Physics GRE Discussion Forums

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?

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.

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.