# Electromagnetic wave

1. Oct 21, 2012

### Cryptocatron

I can't represent me mentally what is an electromagnetic wave.

I know that a mechanical wave is the propagation of a disturbance in a material medium and mentally I can see what it is. But electromagnetic ....

Last year, I did electromagnetism. We studied Maxwell equations and we saw a variation of the field E => B field variation but we never spoke of electromagnetic waves.

what transport an electromagnetic wave? Energy without matter like a mechanical wave? should we see this as a particle (wave-particle duality)?

i'm currently studying quantum mechanics. But before I get into the wave functions, it is best to know what we are talking about before ...

Thank you for your answers which I hope will steer me well

2. Oct 21, 2012

### AJ Bentley

If you know about the electric field and the magnetic field, you know that neither is made of matter, yet they can both transmit a force.

In fact, matter (solid objects) actually interact with each other, not by actual contact of the atoms, but though the repulsive electrostatic force between their orbiting electrons.

So matter isn't more 'real' than electric/magnetic fields.

It's difficult to get a grasp on these ideas, the best you can do is to build mental images of models in the 'real' world that behave a bit like the thing you are trying to study. That's particularly hard with em waves because there is nothing quite like them in ordinary experience. The idea of a 'transverse' wave is hard to grasp - the nearest you can get is the motion of waves on a string - but that's only one-dimensional.

An electromagnetic wave forms when there is a sudden change in an electric or magnetic field. The change of the E field gives rise to a B field nearby As the B field rises and collapses, it gives rise to a new E field which generates another B field and so on. There is no friction involved so the disturbance propagates outwards like ripples on a pond.

Quantum mechanical waves are even more difficult to picture because they vibrate in the complex plane (whatever that means!) Don't confuse QM waves with EM waves. A lot of people make that mistake. Apart from both being waves, they are as chalk and cheese. They have nothing to do with each other.

3. Oct 21, 2012

### sophiecentaur

Remember, if you want to set up a magnetic or electric field, that takes Energy. It takes time to build up. There will be a delay in the field you set up 'here' and the resulting field you will get 'over there'. That delay will give you a wave behaviour when you start with an alternating field 'here' and look at the way the wave 'over there' varies with time.
As it happens, the magnetic and electric fields are at right angles to the direction that the wave eventually travels (after it has settled down) and, also, the two fields vary 'in phase' with each other.
You don't need a medium for an electric or magnetic field to exist - hence EM waves can propagate through empty space.

4. Oct 21, 2012

### Studiot

Well you do need the (empty) space. Whatever that means.

Oh and welcome to Physics Forums Cryptocatron. Are you some kind of robot feline?

5. Oct 21, 2012

### sophiecentaur

OMG
It's the way he tells 'em!.

6. Oct 22, 2012

### infomike

So, does that mean that the 60 Hz alternating electric field in my home wiring is emitting EM radiation into the matter and space in my home?

7. Oct 22, 2012

### davenn

yup indeed,

the radiated RF is easily picked up on a AM radio amongst other things

Dave

8. Oct 22, 2012

### infomike

But the AM radio band is in the KHz range. How can 60 Hz be picked up by the antenna?

9. Oct 22, 2012

### davenn

because there are lots of harmonics that go higher up the RF spectrum and the radio can pic up some of those harmonics.
thats the main reason

another reason would be that because the radiated RF of the 60Hz is so strong, the sensitive receiver in the radio just gets overloaded with the signal when the radio is brought near the power wires.

Dave

10. Oct 22, 2012

### infomike

That makes sense.

I'm guessing that's why electric current can conduct to the ground, since that 60 Hz electric field is transmitting into the earth. Hmm...so if I am touching the ground, then I am also part of that alternating field. No wonder you get shocked if you touch the hot wire!

11. Oct 22, 2012

### Drakkith

Staff Emeritus
No, that is purely due to the electrical circuit. The EM radiation is not transferring a current. In an AC power circuit in a house the live wire comes in from the power lines and runs back out to them, completing the circuit. If there is a short or you grab a live wire and short yourself, that's when the current gets out of the wire and into something it shouldn't be in.

12. Oct 22, 2012

### infomike

Right, but isn't the EM field necessary to create the potential difference (voltage) so that the current can flow? The EM field in the hot wire is out of phase with the EM field in the neutral wire/ground, which creates the potential difference allowing current to flow.

13. Oct 22, 2012

### davenn

No, its the flowing current between a potential difference that produces a radiated EM field

Think of a lightning strike .... Huge potential difference builds up between the cloud and the earth. no EM field present. Then the lightning discharges and the current flows, a HUGE em field is created that radiates for many 100's of km's

Dave

14. Oct 22, 2012

### infomike

So which came first, the chicken or the egg?:tongue:

In order to get current to flow, you must first have an electric field. The way I learned it, the generator produces an electric field (EMF) by its turning through a magnetic field. This alternating electric field is transmitted through wires, which produces the electric current that flows into your home. Now I'm guessing that alternating EMF is different from the electromagnetic wave produced by the subsequent current? So maybe the alternating EMF is more confined to the conducting media whereas the EM waves radiate everywhere?

Lightning is different because there is no alternating field (which you need for a EM wave), but a direct electric force field is built between earth and the cloud. As you said, that is not an EM field.

Last edited: Oct 22, 2012
15. Oct 22, 2012

### Drakkith

Staff Emeritus
He means there is no changing field to produce the EM waves until the current flows.

16. Oct 22, 2012

### davenn

thanks Drakkith :)

yes to clarify ...
you can have a large ELECTRIC field build up thats NOT and EM field
for an electromagnetic field to form there must be an oscillation....
in a lighhtning strike it is many microsecond pulses back and forward between the cloud and the ground

in an electronic circuit it can be the rising and collapsing of the electric and magnetic fields around a coil ( inductor) as a DC voltage is applied and then removed or an AC voltage is applied that has a positive and negative cycling of its voltage

cheers
Dave

17. Oct 22, 2012

### davenn

incorrect, its no different .... see my previous post :)

Dave

18. Oct 22, 2012

### infomike

So, now the issue that still remains for me is the difference between the alternating field (EMF) produced by the generator/transformer that creates the current and the EM waves that are generated after the current is created.

19. Oct 22, 2012

### davenn

OK Im wondering if you dont have a clear understanding of what an EMF is ?

I like Jim Hardy's explanation on this forum earlier in the year......

An EMF is not a field

Dave

20. Oct 22, 2012

### infomike

Okay, from what I've just read, EMF is not a force, either! The confusion being that EMF can stand for "electromotive force" or "electromagnetic field" Sheesh.

It was always clear to me that it was an electric force, the kind produced when a wire moves through a magnetic field in a generator. I thought that the electrons in the wire moved in response to this electric "force." I guess I incorrectly believed that this was a force. I guess I got it wrong! So what makes the electrons move if there is no force?!

Last edited: Oct 22, 2012
21. Oct 22, 2012

### davenn

Last edited by a moderator: May 6, 2017
22. Oct 23, 2012

### FredericGos

I was watching the first episode of one of the susskind lectures on quantum mechanics the other night, and he said something that he calls his 'sermon'. Basically he said that people should stop trying to visualize advanced physics since it ultimately gets you into all kinds of trouble.

Just learn to trust the maths and you'll be fine. Nobody can visualize these things, since our brain is prejudiced on the everyday world we can experience. I think this applies here.

/Frederic

23. Oct 23, 2012

### infomike

I looked through your links in order to refresh my knowledge of EMF and noticed this:

"Formally, EMF is classified as the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. By separating positive and negative charges, electric potential difference is produced, generating an electric field. The created electrical potential difference drives current flow if a circuit is attached to the source of emf."

This was the electric field to which I was refering in my previous post. Yes, EMF is a not a force, but a measure of electrical potential. Therefore, there MUST be an electric field of some kind, which alternates in wires in the electric grid. Granted, this may not be electromagnetic waves, but my question remains:

How can we distinguish between the alternating electric field which CAUSES the current flow from the electromagnetic waves that are CAUSED by the current?

24. Oct 23, 2012

### Drakkith

Staff Emeritus
You cannot. When you are near a circuit with alternating current, both the EM waves and the EMF are indistinguishable. It is only once you are away from the circuit that EM waves are able to be distinguished.

http://en.wikipedia.org/wiki/Near_and_far_field

25. Oct 23, 2012

### sophiecentaur

If you want to associate EMF with a field then where do you want this field to be? Is it between the battery terminals is it between the terminals on the motor or heating element? Field is measured in Volts per Metre and, if you want to discuss fields in a circuit, you would need to consider the size of the battery, for instance. The field across an AA cell would be different from the field across a D cell.
You could insist that it's fields that cause individual electrons to move about but each electron is subjected to a different field. True in principle but do you really think it helps to analyse what goes on in a circuit by considering every microscopic bit of every piece of material it's made of? Potential Difference deals so nicely with it all so why not use it?

Why should you want to? It's just a matter of chicken and egg. The two go together. Neither is the 'cause' and neither is the 'effect'.