Question

A group of students while coming from the school noticed a box marked “ Danger H.T 2200V” at a substation in the main street. They did not understand the utility of such a high voltage, while they argued the supply was only 220V.They asked their teacher this question the next day. The teacher thought it to be an important question and therefore explained to the whole class.
(i)What device is used to bring the high voltage down to low voltage of a.c. current and what is the principle of its working?
(ii) Is it possible to use this device for bringing down the high d.c. voltage to the low voltage? Explain.
(iii) Write the values displayed by the students and the teacher.

Answer 1

Hint: Here we need to know about the concept of mutual induction. How this is implemented to construct transformers. Transformers are used to increase or decrease the a.c voltage. We apply the voltage in the primary coil of the transformer and we get the output voltage from the secondary coil.
Formula used: $\dfrac{{{V}_{s}}}{{{V}_{p}}}=\dfrac{{{N}_{s}}}{{{N}_{p}}}$

Complete answer:
(i) A step down transformer is used to bring down high voltage a.c. to low voltage. The opposite is done by a step up transformer. These are based on the principle of mutual induction. In mutual induction with changing current associated magnetic flux changes. In case of a transformer when the a.c. changes in the primary, the flux related to the secondary changes and an induced emf is generated in the secondary coil.The diagram of a transformer is shown.

If the numbers of turns in primary and secondary be ${{N}_{p}}$ and ${{N}_{s}}$ respectively and voltage in the primary and in the secondary be${{V}_{p}}$ and ${{V}_{s}}$ respectively, then
$\dfrac{{{V}_{s}}}{{{V}_{p}}}=\dfrac{{{N}_{s}}}{{{N}_{p}}}$
(ii) No it is not possible to use this device to bring down high d.c. voltage. Because in the case of d.c there is no change in the current in the primary coil,so there is no change in the magnetic flux associated with the secondary coil. So there will be no mutual induction and no emf will be induced in the secondary coil.
(iii) The values shown by the students are consciousness and inquisitiveness. The teacher showed wisdom and humbleness.

Note:
In this case we will use a step down transformer which decreases the voltage. A step up transformer increases the voltage. Low voltage a.c cannot be supplied to distant places effectively. Thus very high voltage a.c. is transmitted from the power stations and with then we use step down transformers to have low a.c. voltages in our household.