CBSE Class 12 Physics 2015 Delhi Set 1 Paper

Question : 23

Total: 26

SECTION - D

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 a 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.
Answer the following questions:
(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 dc voltage to the low voltage? Explain.
(iii) Write the values displayed by the students and the teacher.

Solution:

(i) Transformer is used to bring down high voltage ac to low voltage.
Working principle: Mutual induction.
Transformers have two coils - primary and secondary. When ac voltage is applied to the primary an emf is induced in the secondary coil.
(ii) No. Continuous change is voltage level in the primary coil is required for emf to be induced in the secondary coil. But in dc, there is no change in voltage level. Hence, no emf is induced in the secondary coil.
(iii) Values displayed by students: Inquisitive nature, scientific temperament.
Values displayed by the teacher: Concern for students, helpfulness, professional honesty.

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