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Old Jul 18th 2009, 08:35 PM   #1
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Circular coil and solenoid

It may be a silly question. But please tell me what's the difference between circular coil and solenoid. As I saw circular coil consisting of many turns.
For the equation representing magnetic flux density of centre of circular coil is$\displaystyle
B=\frac{\mu_0NI}{2r}$
r is the radius of coil
N is the number of turns
I is the current passing

But for the solenoid,
$\displaystyle B=\frac{\mu_0NI}{l}$
l is the length of solenoid
N is the number of turns
I is the current passing

So what's the difference between them? Thanks for any help
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Old Jul 19th 2009, 12:57 AM   #2
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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 01:02 AM.
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Old Jul 19th 2009, 10:33 PM   #3
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Really well explained with the spring analogy.
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Old Jul 20th 2009, 07:34 AM   #4
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A coil is a series of loops. A solenoid is a long tightly wound helical coil. An example of a coil which is not a solenoid is the
Helmholtz coil - Wikipedia, the free encyclopedia
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