About johndangerx

I am a college student, currently volunteering at the Exploratorium, where I get distracted by cool science toys. I make some things and not other things. Sometimes I think things, too, but that is a little too hard so I just try them and see what happens.

Homopolar Motors

Homopolar motors are very simple motors that consist of a battery, a magnet, and a bit of wire connecting the two. I made one with a coat hanger serving as part of the circuit, and the battery freely hanging off of it. Here are some pictures, and a very short video:

Michael Faraday Defeats Cell Phones From Beyond the Grave – and more!

The theme is aluminum foil. Aluminum is a conductor! Foil is easy to shape!

To my mind, it’s pretty obvious what to do next: Faraday cages! Faraday cages are basically conductive boxes that shield whatever is inside it from electromagnetic radiation – basically, since EM radiation is just changing electric and magnetic fields, and these fields push electrons around, the electrons in a conductor naturally settle in a configuration where the net force (and thus the fields) is zero. Then, inside the box, there is no field. My first encounter with Faraday cages was in high school robotics, when we accidentally put our electronics board inside one, making it impossible for us to communicate with the robot! We fixed that up right quick…

I made a Faraday cage to shield my phone from the carrier signal. Faraday cages are very good at blocking DC fields – ones that don’t change. But cell phone signals (and, really, signals in general, such as mind-control ones) are AC – they change. If the conductor is too thin, the signal will be able to penetrate the cage. The skin depth is the thickness at which the signal drops to about 1/3 its original strength; it goes up with the resistivity of the material and down with the frequency of the signal. Since I’m using aluminum (low resistivity) and a cell phone (~100kHz frequency), the skin depth is very low – if you do the math all the way out it’s about .06 millimeters, or about 3 mils. I figured I should get at least 3x the skin depth (so that the signal goes to 1/27 of its original strength) – so about 0.2 millimeters. This turned out to be 12 layers of the aluminum foil we have at the Learning Studio; aluminum foil is surprisingly thin!

My beat-up phone gets 5 bars of signal in the Learning Studio.

I made a little pocket for my phone, and put it in.

I didn’t close off the pocket at first; even like that I was able to see that my signal had gone down from 5 bars to 1 bar. I took my phone out of the aluminum foil and texted it from Google Voice – within seconds, my phone buzzed. “Ooh, a text!”

I then put my phone in the aluminum foil, closed it up real good, and texted it again.

Minutes passed with no activity. I gave in and took the phone out of the foil. The signal had completely gone away – the indicator said “Searching…” After a few seconds, I connected to the T-Mobile network and got another text: “faraday is a chump”.

You win, Faraday. Our advanced cell phone technology is no match for your discoveries about electromagnetism.

So does this mean that tin-foil hats would actually stop you from being mind-controlled by the government? Not really. I made a cell-phone-sized tinfoil hat with the aluminum foil, and the signal only went from 5 bars to 4 bars.

It simply does not cover enough area, even though I simulated a full-face motorcycle helmet. I guess we all have to resign ourselves to being government slaves forever…

Marbles in Water, Part 2: Corn Syrup

To continue the long-standing tradition of dropping marbles in fluids and documenting the results, here are some pictures and video of marbles being dropped into corn syrup. Corn syrup is a lot more viscous than water, so the results are different!

Of note:
– there are a lot of air bubbles suspended in the corn syrup that are fairly permanent and were not caused by the marbles
– the marbles actually seem to create no bubbles on impact; just a crater that then fills up with corn syrup

Here is some video:

I think one of the coolest things is when the fast marble slows to nearly a stop after it enters the corn syrup, then starts to accelerate downward again. This is possibly because right after the marble impacts, there is corn syrup below it but not above it, which slows it down more in the beginning. Once it sinks in a little more and the corn syrup gets on top of it, there is less of a buoyant effect. In addition, the resistive force of the corn syrup depends on the velocity of the marble, which helps explain the drastic deceleration of the marble on impact. You can see that, no matter what speed the marble impacts the corn syrup at, the final speeds are very similar.

Picasa: Summer Explorations: Marbles in Corn Syrup


You can see where the marbles disturbed the bubbles on the way down, above and to the right of the marbles.


The clear one descending through the corn syrup. You can see the dent in the surface that it made. It looks like there is a thin filament of air dragging behind the marble.


The brown marble descending. You can really see how pointy the dent in the surface is. This is because the corn syrup is rushing in from the sides to fill the hole.





These two marbles have been captured right after impact. The tops of the marbles aren’t in the corn syrup yet. Note the lack of a splash. You can also see the convex surface of the corn syrup coming in to fill the hole.




In this sequence (actually taken during 3 different drops from the same height), you can see how quickly the surface returns to normal.

Marbles in Water

I wanted to see what happens when I drop marbles into water. What does the splash look like? Can you see the path the marble traces through the water? What does that look like? Do I have enough depth perception to drop marbles 3 feet into a cup consistently? (No. I had to use a table corner as a reference point.)


A vortex! I have no explanation.


The initial impact of a yellow marble with a rough finish. There are already two marbles in the water: one is brown and speckled, and one (right behind it) is clear and smooth.


The only picture I took with flash captures the air bubbles surrounding the brown speckled marble’s descent, as well as some nice glare. You can see that the bubbles form clumps, and seem to stick to the marble’s surface. The clumps are probably formed when the shock of hitting something shakes the bubbles off of the marble.


The brown marble hits the water while the yellow one is on the way down.


I thought some food coloring would be interesting… Here we see the yellow marble entering the water.


And this is the descent of the brown one.

Lessons learned:
– Flash, though it makes the cup look ugly due to glare, is nice for seeing really crisp pictures of the bubbles.
– Food coloring doesn’t add anything to the analysis but pretty colors, which I guess is worthwhile in itself.
– Pressing the trigger at the right time is hard, as is dropping marbles.

Next time I’ll drop some marbles into corn syrup and see what happens.