Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 08/10/2010 10:07:09
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I was asked today by someone what would happen to a block of aluminium placed inside an AC coil? How would this differ from iron?
Anyone know?
Chris
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There used to be a nice demonstration piece in the kids section of the London science museum where an Aluminium plate was sat above a transformer core that you could switch on and make it float.
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Iron would be attracted to the coil - it is a magnet, and incidentally get warm, as you would be reconfiguring its domains all the time which is lossy.
The aluminium is non-magnetic, so the only effects are electromagnetic. The changing magnetic field will induce voltage and therefore current in the aluminium forming an electromagnet which will repel the coil.
The eddy current experiment is very related to this.
http://www.thenakedscientists.com/HTML/content/kitchenscience/garage-science/exp/mysterious-forces-eddy-currents/
In fact you could get the aluminium to levitate as the changing field keeps putting energy into it to overcome resistive losses, and it would levitate like a superconductor.
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I was asked today by someone what would happen to a block of aluminium placed inside an AC coil? How would this differ from iron?
Anyone know?
Chris
Chris, the way I read your question is that there is an induction coil already energized and you want to know if the aluminum will have some kind of effect when placed in this field.
This can be simulated using an induction stove top.
Since aluminum is non ferrous, the stove manufactures are right up front with this issue.
Aluminum cookware will not work, you need cookware with iron content.
You can take aluminum wire to create a field, but you need an iron core to make the field effective.
"An "interface disc" can be used to make induction stoves work with aluminum and copper pots. This is a flat steel disc. The induction stove heats up the disc, which becomes a conventional hot plate."
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Thanks everyone; the answer I gave was what Dave said above (so I'm relieved!) - that basically a back-EMF occurs in the aluminium, leading to repulsion against the magnetic field.
At school, for an A Level physics project, I designed an experimental system to "brake" a spinning aluminium disc using a controllable elecotromagnetic field. It was a nice demo of electromagnetic braking; the only problem was that the electromagnets kept getting too hot, which affected the resistance in the wire I was using and this made my current graphs go all over the place at high braking powers...
Cheers
Chris
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You can also observe the effect with a rotating permanent magnet. Until electronic instruments became common in cars, speedometers operated on that basis.
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I was asked today by someone what would happen to a block of aluminium placed inside an AC coil? How would this differ from iron?
Anyone know?
Chris
Chris your original question leads one to believe that the Aluminum block was in a static position in a magnetic field.
Now you decide to place it dynamically spinning in a magnetic field.
Is this tailoring the question to fit the answer?
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If you want I will provide the link... Quoted
"590. How does a magnetically levitated transit vehicle work? — LB, West Palm Beach, FL
Although there are a variety of schemes for magnetically levitating trains, perhaps the most promising is a technique called electrodynamic levitation. In this scheme, the train contains very strong magnets (probably superconducting magnets like those used in MRI medical imaging systems) and it travels along an aluminum track. The train starts out rolling forward on wheels but as its speed increases, the track begins to become magnetic. That's because whenever a magnet moves past a conducting surface, electric currents begin to flow in that surface and electric currents are magnetic. Thus the moving magnetic train makes the aluminum track magnetic. For complicated reasons having to do with electromagnetic induction, the track's magnetic poles are oriented so that they repel the magnetic poles of the train—the two push apart. While the track can't move, the train can and it floats upward as much as 25 cm (10 inches) above the track. Once the magnetic forces can support the train, the wheels are retracted and the train floats forward on its magnetic cushion. To keep the train moving forward against air resistance (and a small magnetic drag force), there is also a linear electric motor built into the train and track. This motor uses additional electromagnets in the train and track to push and pull on one another and to propel the train forward (or backward during braking)."
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At school, for an A Level physics project, I designed an experimental system to "brake" a spinning aluminium disc using a controllable elecotromagnetic field.
My old Garrard 401 turntable (I think it's still up in the loft somewhere, with its SME3009 arm) used a magnetic brake for varying the platter speed. It used a permanent magnet in conjunction with the aluminium brake disk though, the the speed control knob just moving the permanent magnet in or out across the radius of the aluminium disk to vary the degree of braking.
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BTW - The effect is not limited to aluminum. Any conductor will exhibit similar behaviour.
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Yes, that's a good point Geezer.
Chris