Magnetism and electricity question

Amps.....The magnetizing force H = .4*Pi * N * I /lm . In cgs units, H is in Oersteds, N is the number of conductor turns, I is the current in Amps, lm is the magnetic path length in cm.

Of course, voltage creates the flow of current by acting on the resistance or impedance by ohms law so you can't have Amps without Volts, but it is the current that creates the magnet.

Just to help you get a feel for the symmetry in electronics there are two type of fields utilized in electronics.

ELECTROSTATIC - Field strength varies with voltage and some other factors.

It is possible to have an electrostatic field without current such as between the plates of a charged capacitor (actually a tiny leakage current will usually flow).

Using a mechanical system as an analogy this is similar to a force with no movement such as pushing on an object that won't move.

MAGNETIC - Field strength varies with current and some other factors.

It is possible to have a magnetic field without voltage as in the case of a superconducting magnet.

Using a mechanical system as an analogy this is similar to movement with no force such as an object moving on a frictionless surface or in space.

As far as magnetic fields are concerned I once maintained a few MRI units. To "ramp up" or activate the superconducting magnet we connected it to a variable power supply capable of providing 40 volts at 400 amps and slowly brought the current up to 360 amps. Once the magnet was brought up to 360 amps a superconducting "short" was activated to allow the current to circulate only in the magnet. The power supply was then slowly brought back down to 0 amps again and turned off. The 360 amps would continue to circulate in the magnet indefinitely without any voltage.

In electronics we typically deal with systems the have both voltage and current and therefore consume power. Remember that power is used or produced in a system only if both voltage and current are present. P = V x I so if either V or I are zero then no power is consumed or produced.

Dorian

Volts have nothing to do with magnetic fields- okay, you can't have current without voltage, err in most cases anyways, so you usually need *some* voltage to get it moving. But magnetic field is always proportional to current.

Where (AC) voltage comes in is because XL = 2*pi*F*L and I = V / XL. Transformers, and well anything operating on Faraday's laws of induction, depends on a *change* in magnetic field to produce an electric potential.

To answer your question, volts alone will not produce a magnetic field, so by default the answer is amps.

Tim

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Bingo... Just what I needed.

Thanks.

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For a practical electromagnet running at maximum limits, the magnetic field strength is set by the core material and core shape, and by the wattage thermal dissipation, and by the grams of copper in the coil winding ...*not* by the current.

If you try to increase the current, you'll exceed the maximum wattage unless you replace your coil with one which has fewer turns of thicker wire, and runs at lower voltage and higher amps. But with fewer turns, the total amp*turns remains the same as before! So any attempt to increase only the current is useless.

To make a stronger field, you can add a core, or change its shape. Or you can crank up the wattage after installing a heat sink. Or you can add more copper to the coil (replacing the coil with thicker wire of the same no. of turns, or just adding more turns of the same gauge already there.) Then change the drive voltage to get back to max wattage.

Example: the number of turns are irrelevant because if max wattage is fixed at 10 watts, then 1 amp and 10 volts needs 10 ohms of wire, which might require 100 turns of thin wire. Or 100 amps at 0.1 volt would require one single turn of very thick wire to maintain 10 watts max. Amp-turns is 100 in both cases, so changing the number of turns can only re-program the volts-amps ratio of the power supply. It doesn't get you a stronger field.

All this just means that bigger, heavier electromagnets give stronger fields. And water-cooled electromagnets give stronger fields. But we already knew that.

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph425-222-5066 http//staff.washington.edu/wbeaty/

All other things being equal the best ansewer is "Yes".

Bye. Jasen

Boy, now there's a useful (for absolutely nothing) answer...

You seem to be good at those lately, Jaysen. Have you been practicing or something?

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Well, he did say that it was an "An-sewer", didn't he?

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Michael A. Terrell
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I'll grant you that point. (though I hadn't noticed it previously)

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Magnetic field is due to the relative motion of charge and an observer. There is no such stuff as "magnetic stuff". All E/M efects are due to photon momentum exchange between charges. So...

How do we get charge moving? Apply an accelerating potential, i.e. a voltage. It is thus the *voltage* or technically, an electric field, that actually "makes" the charge move, i.e produces a current, by way of:

F=qE=d(mv)/dt

Force is charge times Electric field (V/d), which is equal to the rate of change of momentum.

So... to get a bigger magnetic field, i.e. more charge flowing, you need a bigger voltage. Of course, this voltage always originates from other charge.

For a loudspeaker, the basic design principle is that the output is proportional to applied voltage, not current. The impedance of a speaker is all over the place with frequency, causing wide variations in current over frequency, yet the sound output response can be reasonable flat.

This can be summed up by the notion that everthing is controled by way of charge -> voltage -> current.

Kevin Aylward B.Sc. snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers on their knees, with their eyes closed"

Ahmmmm.

Kevin Aylward B.Sc. snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers on their knees, with their eyes closed"

...

Well, it's good to know that you see yourself as the rest of the world sees you.

Cheers! Bobo

Whichever gives you higher ampere*turns. Ed