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A solenoid is a long coil of wire tightly wound in helical form.
A solenoid is a loop of wire, wound around a metallic core in a helical form, which produces a magnetic field when an electric current passes through it. A solenoid is ideal if it is infinitely long with tightly packed turns.


A toroid consists of a circular ring-shaped magnetic core around which wire is coiled. A toroid may be considered as a solenoid wrapped around with its ends connected. Thus, the magnetic field is completely confined inside the toroid and the field points in the azimuthal direction (clockwise due to the way the current flows, as shown in Fig.).
Since N wires cut the Amperian path, each carrying a current I, the net current enclosed by the Amperian path is,
Description: 57543.png

A toroid with N turns
Applying Ampere’s law to the Amperian path, we obtain,
Description: 57533.png

Description: 57524.png
where r is the distance measured from the centre of the toroid and is known as the mean radius. If the thickness of the toroid is much less than its mean radius then Description: 57517.png. Hence,
Description: 57508.png inside the toroid
where L is the length of the wire.
Outside the toroid, net current enclosed = (NI – NI) = 0, and thus, H = 0.

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