# Magnetism and electricity question

Discussion in 'Electronic Basics' started by Don Bruder, May 31, 2006.

1. ### Don BruderGuest

Simple (seeming...) question that I can't find a simple, understandable
answer to. Everything I've managed to encounter so far has left me more
confused.

So let's try asking a group of humans:

When speaking of electromagnets such as a winding in a motor, or the
voice-coil of a speaker, and all other things being equal, which gives a
stronger magnetic field - higher volts, or higher amps?

2. ### Bob EldGuest

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.

3. ### DorianGuest

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

4. ### Tim WilliamsGuest

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

5. ### Don BruderGuest

Bingo... Just what I needed.

Thanks.

6. ### Guest

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

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

7. ### Jasen BettsGuest

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

Bye.
Jasen

8. ### Don BruderGuest

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?

9. ### Michael A. TerrellGuest

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

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

10. ### Don BruderGuest

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

11. ### Kevin AylwardGuest

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.

http://www.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"

12. ### Kevin AylwardGuest

Ahmmmm.

Kevin Aylward B.Sc.

http://www.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"

13. ### Bobo The ChimpGuest

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

Cheers!
Bobo

14. ### ehsjrGuest

Whichever gives you higher ampere*turns.
Ed