Question:

A group of students while coming from the school noticed a box marked "Danger H.T.2200 V"at a substation in the main street. They did not understand the utility of a such a high voltage, while they argued the supply was only220 V. 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?

Solution:

(i) A step down transformer is used to bring the high voltage down to low voltage of an a.c. current and it is based on the principle of mutual inductance (ii) A transformer cannot be used for a dc voltage , because if we use dc voltage it will not change the flux linked with secondary coil and voltage will not be induced in the secondary coil (iii) Value displayed by the students = (2200V) voltage across primary coilValue displayed by the teacher = (220V) voltage across secondary coil .