NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 5

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Question: 2.7

A student measures the thickness of a human hair by looking at it through a microscope of magnification 100. He makes 20 observations and finds that the average width of the hair in the field of view of the microscope is 3.5 mm. what is the estimate on the thickness of hair?

Answer: 2.7

  • Magnification of the microscope = 100

  • Average width of the hair in the field of view of the microscope = 3.5 mm

Actual thickness of the hair is

  • Microscope looks like

Showing the Microscope in image

Showing the Microscope in Image

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Question: 2.8

Answer the following:

(a) You are given a tread and a metre scale. How will you estimate the diameter of the thread?

(b) A screw gauge has a pitch of 1.0 mm and 200 divisions on the circular scale. Do you think it is possible to increase the accuracy of the screw gauge arbitrarily by increasing the number of divisions on the circular scale?

(c) The mean diameter of a thin brass rod is to be measured by vernier calipers. Why is a set of 100 measurements of the diameter expected to yield a more reliable estimate than a set of 5 measurements only?

Answer: 2.8

(a) Wrap the thread on a uniform smooth rod in such a way that the coils thus formed are very close to each other. Measure the length of the thread using a metre scale. The diameter of the thread is given by the relation,

(b) It is not possible to increase the accuracy of a screw gauge by increasing the number of divisions of the circular scale. Increasing the number divisions of the circular scale will increase its accuracy to a certain extent only.

Screw gauge looks like

Shows the Screw gauge in image

Shows the Screw Gauge in Image

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(c) A set of 100 measurements is more reliable than a set of 5 measurements because random errors involved in the former are very less as compared to the latter.

Vernier caliper looks like

Figure shows the vernier calipers

Figure Shows the Vernier Calipers

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