Conceptual Questions - Chapter 1 Nature of Physical World and Measurement 11th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated On May 15, 2024

By SaraNextGen

Conceptual Questions
Question 1.
Why is it convenient to express the distance of stars in terms of light year (or) parsec rather than in $\mathrm{km}$ ?
Answer:
A parsec is 206, $265 \mathrm{AU}$ and is roughly the distance to the nearest stars. If we were to view a giant star with a diameter of $1 \mathrm{AU}$ at a distance of one parsec, it would appear to be just $1 / 3600^{\text {th }}$ of a degree in angular size. For comparison, the sun and moon are both half a degree in angular size when viewed from Earth.

Question 2.
Show that a screw gauge of pitch $1 \mathrm{~mm}$ and 100 divisions is more precise than a vernier caliper with 20 divisions on the sliding scale.
Answer:
Least count of screw gauge $=\frac{\text { Pitch }}{\text { No. of divisions }}=\frac{1 \mathrm{~mm}}{100}=0.01 \mathrm{~mm}$
Least count of vernier caliper $=\frac{1}{20} \mathrm{~mm}=0.05 \mathrm{~mm}$
As shown, the least count of screw gauge is lesser then vernier caliper, hence screw gauge is more precise.
Question 4.
Having all units in atomic standards is more useful. Explain.
Answer:
An atomic mass unit (symbolized AMU or amu) is defined as precisely $1 / 12$ the mass of an atom of carbon-12. The carbon-12 (C-12) atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass. This is approximately $1.67377 \times 10^{-27}$ kilogram (kg), or $1.67377 \times 10^{-24}$ gram (g). The mass of an atom in AMU is roughly equal to the sum of the number of protons and neutrons in the nucleus.
The AMU is used to express the relative masses of, and thereby differentiate between, various isotopes of elements. Thus, for example, uranium-235 (U-235) has an AMU of approximately 235 , while uranium-238 (U-238) is slightly more massive. The difference results from the fact that U-238, the most abundant naturally occurring isotope of uranium, has three more neutrons than U-235, an isotope that has been used in nuclear reactors and atomic bombs.

Question 5 .
Why dimensional methods are applicable only up to three quantities?
Answer:
Understanding dimensions is of utmost importance as it helps us in studying the nature of physical quantities mathematically. The basic concept of dimensions is that we can add or subtract only those quantities which have same dimensions. Also, two physical quantities are equal if they have same dimensions. these basic ideas help us in deriving the new relation between physical quantities, it is just like units.