- #1
BOAS
- 552
- 19
Hello,
I am trying to find the magnetic field that accompanies a time dependent periodic electric field from Faraday's law. The question states that we should 'set to zero' a time dependent component of the magnetic field which is not determined by Faraday's law. I don't understand what is meant by this.
1. Homework Statement
Consider the following periodically time-dependent electric field in free space, which describes a certain kind of wave.
##\vec E (r, \theta, \phi, t) = A \frac{\sin \theta}{r} \cos(kr - \omega t) \hat \phi##, where ##\omega = ck##
(a) Show that, for r > 0, E~ satisfies Gauss’s law with no charge density.
From Faraday’s law, find the magnetic field. Ignore (set to zero) a time dependent part of the B~ -field not determined by Faraday’s law.
(b) Compute the Poynting vector ##\vec S##.
(c) Calculate ##\bar S##, the average of ##\vec S## over a period ##T = 2π/ω ##.
(d) Find the flux of ##S##through a spherical surface of radius ##r## to determine the total power radiated.
[/B]
Part (a) is obvious because the ##\hat \phi## component has no dependence on ##\phi##
part(b)
Given ##\vec E (r, \theta, \phi, t) = A \frac{\sin \theta}{r} \cos(kr - \omega t) \hat \phi##.
I use Faraday's law ##\vec \nabla \times \vec E = - \frac{\partial \vec B}{\partial t}## and the expression of the curl in spherical polar coordinates to find that;
##\vec \nabla \times \vec E = \frac{2A \cos \theta}{r^2} \cos(kr - \omega t) \hat r + kA \sin \theta \sin(kr - \omega t) \hat \theta##.
Integrating with respect to time to find ##\vec B## yields;
##\vec B = - [\frac{2A \cos \theta}{r^2} \hat r \int \cos(kr - \omega t)dt + kA \sin \theta \hat \theta \int \sin(kr - \omega t)dt]##
##\vec B = \frac{2A \cos \theta}{r^2 \omega} \sin(kr - \omega t) \hat r - \frac{kA \sin \theta}{\omega} \cos(kr - \omega t) \hat \theta + C##
I think that this is the magnetic field, but I haven't used the piece of information given in the question about 'setting the time dependent component to zero'.
What does that piece of information mean here?
Thank you.
I am trying to find the magnetic field that accompanies a time dependent periodic electric field from Faraday's law. The question states that we should 'set to zero' a time dependent component of the magnetic field which is not determined by Faraday's law. I don't understand what is meant by this.
1. Homework Statement
Consider the following periodically time-dependent electric field in free space, which describes a certain kind of wave.
##\vec E (r, \theta, \phi, t) = A \frac{\sin \theta}{r} \cos(kr - \omega t) \hat \phi##, where ##\omega = ck##
(a) Show that, for r > 0, E~ satisfies Gauss’s law with no charge density.
From Faraday’s law, find the magnetic field. Ignore (set to zero) a time dependent part of the B~ -field not determined by Faraday’s law.
(b) Compute the Poynting vector ##\vec S##.
(c) Calculate ##\bar S##, the average of ##\vec S## over a period ##T = 2π/ω ##.
(d) Find the flux of ##S##through a spherical surface of radius ##r## to determine the total power radiated.
Homework Equations
The Attempt at a Solution
[/B]
Part (a) is obvious because the ##\hat \phi## component has no dependence on ##\phi##
part(b)
Given ##\vec E (r, \theta, \phi, t) = A \frac{\sin \theta}{r} \cos(kr - \omega t) \hat \phi##.
I use Faraday's law ##\vec \nabla \times \vec E = - \frac{\partial \vec B}{\partial t}## and the expression of the curl in spherical polar coordinates to find that;
##\vec \nabla \times \vec E = \frac{2A \cos \theta}{r^2} \cos(kr - \omega t) \hat r + kA \sin \theta \sin(kr - \omega t) \hat \theta##.
Integrating with respect to time to find ##\vec B## yields;
##\vec B = - [\frac{2A \cos \theta}{r^2} \hat r \int \cos(kr - \omega t)dt + kA \sin \theta \hat \theta \int \sin(kr - \omega t)dt]##
##\vec B = \frac{2A \cos \theta}{r^2 \omega} \sin(kr - \omega t) \hat r - \frac{kA \sin \theta}{\omega} \cos(kr - \omega t) \hat \theta + C##
I think that this is the magnetic field, but I haven't used the piece of information given in the question about 'setting the time dependent component to zero'.
What does that piece of information mean here?
Thank you.