In this article we will discuss about the magnetic effect of electric current, explained with the help of a suitable diagram.
When electric current flows through a conductor, a circular magnetic field is set up around the conductor along its length. This is called magnetic effect of electric current. The magnitude of the field depends on the current. If the current is increased, the magnetic field becomes stronger, and a weaker field results when the current is decreased.
The direction of the flow of current is always perpendicular to the direction of the field, and when the current is reversed, the magnetic field is also reversed. The magnetic field disappears when the flow of current is stopped.
When electric current flows through a single straight conductor, a very weak magnetic field is produced. Such a field cannot serve any useful purpose. But if the same conductor is coiled into several turns, a stronger field will be produced. The magnetic lines of force complete their paths through and outside the coil as shown in fig.51.
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Owing to the direction of current flowing through the coil, the left-hand side of the coil will develop south polarity and the right-hand side north polarity. Such a coil is called a solenoid. When the turns of the coil are very close to each other (i.e. each turn is in touch with its adjacent turns), the coil is called a toroid.
A solenoid may be air-cored or iron-cored. When there is air inside the coil, it is called air-cored solenoid. This shown in fig. 52(a). But when a coil made of insulated wire is placed around an iron bar as shown in fig. 52(b), it is called iron-cored solenoid. Such a solenoid can produce much more number of lines of force than an air-cored solenoid.
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The iron piece is called magnetic core, the coil placed around the core is called magnetising coil and the current flowing through the coil is called exciting current or magnetising current. If the core is made of soft iron, it becomes a strong magnet with comparatively small exciting current. But it loses its magnetism almost completely as soon as the flow of current is stopped.
If the core is made of steel, it requires larger exciting current to produce the same number of line of force as soft iron. But owing to its high retentivity and coercivity, it can retain its magnetism even when no current flows through its magnetising coil. For this reason a soft iron core is usually used for an electromagnet, while steel core is used for a permanent magnet.
The polarity developed at any end of an iron core or solenoid is determined by the direction of the flow of current in the coil. If on looking at one end of the magnet the current appears to flow anti-clockwise, that end of the magnet behaves as a north pole, but if the current appears to flow clockwise, the end behaves as a south pole.