Physics 2000 Science Trek Electromagnetic Waves

Loop of Lines of Force

So why do we bother to visualize lines of force at all?

They give us other useful information. Let me show you one example. Suppose the vibrating electron is near a fixed unmoving proton, like this:

This is more realistic than having a proton far from a vibrating electron because it is hard to pull apart unlike charges over any large distance.

So what do the lines of force look like if we combine the wiggling line from the vibrating charge with the stationary line from the opposite charge?

We now get loops of electric lines of force which move away from the charges along rays.

Loops? Where do they come from?

Well, it's a little like blowing bubbles. When the vibrating electron is above the stationary proton a line of force looks like this:

When the charges are very close together the loop closes on itself and moves out.

Finally, when the electron is below the proton another loop is emerging with forces in the opposite direction.

Isn't that the way an antenna works?

That's right, Kyla, only it's not just one charge moving relative to another in an antenna, but a whole bunch of electrons moving -- a current. It looks something like this --

That would make a great movie!

You bet -- we can look at it right now.

Is there any way we can tell how strong the electric force would be on those lines of force in the movie? I'm trying to visualize that.

Actually, there is a way. Wherever the lines of force are closely packed the electric force is strong. Where they are far apart, the electric force is weak, or even zero.

What are those other loops in the movie? The ones encircling the antenna like this:

Those are magnetic field lines -- the magnetic part of the electromagnetic wave.

Oh -- something different. Tell me about the magnetic field part of an electromagnetic wave.