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Uber Member
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Apr 23, 2010, 11:53 PM
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Oscillations and ionosphere
I had a question in my physics book which I can't solve... I don't even know where to start :(
The ionosphere contains free electrons. What is the amplitude of these electrons when subjected to a 200 kHz electromagnetic wave in which the oscillations of electric field have amplitude 5 x 10^-3 V/m?
A. 3.2 x 10^-15 m
B. 4.0 x 10^-9 m
C. 2.5 x 10^-8 m
D. 5.6 x 10^-4 m
E. 2.2 x 10^-2 m
~~~~
Electrons have a charge of 1.6 x 10^-19 C... So, force is 8 x 10^-22 N. I don't even know if that force will be needed :(
I tried using
but not only I get a negative number, but the number lies between the answers of D and E.
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Pets Expert
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Apr 24, 2010, 12:12 AM
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Okay, Unky, all I can say is this. If you can't solve it, I have no hope in hell of even getting close.
I hope someone can help. I know it's not me.
Good luck kiddo. :)
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Uber Member
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Apr 25, 2010, 01:44 AM
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Ok, so you're probably suggesting this then:
?
I know this formula, and epsilon nought is the permittivity of free space.
The problem is, I can't find the link between the question and the solution. I know I can use that formula in cases for example a charged sphere, and then, I find the electric field at distance 'r' from the sphere. Here, there is no 'charge centre', and I'm at a loss... =/
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Uber Member
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Apr 25, 2010, 10:11 AM
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K varies with f. Find k.
See link.
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Uber Member
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Apr 25, 2010, 10:36 AM
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Um.. still confused. K is a constant...
If you're telling me to find the force... wait, there aren't any information to find that. I need to get either Q or r to get that, and that charge is not due to the electron but to the source of the electromagnetic wave...
Ebaines, some input from you would be welcome too!
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Uber Member
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Apr 25, 2010, 11:52 AM
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From the link, I woas sort of thinking of this:
instantaneously to an applied field. The response must always be causal (arising after the applied field) which can be represented by a phase difference. For this reason permittivity is often treated as a complex function (since complex numbers allow specification of magnitude and phase) of the (angular) frequency of the applied field ω, . The definition of permittivity therefore becomes
?
where
D0 and E0 are the amplitudes of the displacement and electrical fields, respectively,
i is the imaginary unit, i 2 = −1.
Problem is, what is
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Uber Member
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Apr 25, 2010, 12:06 PM
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Uh... I don't think my syllabus requires me to go that far... :eek:
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Ultra Member
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Apr 25, 2010, 04:45 PM
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1 N/C = 1 V/m Helpful?
I know I can't solve it, but I am going to speculate on some problems I see with this problem.
The amplitude of the radio frequencies is in V/m while the ionosphere is only given in m (answers).
If you're talking force, wouldn't the electron density of the ionosphere be helpful.
Some other things I know that aren't mentioned. 250kHz is a strange frequency. Its right on the border between VHF and UHF. This makes it very susceptible to density. The electro-density of the ionosphere would determine weather it is bounced off, absorbed, or let through. There is nothing mentioned about the inverse square law.
I really don't have much of a clue, but these are a few things that popped into my mind as I read through the thread.
Good Luck.
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Uber Member
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Apr 26, 2010, 06:06 AM
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Thanks for your input. Yes, I knew that 1 N/C = 1 V/m, but I can't see where the electric field can be connected to the amplitude and the frequency.
I guess I'll have to wait until the day my teacher's going to give the correction for this number...
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Uber Member
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Apr 26, 2010, 06:03 PM
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If I get a chance, I'll check my "Field theory" book if I can find it.
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Uber Member
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Apr 26, 2010, 10:56 PM
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Sure, thanks KISS :)
By Thursday, I'll be having the correction and I'll post it here, just in case you wanted to know :)
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Uber Member
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May 8, 2010, 09:00 PM
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Sorry for the late reply, I got the solution, then did a wrong calculation and thought that I missed something in the solution, but it was a mere calculation mistake. :o I really need to revise this chapter because the answer is in fact really easy...
Ok, here we go, KISS, you were nearer with your first idea ;)
Given the electric field, we can find the force that the electron experiences using:
Then, using the equation in oscillations, or circular motion, we know that:
where r is the amplitude here.
Equating both, we get:
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Uber Member
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May 10, 2010, 02:48 PM
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Seems simple, NOW!
Thanks.
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