# Moving charges and magnestism.

1. Jun 2, 2010

### middling

Current in a wire produces a detectable magnetic field. Of course the actual situation
is TWO counter-flowing streams of electrons and protons from any general
reference frame, with always a relative velocity equal to the "drift velocity"
of the physics textbook, of something on the order of meters per hour.

So would ONE stream of charge reliably produce an accessible magnetic field?
Imagine the electrons as a net electrostatic charge on a body being moved mechanically.
If the body starts moving relative to me surely,by the theory,a compass needle
in my handle will come "alive" and "die" according to how I track the body?

What is the appropriate lab experiment?

2. Jun 2, 2010

### Naty1

Current in a wire is the flow of electrons alone...loosely bound in a conductor, they are able to drift at relatively slow speed but produce electricty close to the speed of light at a very distant end....protons are too tightly bound in nuclei to move except in special circumstances.

A stationary charge measured by a stationary observer sees an electric field; if either moves, a magnetic field is also detected....

here is some explanation and further background: http://en.wikipedia.org/wiki/Electromagnetic_field#Dynamics_of_the_electromagnetic_field

I'm not sure what experiment you want to do, but if you build up a static charge on an insulator, say maybe rubbing it on a rug, pass it by and also hold it stationary adjacent to a mangentic compass....

lots more here: http://en.wikipedia.org/wiki/Static_electricity

Last edited: Jun 2, 2010
3. Jun 2, 2010

### middling

Thank you.

You say "if either moves"...

My question was, What if BOTH move similarly?

4. Jun 2, 2010

### Studiot

It's about relative motion. What Naty meant was that it does not matter if one od both charges move relative to each other a magnetic field will be detected. If their movement is such that there is no relative motion then it will not.

I do however suggest you review your ideas about what 'moves' both when a current flows and just due to general lattice vibrational energy.

5. Jun 3, 2010

### middling

Thank you.

That is a "yes" then to both my questions. But no reference to a scientific paper
which contains a practical demonstration and experimental proof?

A metre-length copper wire of 1 sq mm carrying a current of 3 Amperes contains
about 10,000 Coulomb of free electrons with a drift velocity of about 1 metre per hour.
Therefore one might want to test the magnetism around a body, carrying 10,000
Coulomb of net charge and moving at 1 metre per hour. The effects around a body,
carrying 3 Coulomb of net charge at 1 metre per SECOND should be similar. What is
the maximum static charge obtainable in an ordinary laboratory? Obviously, the
experiment would have to be conducted in vacuum as otherwise the voltage gradient
would likely exceed the breakdown threshold.

6. Jun 3, 2010

### Studiot

No the effect would not be quite the same.

A metre length of copper wire cannot carry 3amps in isolation. Current must be 'fed in' at one end from somewhere and 'withdrawn' at the other.
Further the charge is spread right along the length of the wire so the magnetic field is a series of concentric cylinders axial to the wire and of equal length to it.

In the case of a single moving body carrying charge the magnetic field will have the shape of a series of rings or annuli and moving with the body. Because the body must have finite extent the rings are actually likelly to be (very) short cylinders.

I am sorry I do not know of any reports of anyone having done this experiment, perhaps yopu could try it with a Van der Graff generator?

7. Jun 4, 2010

### middling

I do not think a standard Van de Graaf can hold that much static charge.

Interesting how difficult it is to find a witness to an obvious experiment,
to test a simple theoretical prediction.

I believe the manifestation of magnetic force, even though only electric charge
and not magnetic charge "exists", is conventionally explained in terms of a
relativistic correction. In the case of a drift velocity of 1 meter per hour the "gamma"
adjustment is easily calculated as:

5 x 10^(-25) (i.e. gamma itself = 0.99999999999999999999999995)

Hard to comprehend how such a small correction can make the diference between
a compass needle swinging and not swinging in my not very steady hand.