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EM waves - energy transfer

by dayan83
Tags: energy, transfer, waves
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dayan83
#1
Jul14-09, 12:21 AM
P: 5
I have some basic doubts regarding how the energy is transfered (e.g. transmission of radio waves) as EM waves. As I know, the wave is not anything that can be practically seen in the space, but it is a model that we use to describe how energy is transferred. But if we think practically, how does actually the energy transfer through vacuum or a medium? In vacuum, as there's nothing inside, how does the energy transfer? I've read about 'radiation', but what is actually used for the energy transfer?
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Born2bwire
#2
Jul14-09, 03:51 AM
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In classical electrodynamics, nothing propagates the energy but the fields. The energy is stored in the time-varying electric and magnetic fields which are self-supportive and thus do not need a medium to propagate. The fields can give up their energy, among many ways, by exciting currents in a conducting medium. They can also create vibrations in polar molecules that create heat.

In quantum electrodynamics, the energy is propagated by photons, energy/momentum packets.
dayan83
#3
Jul14-09, 07:35 PM
P: 5
Thanks for the explanation..But I have some more doubts.

If we think of a practical application of a radio communication link of 10km, does the receiving antenna directly experience the field which is created by the transmitting antenna 10kms away? Is it what then induces a current in the receiving antenna? If a medium is not required to create a field, why would some links fail if there are obstacles in between?

Tks!

PiratePhysicist
#4
Jul15-09, 12:03 AM
P: 68
EM waves - energy transfer

Why would some links fail if there are obstacles in between? Think of it this way: Why playing ultimate frisbee, why does the frisbee not get your team mate when the other team is in the way? It's because they're in the way :)

Probably the best way to think of this is with the frisbee analogy, one person (antennae) is throwing the frisbee (photons, conveying the E/M forces) to the other person. The photons can be blocked just like a frisbee can.
granpa
#5
Jul15-09, 01:41 AM
P: 2,258
Quote Quote by dayan83 View Post
Thanks for the explanation..But I have some more doubts.

If we think of a practical application of a radio communication link of 10km, does the receiving antenna directly experience the field which is created by the transmitting antenna 10kms away? Is it what then induces a current in the receiving antenna? If a medium is not required to create a field, why would some links fail if there are obstacles in between?

Tks!
it isnt simple induction. the first antenna creates a wave which propagates to the second antenna. if the second antenna was in the near field of the first antenna then it would be simple induction

define 'medium'. when you get to the fundamental limits of existence itself its not so easy to define.
cmos
#6
Jul15-09, 02:01 AM
P: 367
Quote Quote by PiratePhysicist View Post
Probably the best way to think of this is with the frisbee analogy, one person (antennae) is throwing the frisbee (photons, conveying the E/M forces) to the other person. The photons can be blocked just like a frisbee can.
More formally, EM waves may sometimes be reflected, absorbed, and/or scattered. These phenomena may prohibit a wave from traversing from point A to point B. If you have any doubts of the energy transfer, go inside your house and look at a light bulb after you turn it on. Then go outside and look at the sun. Which can you look at for a longer period of time? (Note: I do not suggest you actually do this experiment; it should suffice to simply hypothesize the results).

Also note that (referring to the second sentence of the original post), in a manner of speaking, the experiment I described allows you to "see" EM waves (although it takes some additional, clever experiments to actually see the "wave nature").


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