1. May 1, 2013

pkc111

What law is being obeyed by emr travelling out from a source eg a magnetic field does not get emitted from a magnet, why does a magnetic field get emitted (eg travel outwards at speed of light) from the alternating magnetic field present in a light bulb for example?

I would appreciate any help with ideas :)

Thanks

2. May 1, 2013

Simon Bridge

Electromagnetic radiation follows from Maxwel's equations, and is a consequence of the rules for how electricity and magnetism work. All that stuff about charged particles and currents.

You know that a changing electric field produces a magnetic field.
You know that a changing magnetic field can make an electric field.

So it is reasonable to suspect that one can manipulate an electric field in such a way that the resulting magnetic field will create an electric field that creates the same magnetic field and so on and on and the setup becomes self-sustaining.

It turns out you can do this - except that, at each "iteration", the new field has to be slightly advanced in space. i.e. the resultant form has to travel. (You can see how this works in a simpler context in cellular automata - look it up.) The (invariant) speed of the travel is a property of space-time. The speed in media is a material property.

The energy comes from whatever made the initial variation in the field ... per your comments, the magnet does not lose magnetism to make radiation, but someone had to shake it to make the B field vary. The energy in the radiation comes from whatever did the shaking.

In the light-bulb, the electricity makes the filament hot and the heat makes the light, and we are approaching QM. It will help to stick to the classical descriptions while you are getting used to how radiation works.

However - this is an area where the best description is the mathematics.
Words are not going to do it very well.

3. May 2, 2013

Bobbywhy

pkc111,
Simon Bridge has given a good verbal introduction to electromagnetic radiation. He's also correct to remind us that mathematics is the only thorough and complete method to fully grasp this phenomenon of nature.

4. May 2, 2013

Staff Emeritus

Because it has momentum.

5. May 3, 2013

mrspeedybob

Different people like different models. There's nothing wrong with that as long as the model you choose makes correct predictions. Personally, I never liked Maxwell very much. I find Purcell's model or the QED model to be more intuitive.

Here's a good explanation of Purcell's model of electromagnetism...
http://physics.weber.edu/schroeder/mrr/MRRtalk.html

And here's a very old, but very well presented explanation of QED...

Last edited by a moderator: Sep 25, 2014
6. May 3, 2013

7. May 4, 2013

Popper

I'd have to strongly disagree with that response because EM radiation has momentum because it moves. Not the other way around.

One can also visualize what EM radiation moves by considering what happens to a test charge as one moves it. The field meaured at a distance from the charge has a certain magnitude and direction. When the charged is moved then the magnitude and direction of the field must also move since the later configuration results in a changed magnitude and direction of the force on the test charge. This disturbance in the field propagates at a finite speed, i,e, the speed of light.

Regarding the following statements
It seems like almost all textbooks explain things like this. The problem is that you can't determine by experiment why a magnetic/electric field is produced. All that you can do is to say that where there is a time varying E field you will find a time varying B field and vice versa. I recall an journal article pointing this out. I'll see if I can dig it up and post the reference and hopefully it will have an abstract I can quote.

8. May 4, 2013

Simon Bridge

You mean, you cannot determine what, in particular, gives rise to the EM field?
The chicken and egg thing?

This is just a way of impressing the starting student with the existence of a self-sustaining solution.
You are right - one should not think of it as one after the other.
I usually think of that as the next stage in the thinking - but if you have had success getting these ideas across in one go - more power to you :)

9. May 4, 2013

Popper

I love it when that happens. :)

I've learned from many years of experience talking to people trying to learn physics that its best not to give them ideas for simplicity merely to get it across faster if its something that they'll have to unlearn in the future. It is my belief that it's much better to explain things correctly the first time even if its hard to grasp so that later they won't have to unlearn anything, which might be hard to do. For example; I can't count how many times I've had to explain to people that radiation is not pure energy. Just hearing that gives me the willies.

I even heard an old coworker of mine, who had an MS in medical physics, once answer the questrion "What is light" with "It's energy!" .... sigh! :)

10. May 4, 2013

Simon Bridge

There is a nasty habit in some quarters of giving glib "throwaway" answers in the name of getting a simple version of the idea across ... and this can cause problems later. The questions we get here from people watching Discovery Channel being great examples of the problems that arise.

On the other hand - it also leads to problems if we try to deliver too much in one go.
Established pedagogy favors a scaffolding approach - where simpler ideas are used to lead into more complicated, or just more complete, ideas. Students often have to throw away old scaffolds when they have outlived their usefulness.

So much fr being general.

The "feedback" description for how Maxwel's equations lead to radiation is one such. It is a small jump from iterative feedback idea to the simultaneous self-sustaining solution so, in this particular case, it is not something that is hard to unlearn. I have found it is a good idea to spell out, every now and again, that the short wordy answers are only simplifications to "get the idea" though - otherwise people tend to run with them just a tad too far.

11. May 4, 2013

Popper

Simon! M'man! I couldn't have said it any better myself.

I agree. I just ran into that in another thread. One person wanted to know the answer to a simple question so I answered it having learned the answer from reading and from a friend who is a well known expert in the field. I know GR fairly well but have never derived such a proof even if on exists. My friend told me that it was what most specialists in that field believe and not that it can be shown/proved. That makes a huge difference. So I posted what I was informed that they believe. Bothering with a derivation is a waste because I have other things I'm more interested in doing plus the OP wouldn't have understood the response because the math of GR is extremely difficult.

Thanks.

12. May 4, 2013

vanhees71

In my opinion there is too much didactics and too little common sense in teaching, particularly from the socalled professional didactics. In physics you should not teach old-fashioned outdated ideas, if the newest views are as simple or even simpler than the old ones.

One example is classical electromagnetism. There you have a quite simple set of equations for the fundamental issues of charges, currents, and the electromagnetic field. These are the Maxwell equations in differential form. Of course, you have to learn vector calculus first, but that can be done very well with the right physics pictures at hand, and electromagnetism is a great arena to learn also these mathematical tools.

This view is underlined if you look into the history of physics. One of the most successful (theoretical) physics teachers ever was Arnold Sommerfeld. I guess, it would be difficult to find someone else with more Nobel-prize winners under his or her students than him. If you read his textbooks on classical theoretical physics, you see that they were approaching the problems with the right mathematical tools and without unjustified "simplifications" but explaining the problems and their solutions in a very clear and concise way. It's an example for a very fruitful unity of research and teaching (the Humboldt ideal of university pedagogics).