Page 44 - Physics - XII
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Q5. What is the frequency of the A.C. mains in your city?
Ans. The frequency of A.C. supply in the city is 50 Hz.
Q6. What do you mean by one cycle of A.C.?
Ans. One cycle of A.C. means current from zero becoming maximum positive, then zero, then maximum
negative, and fi nally zero again.
Q7. What do you mean by A.C.?
Ans. A.C. (Alternating Current) is an electric current whose magnitude changes continuously and
direction changes periodically.
Q8. What do you mean by D.C.?
Ans. An electric current having constant magnitude and same direction is called direct current (D.C.)
Q9. What is an electromagnet?
Ans. A magnet magnetised by electric current is called an electromagnet.
Q10. How is A.C. produced?
Ans. A.C. is produced by A.C. dynamo (alternator) in which a coil rotates in a magnetic fi eld.
Q11. What is root mean square (r.m.s.) value of A.C.?
Ans. It is that value of steady current which when passed through a given resistance for a certain time
shall produce the same quantity of heat as the given A.C. shall produce when passed through the
same resistance for the same time.
Q12. Give the expression for r.m.s. value of A.C.
Ans. For A.C. having maximum value I ,
0
I
I = 0 = 0 707. I
rms 0
2
Q13. Give the formula for the natural frequency of a sonometer wire under tension.
Ans. The frequency ‘n’ of the wire vibrating in one loop is given by
n = 1 T
2l m
where T is the tension in the wire, m is the mass per unit length of the wire, and l is the length of
the wire between the two knife edges.
Q14. What should be the property of the iron for making it a good electromagnet? (NCERT)
Ans. Soft iron is used to make electromagnets because soft iron has good or high magnetic properties. It
can provide a strong magnet.
Q15. Is there any relation between frequency and magnetism of the electromagnet, and frequency
of the alternating current? (NCERT)
Ans. Increasing the frequency will increase the magnetic induction in the electromagnet.
Q16. How is AC diff erent from DC? (NCERT)
Ans. DC has the same magnitude and direction throughout, while AC has to change magnitude and
direction.
Q17. What is the meaning of frequency of AC? (NCERT)
Ans. Frequency is the rate at which current changes direction per second. The frequency of an alternating
current is the number of cycles completed in one second.
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