How to increase the distance of effective reach of a magnetic field from an electromagnet

Question

How can I effectively increase the distance in which some electromagnet can attract a coin (for the sake of simplicity - coin).

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Now to show that I've done my homework: According to Ampere's Law, the strength of the magnetic field is directly proportional to the number of coils. Therefore, more coils (of some sort) right? How about pressing the coils by virtue of hydraulic press? Wouldn't that allow me to coil more wire at less space? The denser the coil is the better eh?

Also the law tells us that more current will also increase the power of the electromagnet. Well I have 4 3000 mAh 20A/3.6V li-ion LG batteries, are they sufficient? How about using OP-amps? I am not afraid of draining the batteries down quickly.

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Best answer will explain why electromagnets loose strength over distance by the cube of the radius distance from the magnet (as corrected by @Trevor), ways to increase the distance, possibly up to the length of a half hand and explain how to measure what can one achieve, given number of coils, current, coil density etc...

Answer 1

Although air can be magnetized, its relative permeability (\$\mu_r\$) is defined as 1 — about the same as copper and aluminum — which is about as weak as you can get. So it does not conduct magnetic flux very well compared to iron (\$\mu_r=5000\$) or annealed iron (\$\mu_r=200k\$).

In order to have a high pull strength, the gap must be very small or the number of enamel insulated turns must be very large.

Fortunately, our bodies are not very magnetic except to very high >4 Tesla levels which can be used for imaging of excited electrons spinning out of orbit. Where I once lived they developed a portable scanner for putting a patient inside the tube in an operating room that did not have metal in it. They can distort the images of old CRT's under a 10 story building and accelerate a metal chair across a room up highway speeds towards the cryogenically cooled coils. But that's how MRI's work.

Unfortunately Op Amps do not conduct much current needed to make an electromagnet. But beware that when you open the circuit, the inductor stores the energy which must be released at the contacts creating a huge voltage. \$V=L\frac{di}{dt}\$ and \$dt\$ can get really small with an open wire. For flux calculations: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html#c3

Now they are are trying to charge mobiles wirelessly and even EV's (electric vehicles) with 4kW at present, 12kW in labs and 18kW in future. In order to do this impedance matching sending and receiving coils need to be designed to minimize leakage and have a diameter at least 2x to 4x the gap.

Less is more, as in flux coupling.

Answer 2

You can shape magnetic fields, but to do so you will in all probability need multiple electromagnets.

The typical magnetic field around a single non-shaped center is like this:

By placing multiple (say N pole) together you can increase the central projection of field ....there is an interesting patent on this which uses 5 electromagnets to do this.

I've done it with Neodynium Magnets (which was much more efficient for me) to extend a static field from just a couple of inches out to a well detectable 12 inches. I used one central magnet and 3 at 120 deg slanted backwards around the central one. It's used to detect magnetic items passing between the magnetic projector and the sensing coil 12 inches away.

Answer 3

Choose your coin carefully.

Around 1987 in the UK, a large number of coin-op vending machines suddenly jammed.

In an excess of paranoia, vending machine designers "validated" coin inputs by a variety of techniques including a permanent magnet, presumably to stop engineering apprentices from filing down iron washers to the correct dimensions in order to get "free stuff".

This long established technique ran into trouble when, in 1987. the "copper" 1p and 2p coins were changed to a ferromagnetic composition to reduce cost.

Vending machines were quickly re-programmed so that everything cost a multiple of 5p, and the offending coins were rejected.

So, you can use British 1p and 2p coins, and the effect of magnetic fields on them will be much greater. (Testing a pocketful of change with a permanent magnet, I am seeing the same effect with "silver" 5p coins this side of 2013, though I don't know how far back the change goes.)

Answer 4

Nitrogen is slightly diamagnetic and oxygen is slightly paramagnetic. A strong enough magnetic field will repel nitrogen and attract oxygen. Air being a mostly non-ionized gas is electrically neutral and has almost no magnetic moment. Air's relative magnetic permeability is approx. 1.00000043 while Neodymium magnet has relative permeability of 1.05 and Ferrite (nickel zinc ferrite) 16-640, but air matters here.

One can place magnetic resonators to sustain magnetic fields, but not in any efficient manner. Also air's magnetic properties can be improved, as long as VERY small, dense and numerous fragments of some good ferromagnetic material travel through it, possibly molecules. Of course this won't make the air itself better conductor, just facilitate the electric charges movement per se.

Increasing voltage to some extent may cause other problems. So this shouldn't be really taken too seriously. The core of coils matters, but this will not serve a good purpose of increasing the distance significantly as well. As for "focusing" the magnetic field - one can take advantage of Quadrupole or pyramid-shaped ("pointy") magnets and see the magnetic field with one of these.