Page 26 - Physics - XI

Q24. Why do you measure the diameter of the wire in two perpendicular directions?
Ans. It is done to eliminate error due to non-uniformity of the thickness.

Q25. Which is more accurate—Vernier Calliper or screw gauge?
Ans. Screw gauge is more accurate since its least count is smaller than that of Vernier Calliper.
Q26. Why does a screw gauge develop backlash error with use? (NCERT)
Ans. When the thread on the screw and that on the nut is tightly fi tted with each other, then with repeated
use of the threads, the screw and the nut may get worn out. As a result, a gap gets developed between
these two threads which is called play, and this introduces an error in the measurement of the screw
gauge. This error is called backlash error due to which the screw slips a small linear distance without
rotation.

Q27. What are the limitations of the screw gauge if it is used to measure the thickness of a thick
cardboard sheet? (NCERT)
Ans. The limitations of the screw gauge if it is used to measure the thickness of a thick cardboard sheet
are:

(i) The screw gauge may have backlash error.
(ii) The threads of the screw may not be of equal pitch.

(iii) The screw may have friction.
(iv) The divisions on linear scale and circular scale may not be evenly spaced.

Q28. Is the screw gauge with smaller least count always better? If you are given two screw gauge,
one with 100 divisions on circular scale and another with 200 divisions, which one would you
prefer and why? (NCERT)
Ans. Yes, the screw gauge with the smaller least count is always better.
The least count of a screw gauge is defi ned as,

Least count = Pitch
Total no. of divisions on the circular scale

So, if we are given a screw gauge with 100 circular scale divisions and another with 200 circular
scale divisions, then we would prefer the screw gauge with 200 circular scale divisions because as
per the formula, if the number of circular scale divisions is more, then the least count will be less.
Q29. Is there a situation in which the linear distance moved by the screw is not proportional to the

rotation given to it? (NCERT)
Ans. Yes, this can happen in case of a faulty screw gauge. Backlash error because of a fault in screw
represents this fallacy.

Q30. Is it possible that the zero of circular scale lies above the zero line of main scale, yet the error
is positive zero error? (NCERT)
Ans. No

Q31. For measurement of small lengths, why do we prefer screw gauge over Vernier Callipers?
(NCERT)
Ans. The least count of screw gauge is less than that of Vernier Calliper. That's why, we prefer screw
gauge over Vernier Callipers for measurement of small lengths.

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