A solenoid is essentially a device used to create magnetic fields by passage of current through it(Analogous to a capacitor in electricity.) Its shape is somewhat like a spiral spring,like a spiral loop of wire. A solenoid may also be regarded as a circular coil, albeit with a **very** large number of turns. Like a collection of circular coil elements.(Analogy is with a cylinder, which may be regarded as a stack of elementary discs).**So a solenoid may be regarded as made up of elementary circular coils.**
The formula 1 is the expression for the magnetic field at the centre of a tightly wound circular coil.(NOT related to solenoid.)
The second formula is the expression for field in the interior of the solenoid, **but whose length is much larger than its radius, so the field may be assumed to be uniform. It is an approximation. In actuality, the field in the interior portion of the solenoid is never uniform. The actual expression is calculated by advanced integration techniques while the given exp. can be easily derived by Ampere's Law.**
So to put it in very rudimentary terms, a solenoid looks like a spring at its natural length.(or slightly elongated) A **tightly-wound** circular coil is the same spring compressed.
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Last edited by Akshay; Jul 19th 2009 at 02:02 AM.
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