There are so many cool things to look at in this clip from Breaking Bad. All you need to know is that there are some guys in a truck next to a police building with some evidence in it. The truck guys crank up a huge electromagnet to destroy the evidence. Could you really do this?

Hard Drives, How Do They Work? ——————————

Let me assume that these guys are trying to destroy data on magnetic hard drives. Really, I am not certain - but it is an assumption. I guess, they could be trying to destroy some old 8-Track tapes also - but I will go with hard drives. If you have to break the operation of a magnetic hard drive down to a very basic level, it starts with magnetic domains. A magnetic domain is a region in the hard disk has a uniform magnetization. If you think of an atom as a tiny magnet, in each region most of these tiny magnets are aligned in the same direction.

Here is a simplified version of the domains in a disk. Different orientations of magnetic domains can represent either a "1" or a "0" in a binary number.

Of course, you will want to change the data at some point, right? These domains are changed by a read-write head that moves over the disk. Here is a picture from a drive I took apart. Everyone should take apart a drive, it's fun - just be careful. Some of the platters can shatter and make a mess and you shouldn't eat anything in there. However, the platter make AWESOME mirrors and the permanent magnets in there are good for all kinds of stuff.

If you want to erase a hard drive, you could just change all of the magnetic regions to "1" or something. Then all the data is gone.

Estimate of the Magnetic Field ——————————

I googled around for a little bit, but I didn't find a nice value for the magnitude of the magnetic field needed to change a magnetic domain from "1" to "0". So, I am just going to make some wild estimates. Here we go.

The read-write head is a coil of wire consisting of 10 loops. Really, I don't know this is true. After close inspection, I can't see any loops in the head. Of course, I will proceed anyway.

The radius of this loop is about 0.5 mm.

The wires to this head are quite thin. I doubt they could take a current of more than 1 mAmp.

The platter is about 0.1 mm from the head.

With these crazy assumptions, I can calculate the magnetic field from this small electromagnet. If you have a small loop of wire with current, the magnetic field along the axis can be determined by:

Here, the μ is just a constant and z is the distance along the axis of the coil where I want to calculate the magnetic field. If I put in my best guesses for values, I get a magnetic field at the platter with a magnitude of 1.2 x 10-5 Tesla. So, I will assume this is somewhere around the value of the magnetic field needed to delete stuff.

Electromagnet in the Truck ————————–

The real question is: how much current would need to be going into the coil in the truck to produce that same magnetic field but from a greater distance? First, here is a shot of the magnet in the truck.

This looks like a standard electromagnet that is used in a junkyard. I am going to assume it has a radius of 1 meter. How far away is the stuff in the evidence room from the electromagnet? I don't think it could be any closer than 5 meters - so I will go with that.

There is one more estimate: the number of loops in the coil. If I assume the cable inside the coil has a diameter of 1 cm, then maybe it could fit in a 20 cm by 20 cm cross section area of the coil. This would fit about 400 loops. This is just a total guess.

Now, I can solve for the value of current in that electromagnet that would produce that magnetic field. Let me go ahead and assume that that z is large enough compared to R that I can use this simplified version of the magnetic field.

Solving for I:

With the above estimates, this gives a current in the electromagnet with a magnitude of about 6 amps. That isn't crazy high (which I am sort of surprised by). However, look at the distance. What if I just increase the distance to the evidence from 5 meters to 6 meters? This would change the current needed to a value of 10 amps (since there is a cubed dependance on distance). At 7 meters this would jump up to 16 amps. That is fairly high, but from this estimation that still seems reasonable.

Is It Possible Then? ——————–

Although my calculations show it could work, I am still not sure. In every step, I estimated in favor of getting the scene to work. By doing this, I get a value that seems plausible. However, this is not a linear calculation. If I just change the distance by a little bit - and I think the distance could be as high has 10 meters or more, then this becomes an unrealistically high value of current around 47 amps (maybe not completely unrealistic - but still super high).

I really feel unsure about the value of the magnetic field inside the hard drive, but I did the best I could do. Perhaps this would be a great scene for the MythBusters to test.

Other Data ———-

After playing around a bit more, I found useful site from K and J Magnetics. They tested this hard drive erasing idea with some nice strong magnets. Check out this picture.

They put two of the DX0X8 Neodymium magnets on a running hard drive. They list the value of the surface magnetic field at around 1.32 Tesla and found that it did NOT erase the hard drive.

What kind of current would the truck-based electromagnet need to produce just 1 Tesla at a distance of 5 meters? Huge current. 4.97 x 106 amps. This is some serious stuff here. Even 100 car batteries couldn't produce that kind of current. You would need something like a bolt of lightning.

Ok, I feel better now. Let me just chalk this up to my poor estimation of the magnetic field inside a hard drive.

Homework Questions ——————

There is more than this one calculation. Here are some other things you could look at.

At one point, the hanging lamps in the building pull to one side. If you assume these are steel (you will have to look up some stuff here), calculate the magnetic force based on the angle the lights hang. Use that magnetic force to estimate the strength of the magnetic field. Hint: this is tough since the magnetic force also depends on the divergence in the magnetic field.

At the end, the truck with the magnet tips over. What magnetic force on the truck would be needed for this to happen? (this one isn't so bad - remember, I didn't ask for the magnetic field)

If there was a 10 amp current in the electromagnet, what would be the value of the magnetic field 3 meters away? What if there was 50,000 amps in the electromagnet? Really, I am thinking about the guys in the truck here even though they are on the side.

Suppose the cables from the batteries have a diameter of 1 cm. If a current of 10 amps runs through them, how hot would they get? You can assume they are copper cables. What if the current is 50,000 amps? How long would it take for them to melt?

Estimate the total length of copper wire in the coil of the electromagnet. Assuming a diameter of 1 cm for the wire, what would the resistance of this wire be? How many 12 volt batteries would you need to get the current up to 10 amps? How many 12 volt batteries would you need to get to 50,000 amps?

One other quick note. I remember an area in Fermi Lab where they had super high magnetic fields for the ancient bubble chambers they had there (I was just in high school, so I might be remembering things incorrectly). We were told that if you enter a region with super high magnetic fields and turn your head fast you will see stars. The stars a result of currents generated in your brain due to the turning motion. Again, this is just the story some grad student told us. He could have been pulling our leg.