# Magnetic field far from a solenoid

1. Jan 18, 2013

### jaumzaum

I know the magnetic field inside the solenoid is μIn/L and outside is 0 (provided that the point is near the solenoid).

But if we had a long solenoid and choose a point far from the solenoid (and perpendicular to it), would it behave like a wire? Would the magnetic field be μI/2πR?

2. Jan 18, 2013

### rude man

If the solenoid is wound as a spiral, which it usually is, then the field outside it would be the same as a long straight wire. That's because the coil inside has to be wound continuously from end to end. You can think of the coil as the sum of N (N→ ∞) perfectly perpendicular turns, each with a small gap, and a long single wire joining all the turns at the gaps. Whether the gaps are all lined up or random or any other orientation makes no diff.

In fact, even if the solenoid is not of infinite length, the field outside would be the same as if it was just a single wire, again if it was wound in a spiral.

http://en.wikipedia.org/wiki/Solenoid

3. Jan 18, 2013

Thanks man