Units & Measurement Class 11 Notes Physics Chapter 1

In order to make the measurement of a physical quantity we have, first of all, to evolve a standard for that measurement so that different measurements of the same physical quantity can be expressed relative to each other. That standard is called a unit of that physical quantity.

• System of Units
  (a) C.G.S (Centimeter-Grand-Second) system.
  (b) F.P.S. (Foot-Pound-Second) system.
  (c) M.K.S. (Meter-Kilogram--Second) system.
  (d) M.K.S.A. (Meter-Kilogram-Second-Ampere) unit.
• Dimensional Formula
  The dimensional formula of a physical quantity is the formula that tells us how and which of the fundamental units have been used for the measurement of that quantity.
• How to write dimensions of physical quantities
  (a) Write the formula for that quantity, with the quantity on L.H.S. of the equation.
  (b) Convert all the quantities on R.H.S. into the fundamental quantities mass, length and time.
  (c) Substitute M, L, and T for mass, length, and time respectively.
  (d) Collect terms of M, L, and T and find their resultant powers (a,b,c) which give the dimensions of the quantity in mass, length, and time respectively.
• Characteristics of Dimensions
  (a) Dimensions of a physical quantity are independent of the system of units.
  (b) Quantities having similar dimensions can be added to or subtracted from each other.
  (c) Dimensions of a physical quantity can be obtained from its units and vice-versa.
  (d) Two different physical quantities may have the same dimensions.
  (e) Multiplication/division of dimensions of two physical quantities (may be same or different) results in the production of dimensions of a third quantity.
  

• Mechanical Physical Quantities (derived)
  
  
• Electrical Physical Quantities (derived)
  
  

• Principle of Homogeneity
  It states that “ the dimensional formulae of every term on the two sides of a correct relation must be same.” 
• Types of Error
  (a) Constant errors: An error is said to be a constant error if it affects, every time, a measurement in a similar manner.
  (b) Systematic errors: Errors which come into existence by virtue of a definite rule, are called systematic errors.
  (c) Random error or accidental error: Error that takes place in a random manner and cannot be associated with a systematic cause are called random or accidental errors.
  (d) Absolute error:
  (e) Relative Error:
  (f) Percentage Error: