(GR0177 #61) Please help (GRE problem)

elimes
Posts: 2
Joined: Fri Dec 03, 2004 11:30 am

(GR0177 #61) Please help (GRE problem)

Postby elimes » Fri Dec 03, 2004 11:38 am

Does anyone know how to get the right answer (c) for problem 61 on the newest available GRE test. It goes like this:
61.An electromagnetic plane wave, propagating in vacuum, has an electric field given by E=Ecos(kx-wt) and is normally incident on a perfect conductor at x=0, as shown in the figure above. Immediately to the left of the conductor, the total electric field E and the total magnetic field B are given be which of the following?
E B
A) 0 0
B) 2Ecoswt 0
C) 0 (2E/c)coswt
D) 2Ecoswt (2E/c)coswt
E) 2Ecoswt (2E/c)sinwt

// How do you get a nonconstant magnetic field with no electric field?

Grant
site admin
Posts: 189
Joined: Tue May 11, 2004 7:55 pm

Postby Grant » Sun Dec 05, 2004 5:46 am

Hi elimes,

Here is one way to quickly approach the problem.

What is going on in this problem is that the wave is being reflected by the conductor. When this happens the E fields of the incident and reflected wave cancel in the conductor and so immediately to the left of x=0, E=0. Since you know the direction of the E fields of the incident and reflected wave and you know the direction the waves are propagating (i.e. in opposite directions), you can determine the direction of the B fields and can see that The B fields point in the same direction immediately to the left of x=0.

In summary, E fields point in opposite directions immediately to the left of x=0 and add to zero whereas B fields point in the same direction immediately to the left of x=0 and double.

elimes
Posts: 2
Joined: Fri Dec 03, 2004 11:30 am

Thanks Grant

Postby elimes » Sun Dec 05, 2004 5:56 pm

Thanks for that, it makes perfect sense now. That automatically answers my second problem for that also, which was how were they getting a 2 at all in their answer. Thanks again for your help.




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