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1. Ampere's Law and current density

Not sure I understand.. Should I get: B = mew0*I*r / 2*pi*R2 Where r is the distance to the point in the wire we're looking for, and R is the radius of the whole wire? How does J fit into it? The part of the question I don't get is this bit: axis according to J = J0 r / a What...
2. Ampere's Law and current density

The current density inside a long, solid, cylindrical wire of radius a = 10 cm is in the direction of the axis and varies linearly with the radial distance r cm from the axis according to J = J0 r / a with J0 =0.5 A/cm2 . Find the magnetic field in μT at r = 0.045 m Ok I know from amperes law...
3. Electric field help

Well, I got them all except the last one now. Still have absolutely no idea how to do it, I've gone through my text book, lecture notes and everythnig but can't find anything on it.
4. Electric field help

Oh yeah I forgot the square after the r... I wrote it down on the sheet, just missed typing it.
5. Electric field help

I have these questions on electric fields that I'm a bit confused on.. A flat circle of radius 8 cm is placed in a uniform electric field of 8.5 × 10^2 N/C. What is the electric flux (in Nm^2/C) through the circle when its face is at 51° to the field lines? I just use EAcos(theta) don't I...