REALLY quick questions about solenoids (no calculuations)

[Bkas]

For an infinite solenoid, the magnitude of the magnetic field is independent of the radius of the solenoid, right? If that's the case, and you take two concentric solenoids, both with currents (same magnitude of current) in opposite directions, B-field would be zero everywhere, right? That seems kind of odd, if you ask me. I just want to double check.

What if they're finite solenoids? Would that change anything? Also, is the expression for magnetic field of a finite solenoid the same as that of an infinite solenoid? Or does it actually take radius into account?

'Cause to me, it seems having concentric solenoids (finite or inifinite) of equal magnitude of current flowing through them in the opposite directions would produce 0 B-field everywhere.

But I don't think that's right (since I have to use it to calculate self flux and other stuff later on).

I really appreciate any responses!

 

[DivideBYZero]

Highly technical forum will get you better responses than OT.

 

[theNEOone]

isn't this your third or fourth physics question? just read your damn book. if not, just email your TAs. that's their purpose.

 

[dowxp]

your two selonoids need a direction, assuming they are parallel, the total flux would be zero... just use ampere's law. take a surface enclosing both solenoids..

B*dl = u(sum of i) .. since summation of i is 0 ... RHS=LHS=0;

been a while, but thats what im guessing

 

[dowxp]

also, most problems use a finite solenoid but also says to ignore edge effects. so, it doesnt matter...

 

[Bkas]

Originally posted by: Bangsailio
your two selonoids need a direction, assuming they are parallel, the total flux would be zero... just use ampere's law. take a surface enclosing both solenoids..

B*dl = u(sum of i) .. since summation of i is 0 ... RHS=LHS=0;

been a while, but thats what im guessing

They're concentric. That implies that they're coaxial, so they are laid out in the same direction. However, their currents flow in opposing directions (e.g. if you were to take a crosssection, current flows counter on one, clockwise on the other).

That looks about right, I think. It's the same for finite solenoids too, right?

 

[Bkas]

I can't seem to find an equation for a finite solenoid other than a very similar equation to an infinite solenoid...still independent of radius of solenoid.

I think something's wrong.

 

[dowxp]

i never learned how to caculate edge effects. finite sol.s with a definite length are assumed to be ideal.