it would be decreasing in magnitude at the location of the coil. This means that the induced current (and thus the magnetic field due to the loop) would be in the opposite direction as shown in the diagram. Yes, I know this is hard to picture.

Using an Electromagnet

Moving a magnet makes a changing magnetic field. But what about an electromagnet? If I replace the magnet in the diagram with a coil of wire, I can change the magnetic field without even moving coil. Just by changing the current in the one coil, I can induce a current in the other coil. I can continually change the magnetic field in the electromagnet by just having the current oscillate back and forth. It’s actually not that difficult.
Check this out. Here is a large coil of wire hooked straight into an AC outlet. Yes, you probably shouldn’t do this since you could really make something hot. The AC outlet oscillates the current in the wire and produces an oscillating magnetic field on the top. Above that, I have small copper plate. This is what happens when the current is on.

Image: Rhett Allain

Boom. Electromagnetic repulsion.  The only difference for the hoverboard is coil of wire is on the top and the copper plate is on the bottom (as the conducting surface). Pretty cool and pretty simple. Ok, I’ll admit that it’s a little bit more complicated than that – but you get the idea. Of course, this is a small demonstration of electromagnetic repulsion. In this following video, Derek (from Veritasium) shows a MUCH bigger and more awesome demonstration of electromagnetic levitation.
The physics for levitation is all there. You just need to work on the engineering to get a hoverboard to work.

Looking at the Hendo Hoverboard

Does this hoverboard need a battery? Yes, it needs a battery. I assume that there is a battery in the board to run the 4 coils on the bottom. How much current to do you need to run through them? I have no idea. It might be fairly large.
I didn’t do a full exploration, but I did take a look at the power needed for my electromagnetic levitation demonstration. Using the Kill-a-Watt, I got about 75 Watts while the thing was hovering. Where does all this energy go? Well, a tiny fraction of it goes into increasing the gravitational potential energy of the disk. This is just a tiny bit. The rest of this 75 watts is lost through an increase in temperature of the wires. Oh, the wires in the coil get hot as well as the conducting surface.
So what kind of power does the Hendo need? I have no idea. I suspect that if they use some clever engineering they can get the power consumption to reasonable levels. If I had to guess (which apparently I do), I would say it’s probably in the 300 Watt range. Of course if they used superconductors, the power requirements would be minimal – except for the power needed to keep the superconductors cold (we don’t have room temperature superconductors yet).
What if you used a ferromagnetic surface instead of something like copper? In that case you would still induce a current in the material. However, you would also cause the magnetic domains in the ferromagnetic material to line up with the magnetic field from the electromagnet. This would cause an attraction between the two and it wouldn’t make the thing hover.
Ok, so the physics for this type of hoverboard seems possible. Looking at other sites talking about this online, I am fairly certain it’s real.  One last physics note: I’m really not sure if a hoverboard powered by electromagnetic repulsion would be frictionless.  I suspect there would be some type of electromagnetic drag as the coil moved over the metal surface – but I could be wrong.
Hendo also has these small developer kits called the Whitebox.  This Whitebox is a small box (and it’s white) with the same hover technology as the hoverboard.  It also has some type of propulsion system in it so that it can move around while hovering (over a conducting surface).  It looks pretty cool.  If Hendo wants to send me one of their little “The Whitebox” developer kits, I would be happy to test it out and give a report.


http://www.wired.com/2014/10/physics-hendo-hoverboard/