Page 41 - Physics - XII

Alternating Current
3 Alternating Current

EXPERIMENT - 6

Aim
To fi nd the frequency of A.C. mains with a sonometer.

Apparatus and Materials Required

A sonometer (with soft iron wire stretched over it), an electromagnet with a step-down transformer, a
hanger, slotted half kilogram weights, a physical balance, a weight box, and two sharp edge wedges

Principle/Theory

If a wire of length l having mass per unit length m is stretched by applying a tension T in the string and it
is set into transverse vibrations in fundamental mode, then the frequency n of the fundamental mode of
vibration is given by,

1 T
n =
2l m
When an alternating current (A.C.) is passed through an electromagnet and length l and tension T in the
wire is so adjusted that it is set into resonant vibrations by A.C., then the frequency of A.C. ‘f ’ will be half
of the frequency of the vibration of the wire ‘n’.
n
Frequency of A.C. = f =
2
Procedure
1. Fix the sonometer on the table as
shown in Fig. 6.1 in such a way so
that pulley projects beyond the edge Step-down
of the table. transformer Electromagnet

2. Test the pulley and make it friction- 200 V A.C. 6 V A.C.
less by oiling it.
3. Stretch the sonometer wire AB by W Wedge (W)
placing a weight of 0.5 kg on the
hanger. A B Pulley

4. Clamp the electromagnet in a stand Peg
and connect it to the secondary of a
step-down transformer. Now adjust Slotted
the position of the electromagnet so weight
that its lower end lies close to the Fig. 6.1: Experimental set up for fi nding frequency of
middle of the sonometer wire AB. A.C. mains by sonometer and an electromagnet.

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