# Electric and magnetic fields

1. Apr 12, 2009

### kashiark

what are electric and magnetic fields? i know that theyre constant exchanges of virtual photons but what properties do each have and whats an example of a change of perspective where an electric field becomes a magnetic field and/or vice versa

2. Apr 13, 2009

### tiny-tim

Hi kashiark!

An electromagnetic field is a 6-component thing (technically, it's called a 2-form, but don't worry about that ) …

(Ex, Ey, Ez, Bx, By, Bz) …

the E components are called the electric field and the B components are called the magnetic field …

changing to a frame of reference with a different velocity mixes up all the components

(and the "virtual photons" are just a bit of maths used in calculations in perturbation theory)

3. Apr 13, 2009

### dx

Imagine a current in a wire, and you are standing still with respect to it. There are equal amounts of positive and negative charges in the wire (even though the negative ones are moving), so there is no net charge. This means that there is no electric field, but there is a magnetic field due to the current. Now if you start moving along the wire at constant speed, the Lorentz contraction of the charges creates a net finite charge density in the wire, and therefore there will be an electric field in this new frame.

4. Apr 13, 2009

### kashiark

ok that makes sense but what do the fields do?

5. Apr 13, 2009

### Bob S

First, the electric field E surrounds anything that has an electrical charge (Coulombs) on it. It can be anything from static electricity to a transformer terminal at a public utility distribution point, or a wire in your home. The electric fields are radial lines.
Second, the magnetic fields B associaled with currents (Coulombs per second) are azimuthal (circular lines) surrounding a wire carrying current.

In more advanced forms of E and H that do not require wires, they can be the components of an electromagnetic wave like broadcast signals from your local FM stations, or photons from a light bulb.

It is true that the E and B fields are coupled by virtual photons, but this concept is not necessary or useful until you get to extreme B or E fields. The E fields near nuclei for example involve virtual photons. The Coulomb field near a nucleus is not a pure a 1/r2 field but modified by virtual particles.

6. Apr 13, 2009

### Staff: Mentor

They exert forces on electric charges.

7. Apr 13, 2009

### kashiark

dont get me wrong this is helpful but im what im asking is what would someone observe when looking at a magnetic/electric field?

8. Apr 13, 2009

### tiny-tim

Nothing …

a magnetic/electric field can exist in a vacuum, and there's nothing there to see …

you can only see the effect of a field, not the field itself

(unlike wind, for example, where you can not only see the effect, of things being blown by it, but you can also see the moevement of air molecules that is the wind … no such thing for a field )

9. Apr 13, 2009

### kashiark

thats what i mean what are the effects?

10. Apr 13, 2009

### dx

The effects are forces on charges.

11. Apr 13, 2009

### cabraham

Two charged bodies mutually exert a force upon one another, which has been empirically observed and measured. However, this force is NOT instantaneous, i.e. not "action at a distance", but propogates at a finite velocity. The field concept was adopted in light of this finite time required for forces to be "felt", and the action at a distance paradigm was discarded.

If a charged body is at some point A, and another charged body is at a point B, and the B body is perturbed, displaced a small amount, the change in force at body A will exhibit a time delay. Although B moved at time 0, it's influence was not felt by A until a moment later.

In other words, fields propogate from charged bodies at a finite speed. When the field reaches a second body, the influence of the force is observed. Action at a distance, OTOH, assumes that the forces are instantaneous, requiring no propogation time.

Does this help?

Claude