Page 130 - Physics - XII

Precautions

1. The parallax should be removed tip to tip.
2. The convex lens of a large focal length should be used.
3. The lens and plane mirror should be cleaned before use.

4. The screw of the spherometer should be moved in one direction to avoid backlash error.

PROJECT - 4

Aim
To investigate the relation between the ratio of (i) output and input voltage and (ii) number of turns in the
secondary coil and primary coil of a self-designed transformer.

Apparatus and Materials Required
A soft iron rod, copper wires of diff erent cross-section, two A.C. ammeters, and two A.C. voltmeters

Principle/Theory
Transformer
A transformer is an electrical device used for changing the A.C. voltage levels. A transformer which
increases the A.C. voltage level is called step-up transformer, whereas a transformer which decreases
A.C. voltage level is called step-down transformer.

A.C. ammeter A.C. ammeter
P S
1 1

220 V A.C. Input voltage N P N S A.C. voltmeter

P 2 S 2
Fig. 4.1: A self-designed transformer

The working of a transformer is based on the principle of mutual induction. Consider an ideal transformer
in which all the fl ux is linked with the primary and secondary windings of the core as shown in Fig. 4.1.
The e.m.f. induced in the primary coil,
d
E = – N
P
P
dt
The e.m.f. induced in the secondary coil,
dφ
E = –N
S S
dt
where, φ is the magnetic fl ux linked with each turn of primary and secondary coil at a time t.

E N
So, S = S = K = transformer ratio
E P N P

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