Self Induction: The number of lines of force in a magnetic field depends upon the strength of the field. A stronger field has more lines than a weaker one.
Interesting information
how some birds, like pigeon, correctly estimate the direction? there is some sort of natural magnet in their head.
We have read that as current passes through a coil, it produces a magnetic field whose lines of force pass through the different turns of the coil (fig. 17.5b). if the current passing through the coil changes, then the magnetic field produced by it also changes which results in a change in the number of lines of force passing through the coil. In other words, the magnetic flux linked with the coil changes.
This causes an induced emf in the coil which is known as self induced emf and this phenomenon is called self induction.
The phenomenon of self induction can be demonstrated by the experiment shown in fig. 17.20.
When the switch is put on, the bulb does not light up because the battery voltage is not sufficient for this purpose. However when the switch Is put off, the lamp flashes lightly for a few moments, indicating that a voltage higher than that of the battery, has been instantaneously supplied by the coil to the bulb due to which the bulb flashes. This large voltage is the self induced voltage produce in the coil at the instant when the switch Is put voltage produce in the coil at the instant when the switch is put off.
Do You Know?
transformer works on the principle of mutual induction. it is an important part of most of the electronic devices.
17. 9 Transformer
This is an electrical device which is used to increase or decrease the value of alternating voltage.
It consists of two coils which are wound on two different sides of a rectangular iron core (fig. 17. 21). One coil is called primary and the second one is known as secondary. The alternating voltage, whose value is to be altered, is supplied to the primary due to which an alternating current begins to flow through it. This current creates a continuously changing magnetic flux through the primary coil.
The iron core enhances the magnetic flux to a very large value and also concentrates it into the core with the result that practically the whole of flux generated by the primary coil is also linked up with the secondary. As this flux is continuously changing, therefore, in accordance with the principle of mutual induction, an alternating voltage is induction across the secondary coil. The value of this voltage depends upon the number of terms in the primary and secondary coils. If
Np = number of turns in the primary coil.
Ns = number of turns in the secondary coil.
Ep = voltage applied across the primary which is to be altered.
Es = the acquired voltage generated across the secondary coil;
Then we can prove that
According to this formula, if the voltage applied across the primary coil is to be decreased, i.e., Es <Ep, then Ns would also be smaller as compared to Np. That is , the number of turns in the secondary would be smaller than the number of turns in the primary. Such a transformer is known as step down transformer (fig . 17. 21a). on the other hand, if the voltage applied to the primary coil is to be increased, the number of turns in the secondary would be larger as compared to number of turns in the primary. Such a transformer is known as a step up transformer. Fig 17. 21 (b).
