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Frequency of AC Mains using Sonometer Viva Questions

Frequency of AC Mains using Sonometer Experiment Viva Questions with Answers

1 Which is more dangerous in use a.c. or d.c.? Why?

Ans: For example, a 220V a.c has a peak value of 220 x 2 = 311 V whereas the peak value of a 220 d.c. is 220 V itself. Hence a.c. is more dangerous than d.c. of the same voltage.

2. Why a d.c. voltmeter and d.c. ammeter can't read a.c.?

Ans: This is because the average value of a.c. is zero over a complete cycle.

3. Why is the core of a transformer made of soft - iron?

Ans: The hysteresis loop of soft - iron has a small area. So there is a small hysteresis loss.

4. In a step down transformer, which of the windings will have thicker wires? Why?

Ans: In the step down transformer, the current in the secondary coil will be larger than in the primary coil. If the resistance is more, loss of energy will occur in the form of heat (I2R). Hence, to reduce the resistance, thick wires are used in the secondary windings.

5. An induced e.m.f. has no direction of its own. Why?

Ans: This is because, according to Lenz's rule, the direction of the induced emf is such as to resist the cause that produces it. Thus the direction of the induced e.m.f. is to be determined by the cause of e.m.f.

6. Will an induced current be always produced whenever there is a change in magnetic flux linked with a coil?

Ans: The induced current is produced only if the circuit is closed. However, the induced e.m.f. will be definitely produced.

7. A rectangular closed loop moves horizontally in a uniform magnetic field.

i. Is there any induced current, if the loop is entirely in the magnetic field?

Ans: There is no change in magnetic flux and no induced current in the loop.

ii. Is there any induced current, if the loop is partially out of the magnetic field?

Ans. The magnetic flux associated with the loop changes and an e.m.f. is induced in it.

iii. Will the induced current remain if the loop becomes stationary and the magnetic field alternates with time?

Ans. Yes, an induced current will be set up in this case also, in confirmity with the phenomenon of electromagnetic induction.

8. What is alternating current?

Ans: Alternating Current is an electric current that periodically reverses direction and changes its magnitude and polarity at regular intervals of time.

9. Explain peak value and rms value of ac.

Ans: The maximum value attained by an alternating quantity, either positive or negative that a waveform attains during one cycle is called its Peak value.

RMS (Root Mean Square) value of alternating current is defined as the direct current which flows through a resistance.

10. What is the relation between peak and rms values of ac?

Ans: Irms = I0/2 = 0.707I0

11. What is meant by the frequency of an ac?

Ans: Frequency is defined as the number of times the current or voltage signal reaches its maximum or minimum in one second.

12. Give the frequency of ac used in India.

Ans: 50 Hz

13. Why do you use a step up transformer to measure the frequency of ac using a sonometer?

Ans: A step down transformer is used to find the frequency of alternating current. This is because the voltage of AC mains for domestic purposes is 220V. Hence a.c. is more dangerous. So there needed the step down transformer to reduce the voltage to 6 volts.

14. Give the expression for the fundamental frequency of vibration of the sonometer wire.

Ans: n = 1/2L√(T/m)


L = length of wire

T = Tension

m = mass per unit length of string

15. Aim of Frequency of Alternating Current Experiment.         

Ans: To find the frequency of alternating current (ac) with a sonometer

16. Apparatus of Frequency of Alternating Current Experiment.

Ans: Sonometer, Step down transformer (9V to 6V), Horse shoe magnet or powerful bar magnets, weight hanger, slotted weights etc.

17. Theory of Frequency of Alternating Current Experiment.

Ans: If a sonometer vibrates under an applied ac, then the frequency of ac is given by,

n = √(g/4m x M/l2)


g = acceleration due to gravity

M = mass suspended

l = resonating length

m = linear density of sonometer wire

18. Procedure of Frequency of Alternating Current Experiment.

Ans: Experimental arrangement is shown in the figure. A suitable mass is loaded (Say 150g) at the free end of the sonometer. A light paper rider is placed on the wire between the bridges A and B. Send a suitable current through the circuit. The bridges are adjusted till the wire AB vibrates with maximum amplitude (resonance condition). As the bridges are moved, the position of the magnet is adjusted such that it is always at the mid way between the bridges A and B. The resonating length is measured. The experiment is repeated for different loads.

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