AC and DC as related to magnetic fields..differences?

[petemoore]

  Shoot ! Any comments welcome.
  Whappens magnetic field wise if DC is put through a coil?
  Or if you had 'AC' in the form of a slowly varying voltage, like a medium speed LFO of varying DC [sub-sonic], could that be put through a coil to make a magnetic field?
 
 

[R.G.]

Every time electricity moves through a conductor, there is a magnetic field generated, whether the electricity is AC or DC. The field is proportional to the current that flows. One way of precisely measuring the ampere is to measure the force on two parallel conductors. This is a freshman physics lab experiment.

DC makes non-varying M-fields, AC makes proportionately varying M-fields.

[george]

... but only a varying magnetic field created by a (AC) current flowing through a conductor will induce a current to flow through another (unconnected) conductor (eg an AC transformer), right?

... or AC flowing through a coil experiences inductance (caused by back-EMF?)

I'm guessing the only interesting or useful effect of EMF is caused by AC current ....

[R.G.]

... but only a varying magnetic field created by a (AC) current flowing through a conductor will induce a current to flow through another (unconnected) conductor (eg an AC transformer), right?

That's true - as long as there is no physical motion. A static - DC - field will happily induce current in another conductor if there is relative physical motion of the two. This is how all motors work, and most generators. Then there's the start up/down transients that can induce current. Induction is caused by any changing M-field. "Change" can be physical as well as magnitude or direction to get induction.

If you don't limit yourself to induction, there are other useful and interesting facets of a nonvarying field. There's the Hall effect. There's compasses, using the earth's M-field. There  are a number of saturation effects in transformers you can cause by DC offsets, including inductor distortion in wah pedals. There are magnetic amplifiers. There is magnetic levitation. We use DC M-fields for switchable magnetic clamps and holders in machine shops and for ferrous scrap moving in scrap yards. You can modulate electromechanical brakes made of iron particles in a chamber with DC. There is magnetohydrodynamics. Rail guns. Probably others that don't come to me right now.

In specific answer to the original question, DC in a coil makes a static M-field proportional to the number of turns, the amperes flowing, and the relative permitivity of the M-field path around the coil. And slowly varying DC as in an LFO does indeed make a proportional slowly varying M-field in a coil.

What can be done with that is limited only by imagination and Mother Nature's Rules.

[mac]

http://en.wikipedia.org/wiki/Maxwell's_equations

mac

[puretube]

then there was that "right hand rule"...

[the_random_hero]

then there was that "right hand rule"...

Or left hand rule, depending on what you were talking about   

[R.G.]

Right hand rule?

Oh, that's easy. Just remember "I before E, except after lunch."