Electromagnetism Questions

Unanswered

• Why does adding a dielectric increase the capacitance of a capacitor?
• Is the gaussian system of units as correct as the SI system?
• If you surround a permanent magnetic with a conductor, does this shield the outside from its magnetic field?
• Does the Poynting vector represent the flow of electromagnetic energy even in the case of an electrostatic plus a magnetostatic field? The Feynman Lectures pg. II-27-8 seems to suggest that that it does despite the fact that this is counter-intuitive. I am still not totally convinced. Is it possible that it only pertains to closed surfaces? Feynman II-27-6 makes it clear that nobody yet knows where the field energy is because the Poynting vector is only one of the many possible expressions. But on page II-27-11 we learn that it is necessary to have this energy flow in a static situation in order to account for conservation of angular momentum. Also, in principle it should be possible to determine the location of the energy because we would need to know in order to accurately calculate the gravitational fields as Feynman points out on page II-8-11.
• Why does current distribute itself evenly across the cross section of a wire as stated in the Feynman Lectures pg. II-14-4?
• How does matter conspire to change the wavelength of light in it as mentioned by Griffiths EM p383? I need to check the citation.
• If acceleration causes radiation, then will all spinning objects eventually radiate off their angular momentum, or does radiation actually require jerk as suggested by some physicists? My EM professor said that everything would eventually stop spinning.
• Is there a cylindrical shell theorem like Newton's Shell Theorem?
• How do you find the power radiated due to more than one accelerating charge?
• What are all the rules for method of images pseudo Green's functions?
• What causes ferrmomagnetism?

Answered

• How can mu_0 be defined to be a precise value when it is constrained by the relation mu_0*epsilon_0 = c^2? Feynman II-13-5 mentions this. Amperes are defined using the force of attraction of wires with the equation mu_0*I/(2*Pi*r), and current is used to define the Coulomb, so changing mu_0 would change the Ampere and the Coulomb, then the change in the Coulomb would change epsilon_0 to preserve the speed of light relation.
• Why don't we use the retarded fields instead of the retarded potentials when calculating the fields of moving charges? As Griffiths implies on page 423, the retarded fields don't satisfy the Lorentz condition. The way to look at it is that the fields transform relativistically between electricity and magnetism, and that would make it more difficult to use the fields directly.
• How can a flame be a plasma if it is at a temperature where most of its atoms should be in their ground state? The degree of ionization is very low, probably somewhere around 10^-13.
• Do the auxiliary fields D and H neglect multipole moments higher than the dipole moments? Yes, and this is justified because there is no simple way to create a net quadrupole or higher moment in most materials.
• Why do we always approximate mu = mu_0 and not epsilon = epsilon_0? This is just the way most materials are i.e. all non-magnetic materials.