# ptr

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Entertainment

Published on January 31, 2009

Author: sheikmohamed

Source: authorstream.com

Points to Remember : ? The measuring process is basically a comparison process. We select the unit of measurement (u) and find number of times (n) this unit is contained in the quantity (q). Then q = nu. ? The units of length (metre), mass (kilogram), time (sec), temp. (ºK), electric current (ampere), luminous intensity (candela) and quantity of matter (mole) are treated as fundamental units. The units of plane angle (radian) and solid angle (steradian) are supplementary units. The units of all other quantities, which can be obtained from the fundamental and supplementary units are called Derived units. For example, derived units of force, work/energy, power, electric potential and electric capacity are Newton (n), joule (J), watt (W), coulomb (C), volt (V) and farad (F) respectively. Points to Remember Slide 3: ? Some important Practical units are: (i) Astronomical unit, 1AU = 1.496 × 1011m (ii) Light year, 1ly = 9.46 × 1015m (iii) parsec, 1 par sec = 3.084 × 1016m (iv) Angstrom, 1Å = 10-10m (v) Fermi, 1F = 10-15m (vi) 1 barn = 10-28m2 (vii) 1 shake = 10-8s (viii) 1bar = 760 torr. Slide 4: ? The order of magnitude of a quantity gives us a value nearest to the actual value of the quantity in suitable powers of 10. For this, a number less than 5 is treated as 1 and a number between 5 and 10 is treated as 10. ? The masses we come across vary form 10-30 kg (mass of electron) to 1055 kg (mass of universe). An inertial balance is used for measuring inertial mass of a body and a common balance is used for measuring weight of a body. ? A quantity like angle has no dimensions, but it has units. The reverse is not true. ? The dimensional formulae of non mechanical quantities can be obtained using all the seven fundamental units and two supplementary units on SI. Slide 5: ? Three uses of dimensional analysis are: Conversion of one system of units into another. Checking the accuracy of various formulae. Derivation of formulae. In the last two advantages, we use the principle of homogeneity of dimensions. According to this principle, a formula is correct when dimensions of mass, length and time on one side of the formula are equal to their respective dimensions on other side of the formula. ? Note that only like quantities can be added on subtracted from one another. ? A coherent system is based on a certain set of fundamental units, from which derived units are obtained by multiplication or division without introducing numerical factors. ? Shortest time interval measured indirectly so far 10-22sec.

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