Weird result/problems with magnetism experiment

[JonoF]

Hi there,

I have just set up / taken measurements from an "Einstein de Haas" experiment I set up, but there were a couple of weird effects I cannot explain nor figure out. FWIW, I am competing in the "International Young Physicists' Tournament" in Switzerland this July - should be a great experience.

Anyways the experiment involved setting up an iron rod, suspended vertically by a wire, in a vertical magnetic field (would the field created by your average classroom size solenoid be worthy of being called "uniform" ?). The magnetic field aligns the domains created by the electron spin, and the change in angular momentum of the electron spin results in a torque being applied on the iron rod causing it to begin to rotate.

Modelled as a torsional pendulum, with the angular displacement measured by way of a laser beam reflected off of a small piece of mirror on the iron rod, we see that the oscillations start off with a maximum amplitude, which slowly dies away to nothing.

HOWEVER <insert weird music> this is where the first weird effect occurs. The laser beam settles on a point slightly to the side of the equilibrium position. It returns to equilibrium position when the magnetic field is switched off. To me, this implies that there is a constant torque applied by the magnetic field on something (perhaps the now-aligned magnetic moments), which is equal to the restorative torque from the wire suspending the iron rod. BUT I do not know from where this torque arises. I cannot find any mention of a torque that would act on the axis of the magnetic field line. Perhaps I am being foolish and overlooking something simple, but any... ANY! input on this would be appreciated.

And now for another weird effect... it would appear that exactly the same thing occurs when the direction of current flow is reversed. I would have expected that the rod would begin to rotate in the other direction, but it seems to be exactly the same... Tres bizzare.

Like I said, any help would be appreciated so much Getting late and I am one confused Kiwi right now.

Cheers
Jono

 

[pmb_phy]

The magnetic field aligns the domains created by the electron spin, and the change in angular momentum of the electron spin results in a torque being applied on the iron rod causing it to begin to rotate.

That is not how ferromagnetism works and that is the only magnetic effect here. Think of this, not as due to electrons spin, but due to the electron orbiting the nucleus.

As far as the rest goes - Not sure. First guess - How much is it off by? Have you measured the directions of the magnetic field of the solenoid and iron rod? If the rod is initially unmagnetized then it will become magnetized when you turn the field on. When you turn the field off the rod will still be magnetized but not as much. This effect is called "hysteresis." Seems to me that you're assuming a perfect rod and perfect solenoid. If anything is non-perfect then there's no reason to assume that the deflection you speak of should be zero.

Pete

 

[JonoF]

Assuming a perfect theoretical rod, and looking only at the magnetic moments due to electron spin, my practical answer is about 500 times larger that the theortical calculations.

Everything I have read about ferromagnetism says that the effect of the moments due to orbital electron 'motion' is practically insignificant compared to the effect of the spin moments.

It's off by so much that it just doesn't seem like small imperfections in the set up of the expt.

Such is the confusion of physics hehe.
Looks like another all nighter as I attempt to *cough* get creative with my results.

Cheers for the input Pete

Jono

 

[Gokul43201]

Assuming a perfect theoretical rod, and looking only at the magnetic moments due to electron spin, my practical answer is about 500 times larger that the theortical calculations.

Everything I have read about ferromagnetism says that the effect of the moments due to orbital electron 'motion' is practically insignificant compared to the effect of the spin moments.

This is true. For most transition metal (including iron) compounds, the orbital contribution to the magnetic moment is negligible compared to the spin contribution (at moderate fields).

The residual torque may arise from off-centering the rod in the solenoid (thereby allowing non-vertical components of B to be non-zero). You might be able to reduce this effect by fixing the midpoint of the rod (midpoint of the length) close to the midpoint (of the length) of the solenoid. This way, non-vertical components may average to zero if the rod is coaxial with the solenoid.

Let's see your calculations, and perhaps someone might spot the error.

 

[pmb_phy]

Everything I have read about ferromagnetism says that the effect of the moments due to orbital electron 'motion' is practically insignificant compared to the effect of the spin moments.

Can you give me an example? Here's what I'm going by at the moment. From Modern Physics, Bernstein et al, 2000, page 26-27

The most important class of magnetic materials is known as ferromagnetic; these materials produce a magnetic field on their own. The source of the field is the alignment of atoms, which, by virtue of the current loops formed by the circulating charge of the atomic electrons, themselves act like tiny magnets.

I'm still quite unclear on this experiment. Are you starting with a magnetized iron rod?

Whatever you're doing - If it was me doing this experiment then I'd run the experiment and then switch the rod around so the ends are switched. You seem to be implying that there is something theoreticalaly screwy going on rather than something experiementaly screwy. You say "assume its perfect" and yet it isn't logical to do so. You're speaking about an actual experiment here and observation beats theory in all cases. How do you know its perfect? For all you know if it was actually perfect then the effect you mention may not be there. Switching the rods end over end and re-running and observing the result may be helpful.

It's off by so much that it just doesn't seem like small imperfections in the set up of the expt.

I was having chest pains for most of the day due to a problem I have with anxiety (or a heart condition). And now you tell me that after you tell us that you observer a ..point slightly to the side of the equilibrium position.. and now you say its off by so much? - You've increased the pain. Its all your fault. Therefore you must get more anti-anxiety drugs to my house asap! Those are the fun kind of drugs!

Pete