# E and B fields and Electromagnetic radiation

1. Oct 24, 2013

### kasap

Hello,

I am having a lot of trouble in determining the difference between an electromagnetic field and electromagnetic radiation. Is an electromagnetic field caused by electromagnetic radiation within the space of which the field acts over? However if I am right a point charge that is not moving has an electric field but emits no radiation, if this is the case then how can it exert a force on another charged particle that is also stationary within a certain distance without the means of electromagnetic radiation for their interaction?

Also what actually happens when two fields meet, e.g with two non-stationary positive point charges, what goes on in terms of the fields/radiation that makes them both feel a force of repulsion? (Are there any theories which could explain this other than boson exchange?)

Sorry if this isn't very clear, thanks in advance!

2. Oct 24, 2013

### mikeph

Boson exchange?

Go back to the fundamentals, if you only just found Maxwell's equations then I wouldn't worry about quantum field theories just yet.

Electromagnetic radiation is just a particular manifestation of the electromagnetic field.

3. Oct 24, 2013

### Crazymechanic

Well like mikeph said the em field has various ways of intraction also the eletric and magnetic side of the field is somewhat trivial if you want to understand it and it usually comes later with Special relativity.Because if you have a point charge it has a electric field and if your other point charge travels past that first point charge at an angle (perfectly at 90 degrees aka perpendicular) then the traveling point charge experiences a magnetic field.That's for short.

A point charge with just an electric field emits no radiation radiation is usually described in EM as something that has frequency , in other words something has to oscillate for it to have frequency , either the strength of field must vary with time or some physical object that has some charge has to move physically , well there are different ways but it's easy to remember that something oscillates when there is EM radiation.
the term which described how fast something oscillates is frequency , in EM you can have starting from very low , a few hz say up to the speed of light.Which is the shortest wavelength aka the highest frequency.

4. Oct 24, 2013

### Khashishi

Electromagnetic radiation is the part of the electromagnetic field that propagates and carries energy and momentum. If you do a multipole expansion of the sources, you might be able to separate the static part from the propagating part. A stationary magnet has a magnetic field but doesn't emit radiation. If you spin the magnet, you have a static part of the field and also radiation. The radiation intensity drops off as 1/r^2, but the static field drops off as 1/r^3 for this case (of a magnetic dipole), so the radiation is more important at far distances.

5. Oct 24, 2013

### WannabeNewton

The energy-momentum of an electromagnetic field is a property of the field itself. The energy-momentum carried by a radiation field (a null field if you want to think of it in terms of SR) is a specific case.