Thursday, January 1, 2009

MRI Magic



This is a video of a couple of guys tipping over a block of aluminum inside of an MRI machine. Because of the huge magnetic field the Lenz force opposing gravity is enough that the block falls very, very slowly.

19 comments:

Harold Fowler said...

Wow, that is truly amazing!

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Anonymous said...

Couldn't find the term "Lenz Force" in wikipedia.
So I have to ask what is Lenz Force - is it a magnetic lens you're referring to? Just curious. Cool video. What was the movie where the hero throws a big Oxygen tank into an MRI machine to mess with the bad guys? And was there any danger of that happening to the aluminum block, or was there none, due to the less magnetic nature of aluminum, or its size? Or was the MRI not 'cranked up' all the way?

JIM said...

It's "Lorenz" force.

Alex said...

I think it's possible he's talking about the Lorentz force, which from what I recall definitely has to do with Magnetic fields.

Dale said...

Does the block heat up from this? Also, is it only rotational, or would the same thing happen if they dropped it?

Lazy Lavender said...

@Anonymous,

Lenz' force -> http://en.wikipedia.org/wiki/Lenz%27s_law

The concept behind the video is explained here -> http://rabi.phys.virginia.edu/106/1999/PS3a.html

Questions 4 & 5. Amazing indeed!

mg said...

It's the lorentz force... it basically quantifies the amount of force is given by magnetic or electric fields.

http://en.wikipedia.org/wiki/Lorentz_force

ttexas said...

Typically what you will hear is "Lenz's Law" which is more of a rule of thumb. There are more fundamental laws (from Maxwell's Equations) that cause the physics to happen.

Before I use a bunch of words when pictures would work best, let me mention: you will find more popular explanations of it under the name "eddy currents".

Explanation: Lenz's Law reminds you that currents in metal will flow so as to preserve the "flux" piercing a plane. Ok. What does that mean. As the metal tips over, the magnetic field from the MRI hits the plane of the metal differently, affecting a sin(angle) term in the "flux". (Magnetic field parallel to a plane would have a term sin(0) = 0 = zero flux)

When the flux would change, currents flow in a circle within the metal so that perpendicular to them, flux is created; the currents increase until the flux is preserved for for whatever falling angle we're at. Remember that Lenz's Law says these currents are created whenever they can prevent the change of flux, and it is gravity which is enabling the change of flux, so the currents try to counteract the force of gravity. Gravity only succeeds to the extent that the currents meet resistance in the metal. If, for example, the aluminum plate had a lot of long slits in it, the eddy currents would be suppressed, and it would fall much faster.

Anonymous said...

It's under "Lenz's law" in Wikipedia.

Anonymous said...

it's lorenz, not lenz

Anonymous said...

it's lorenz, not lenz

Anonymous said...

Lenz Law, not force. there is a difference. the force is the induced electromotive force (emf).

James Harrison said...

I believe the correct term is 'Lorenz Force'.

Anonymous said...

You, sir, fail at google. http://www.google.com/search?q=lenz+force

Anonymous said...

These people are stupid, they meant Lorentz force. There is no such thing as "Lenz" force.

eclectro said...

http://en.wikipedia.org/wiki/Lorentz_force

I believe what is referred to is the Lorentz force. Even though it is Aluminum, it still is conductive and the magnetic force induces a current in the surface of the block which is going to magnetically oppose the force of the MRI. Not really a whole lot to do with gravity other than the fact it pulls the block downward - but more slowly because of the magnetic dynamic taking place in the MRI.

Jess said...

Anonymous:

Here's the article on Lenz force. Magnetism isn't affected by optical lenses.

Aluminum is non-ferromagnetic, which means that it is not attracted by magnetic fields at all. This particular phenomenon in the video is due to induced electromagnetism. To put it an easier way, aluminum is a conductor of electricity, and the motion of the block of aluminum through the MRI unit's magnetic field causes an induction current in the aluminum. The flow of electric current in the aluminum creates a magnetic field around the block, which is what interacts with the magnetic field of the MRI. Ever made an electromagnet with a nail, some copper wire, and a lantern battery? It basically works on the same principle, except in reverse. Magnetism can induce electric current, and vice versa.

Also, an MRI scanner doesn't have different "levels" of strength, they're either on or off.

Jess, your friendly neighborhood x-ray tech

Anonymous said...

http://en.wikipedia.org/wiki/Lenz%27s_law

apparently with a link to the present video (I didn't check).

Essentially, any moving circuit loop moving inside a magnetic field will present an induced current running on it.

Lenz's law says this current will generate an opposing magnetic field to the original one. Kinda like action and reaction, I suppose.

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