[Hannah Cartojano]

Hannah Cartojano asked the Naked Scientists:
   
I really need an electromagnet that can pull a very heavy metal from above (which means the electromagnet has to be very strong and very concentrated in one direction), but also an energy-saver or uses little amount of energy 

Guys , I need some helpful comments right now . Please help !!!!!

What do you think?

[Geezer]

I think the only way to do that is to make the coils out of a conductor that has extremely low resistance, and that likely means you will have to use some type of superconducting metal, or metal alloy, and cool it to an extremely low temperature.


Hmmm?? Why do I have a bad feeling I just answered an assignment question.

[CliffordK]

There are many calculations for electromagnets which depend on the number of coils and the tightness of the coils.  And, the amount of resistance depends on the wire size.  Copper is an excellent conductor and there is no need for more exotics (except in the superconductors, but then you also have to account for cooling.  Most NMR & MRI magnets are supercooled.

However, your power consumption should be more or less directly related to your magnetic field.  One of the variables is the distance between the field and your material which needs to be minimized.

What I would do is at least use the core being a very big rare earth magnet.

The most common are Neodymium (Nd₂Fe₁₄B) magnets. 

Then you just need to figure out how to break the magnetic field. 

There is a type of magnetic base stand that you can buy for laboratories that are really cool.  With virtually no effort, you can shift from a relatively strong magnet to virtually not magnet.  Here is a description on Wikipedia.

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

I think there are other methods to break magnetic fields too.  Airport sweepers often use permanent magnets and have a way to release the debris.

[RD]

http://www.thenakedscientists.com/forum/index.php?topic=33359.0

[Geezer]

There are many calculations for electromagnets which depend on the number of coils and the tightness of the coils.  And, the amount of resistance depends on the wire size.  Copper is an excellent conductor and there is no need for more exotics (except in the superconductors, but then you also have to account for cooling.  Most NMR & MRI magnets are supercooled.

However, your power consumption should be more or less directly related to your magnetic field.  One of the variables is the distance between the field and your material which needs to be minimized.

What I would do is at least use the core being a very big rare earth magnet.

The most common are Neodymium (Nd₂Fe₁₄B) magnets. 

Then you just need to figure out how to break the magnetic field. 

There is a type of magnetic base stand that you can buy for laboratories that are really cool.  With virtually no effort, you can shift from a relatively strong magnet to virtually not magnet.  Here is a description on Wikipedia.

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

I think there are other methods to break magnetic fields too.  Airport sweepers often use permanent magnets and have a way to release the debris.

Hey! No fair. The OP said "electromagnet"  []

But I like your idea!

BTW, the resistance is a function of wire size, but I think it's worth mentioning that the power consumption, and field strength, end up being largely a function of the total cross sectional area of the required coil, whether you make it out of a few turns of really thick wire, or a lot of turns of thin wire.

[CliffordK]

Hey! No fair. The OP said "electromagnet"  []

A permanent magnet is an electromagnet...  it is just at a subatomic level   []

The door magnets sounded interesting.

I was thinking more of a junkyard magnet.

I am curious about the watts per pound, lift. 

[Geezer]

I am curious about the watts per pound, lift. 

We should be able to translate the lifting capacity into field strength by cheating a bit and thinking of the pull a solenoid would exert on a steel plunger. If we know the field to produce one Newton, we can get to the power to produce a force of 1N from the required ampere-turns and the resistance of copper.

[Bored chemist]

"I am curious about the watts per pound, lift. "
Zero

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

[Geezer]

"I am curious about the watts per pound, lift. "
Zero

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

Only if you ignore the power used to keep it cold.

[Bored chemist]

"Only if you ignore the power used to keep it cold."
I keep mine in deep space which solves the problem.
A friend of mine is part of a slightly more advanced civilisation than ours. While we only have "high temperature" superconductors that work up to 133K, they have ones that work at room temperature. The bastard won't give me the recipe- something about some "prime directive".

Anyway, from the definition of the amp we can show that the force is proportional to the current but independent of the voltage. If you get the resistance arbitrarily low by using really thick wires then you can get as close to zero Watts per pound as you like. It's just that the magnet might be rather silly.

[CliffordK]

Anyway, from the definition of the amp we can show that the force is proportional to the current but independent of the voltage. If you get the resistance arbitrarily low by using really thick wires then you can get as close to zero Watts per pound as you like. It's just that the magnet might be rather silly.

It is more complicated than that.
The more loops you make, the more powerful the magnet.
The tighter the loops, the more concentrated the magnet.
The closer the loops to the surface, the more effective force at the surface (I think).
The longer the wire, the more resistance.

Anyway, somewhere there are ways to calculate the force and resistance.

[Bored chemist]

This bloke
http://en.wikipedia.org/wiki/Francis_Bitter
 did a lot of work on the question but, when push comes to shove, resistivity is king. If you drop that then you change the Watts per pound.

[syhprum]

I think that if you want the magnet to lift scrap you should use iron with a fat hysteresis curve so that that it has a high degree of magnetism after the power is switched off.
Of course to dump the load you would need a short pulse of reverse current to demagnetize it but I think as more time will be spent getting it into position than actually carrying a load the overall power consumption will be reduced.

[CliffordK]

I think that if you want the magnet to lift scrap you should use iron with a fat hysteresis curve so that that it has a high degree of magnetism after the power is switched off.
Of course to dump the load you would need a short pulse of reverse current to demagnetize it but I think as more time will be spent getting it into position than actually carrying a load the overall power consumption will be reduced.

Actually, I was wondering about taking a strong permanent magnet.  Then whether one could use a reverse polarity to dump it without harming the underlying permanent magnet.

[Geezer]

when push comes to shove, resistivity is king.

I think that's the problem, and because of that you can't escape from the geometrical limitations. It's the total cross section of the amount of copper that gets you. It does not matter whether it consists of a few turns of very thick wire, or a lot of turns of very thin wire, the power dissipated by the coil will be pretty much the same (the packing of the wires and the insulation on the wires can make a little difference of course.)

The reason for this is that it's all about current as you say BC, and there's no escape from I²R