Calculating Magnetic Field Between Parallel Wires with Opposing Currents

[Parth Dave]

I've been studying for my exam and came across this little problem.

Two parallel conductors each carry 10 A of current in the same direction.

a) What is the magnetic field strength at the midpoint between these wires?
b) What is the field strength at the same point if the current ran in opposite directions?

For a, the answer is zero. The magnetic fields will cancel out.

For part b, I don't see how you can solve the problem without being given the distance. The magnetic field strength of a straight conductor is a function of the distance away from it. However, the textbook has a numerical answer (4.0 x 10^-4 T). Is the textbook wrong or am I just missing something?

 

[vincentchan]

if it is a surface current on a infinite large plate instead of current in a wire, this problem is soluvable...

ps. if it is the case, the problem sould give you surface current density =10A/m,

 

[Parth Dave]

Alright, that's what I thought. Thx a lot!