Prove that emf from E equals emf from dB/dt

  • Thread starter Thread starter Addez123
  • Start date Start date
  • Tags Tags
    Emf
Addez123
Messages
199
Reaction score
21
Exercise:
I'm suppose to prove that e1 = e2
e1 = ∫E*dl
e2 = ∫dB/dt*dS

where S is the surface encircled by the conture c.
c is a box with with a length (in x axis) and b height (in y axis).
33z3fk9.png


for an electromagnetic wave:
E = E0*sin(kx - wt) (in y axis)

I'm ASSUMING this means that
B = B0*sin(kx - wt) (in z axis)

Pathetic Attempt:
e1 calculated on the outside of c becomes
b*E(x + a,t) - b*E(x,t) =
E0*sin(kx + ka - wt) + E0*sin(kx - wt)
This expression is confusing and doesn't lead to any simplification at all.

e2 = ∫dB/dt*dS
dB/dt = B0*-w*cos(kx - wt)

Now here's the tricky part, how do I integrate this infinitly thing vector over a surface?
The wave has no thickness, shouldn't the integral be zero?!
 
Physics news on Phys.org
I'm also suppose to prove this explicitly, I DONT EVEN KNOW WHAT THAT MEANS!
 
Addez123 said:
e1 calculated on the outside of c becomes
b*E(x + a,t) - b*E(x,t) =
E0*sin(kx + ka - wt) + E0*sin(kx - wt)
What happened to the factor of b in going to the last line. Also, what happened to the negative sign between the two terms?

This expression is confusing and doesn't lead to any simplification at all.
You've made a good start once you make the corrections above.

e2 = ∫dB/dt*dS
dB/dt = B0*-w*cos(kx - wt)
OK

Now here's the tricky part, how do I integrate this infinitly thing vector over a surface?
The wave has no thickness, shouldn't the integral be zero?!
Not sure what you mean by "this infinitly thing vector".
##\frac{\partial B}{\partial t}## is a function of ##x## and ##t##. ##t## is some fixed but arbitrary time. So, you can think of ##\frac{\partial B}{\partial t}## as a function of ##x## that you need to integrate over the rectangular area. What would be a good way to break up the area into infinitesimal elements ##dS##? Recall how you did similar integrals in your calculus course.
 
Thread 'Need help understanding this figure on energy levels'
This figure is from "Introduction to Quantum Mechanics" by Griffiths (3rd edition). It is available to download. It is from page 142. I am hoping the usual people on this site will give me a hand understanding what is going on in the figure. After the equation (4.50) it says "It is customary to introduce the principal quantum number, ##n##, which simply orders the allowed energies, starting with 1 for the ground state. (see the figure)" I still don't understand the figure :( Here is...
Thread 'Understanding how to "tack on" the time wiggle factor'
The last problem I posted on QM made it into advanced homework help, that is why I am putting it here. I am sorry for any hassle imposed on the moderators by myself. Part (a) is quite easy. We get $$\sigma_1 = 2\lambda, \mathbf{v}_1 = \begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix} \sigma_2 = \lambda, \mathbf{v}_2 = \begin{pmatrix} 1/\sqrt{2} \\ 1/\sqrt{2} \\ 0 \end{pmatrix} \sigma_3 = -\lambda, \mathbf{v}_3 = \begin{pmatrix} 1/\sqrt{2} \\ -1/\sqrt{2} \\ 0 \end{pmatrix} $$ There are two ways...
Back
Top