Lenz’s law – at 3 tesla

By Marcus Wilson 20/05/2015 3

When I was at school, and introduced to magnetic fields in a quantitative sense (that is, with a strength attached to it), I remember being told that the S.I. unit of magnetic flux density (B-field) is the tesla, and that 1 tesla is an extremely high B-field indeed. Ha! Not any more. Last Friday night  I got to see a MRI machine in action – at Midland MRI at Waikato Hospital – this particular one is a 3 tesla affair. One of my PhD students was making some measurements with it. It needed to be at night – such is the demand for MRI scans we'd never get to play with it during the day. But well worth extending my day's work for. 

Now, what does 3 tesla do? First you are advised to check pockets very carefully and remove keys and the like. No pacemakers? Good. Now enter the room. Interestingly, I didn't really 'feel' anything until very close to the machine – then there was just a hint of something slightly 'odd'. Things a little tingly, but nothing really significant. 

Two events, however, confirmed that there was a sizeable field indeed. First, my belt unbuckled by itself. That prompted a quick retreat outside to take that off, before bits started flying through the air. Then our host demonstrated what 3 tesla does to a sheet of aluminium. 

It's important to remember that alumunium is not ferromagnetic. It is not attracted by a magnet. But it is, most certainly, very conductive. When a conductor moves through a magnetic field, electric currents are induced. These in turn generate magnetic fields, which are such that they oppose the movement. This is Lenz's law. Consequently there is a force felt by the conductor that opposes its motion. And at 3 tesla, that's some force. You can stand the sheet of alumnium on its end. Normally, you'd expect it to fall over, pretty quickly. But not at 3 tesla, it doesn't. Very, very slowly, it topples, taking several seconds to move from vertical to horizontal. I could feel the effect of Lenz's law by trying to flip the sheet over. It was like trying to turn a rapidly spinning gyroscope. Pretty impressive stuff. 

You can see a movie of this experiment (not ours, I should add), here. https://www.youtube.com/watch?v=JNUVfmy-iqM 

3 Responses to “Lenz’s law – at 3 tesla”

  • Wow, that experiment with the aluminium sheet is so cool. Roughly how far from the machine does the magnetic field extend at the strength where it can have that effect?

    I’d kind of like to see what happens if it just gets dropped in mid-air, but I guess that’s unlikely to happen as it could damage the machine?

  • Cool. Oh yes. You have to get your hands on one of these things. New, I think they are about $10 million. Second-hand, just a few million will do it. Plus reinforcing the floor, employing someone to drive it, etc etc.

    I don’t know the exact answers to your questions, but certaintly to get the best effect with the plate you do need to be close to the machine. Just a metre back is enough for there to be much less effect. Also check this one! https://www.youtube.com/watch?v=lszHhU52Mow

  • Wow, that’s really cool! I’ll definitely have to start saving up 😛

    The description of that video says that’s at 1 Tesla, so significantly weaker than the field you were working with.

    I was a bit surprised to see the tube move horizontally so quickly in that video though, especially given how slowly it moves vertically. Is that because the machine used there is oriented differently to a medical MRI machine, so the strength of the magnetic field only changes in the vertical direction, so it’s only vertical movement induces a current and results in the motion being resisted?