# Electromagnetic fields/waves

1. Jul 23, 2009

### Paper

Hello,
I'm confused about the whole electromagnet thing. So far I've got these concepts:
- Passing a current through a wire results in a magnetic field 90 degress to the direction of flow

- If the current is AC, the magnetic field is continuously alternating

- This magnetic field is considered to be a near field and can be used to perform inductive coupling

- In addition, this AC current causes the wire to radiate electromagnetic waves (radio frequency light). This is part of the far field

- If we alternate at the natural resonance frequency of a wire or coil, we set up standing waves in the transmitter

- And so the receiver needs only to be designed with the same resonance frequency, and an incoming electromagnetic wave of the correct frequency will cause resonance and standing waves in the receiver

Now my questions are:
* Is that all correct?
* What is the difference between a near field magnetic field and far field electromagnetic waves?

2. Jul 23, 2009

### negitron

Yes, you pretty much have it, in general.

The near field is close to the radiator such that the effect of an individual pole is strong, and this field falls off with the radius squared. The far field is some distance from the radiator such that the effects of opposite poles begin to cancel and the net field falls with the cube of the radius. There is no single, well-defined distance at which the near field gives way to the far field; it's a continuum.

3. Jul 23, 2009

### Paper

I think it's clicked. Forget the labels near field and far field, I'm getting them mixed I think.

Inductive Coupling depends on the magnetic field shifting electrons in the secondary coil.

I guess that receiving radio waves is a completely separate thing? What is the name for the effect in which a wave causes an alternating current in a wire? The Maxwell Effect? Or perhaps the Tesla Effect?

4. Jul 23, 2009

### Bob S

For a dc current in the wire, tere is a small electric field along the wire due to the resistance in the wire. If there is no resistance, there is no voltage drop in the wire and no electric field. The magnetic field from the current surrounds the wire (in circles). It is only azimuthal (no radial component). If you have an ac current, you will have an ac magnetic field. If the ac frequency is many megahertz, the wire will start radiating energy, like a transmitter antenna.