Question

I'm sure I was taught that the net magnetic field inside a perfect conductor must = 0, but I graduated 39 years ago and have forgotten the reference. Can uou help please?

Answer 1

I think you're confusing the electric field inside a conductor in the electrostatic case with the magnetic field inside a conductor. here's a reference: http://ocw.mit.edu/courses/physics/8-02sc-physics-ii-electricity-and-magnetism-fall-2010/amperes-law/

Answer 2

thanks for the reply; I'm definitely thinking of the magnetic field (though I agree the electric field is definitely zero there) but the main reason I thought the magnetic field inside a current-carrying conductor must be zero is, if you take the 'official' formula for the field outside the conductor,
B = mu-naught I /(2 pi r)
and try to use it inside the wire, then the field should be infinite at the centre. And, I definitely remember, an answer of 'infinity' is nature's way of saying you got it wrong!

Answer 3

No, Ampere says that the field is proportional to the amount of current which penetrated the surface attached to the Amperian loop, and as the radius of the loop gets smaller, the amount of current passing through the attached surface gets smaller...
In other words, the I in the formula is the I which penetrates the surface attached to the Amperian loop, not the entire current in the wire. Going to that reference will help you to see this... without the ability to draw this can be a little opaque.