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Dimensional Formula and Dimensional Equation
How to Write Dimensions of Physical Quantities
Base quantities and their units
Derived units
Some Derived SI Units and Their Symbols
Dimensions
Example
Uses of Dimensional Analysis
Limitations of Dimensional Analysis
Related Resources
The fundamental quantities, irrespective of the system of units used for their measurement and their corresponding quantitative values, characterize different independent groups or classes of basic physical quantities. In other words, a fundamental quantity belongs to a class of physical quantities of its own kind only, each of them having no dependence with other fundamental quantities. The attribute that is common to a class of physical quantities is what is called their dimensionality.
Most physical quantities can be expressed in terms of combinations of five basic dimensions. These are mass (M), length (L), time (T), electrical current (I), and temperature, represented by the Greek letter theta (θ). These five dimensions have been chosen as being basic because they are easy to measure in experiments. Dimensions aren't the same as units. For example, the physical quantity, speed, may be measured in units of meters per second, miles per hour etc.; but regardless of the units used, speed is always a length divided a time, so we say that the dimensions of speed are length divided by time, or simply L/T. Similarly, the dimensions of area are L^{2} since area can always be calculated as a length times a length. For example, although the area of a circle is conventionally written as πr^{2}, we could write it as πr (which is a length) × r (another length).
Dimensions of a physical quantity are the powers to which the fundamental units be raised in order to represent that quantity. A dimension deal with qualitative part of measurement.
By international agreement a small number of physical quantities such as length, time etc. are chosen and assigned standards. These quantities are called ‘base quantities’ and their units as ‘base units’. All other physical quantities are expressed in terms of these ‘base quantities’. The units of these dependent quantities are called ‘derived units’.
The standard for a unit should have the following characteristics.
(a) It should be well defined.
(b) It should be invariable (should not change with time)
(c) It should be convenient to use
(d) It should be easily accessible
The 14th general conference on weights and measures (in France) picked seven quantities as base quantities, thereby forming the International System of Units abbreviated as SI (System de International) system.
Dimensional formula of a physical quantity is the formula which tells us how and which of the fundamental units have been used for the measurement of that quantity.
An equation written in the following manner is called dimensional equation.
Area = [M^{0}L^{2}T^{0}]
Dimensions of a physical quantity can be determined as follows:
(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.
The seven base quantities and their units are,
Base quantity
Unit
Symbol
Length
Meter
M
Mass
Kilogram
Kg
Time
Second
Sec
Electric current
Ampere
A
Temperature
Kelvin
K
Luminous intensity
Candela
Cd
Amount of substance
Mole
We can define all the derived units in terms of base units. For example, speed is defined to be the ratio of distance to time.
Unit of Speed = (unit of distance (length))/(unit of time)
= m/s = ms^{-1} (Read as meter per sec.)
Quantity
Express in base units
Force
newton
N
Kg-m/sec^{2}
Work
joules
J
Kg-m2/sec^{2}
Power
watt
W
Kg-m2/sec^{3}
Pressure
Pascal
Pa
Kg m-1/S^{2}
The following conventions are adopted while writing a unit.
Even if a unit is named after a person the unit is not written capital letters. i.e. we write joules not Joules.
For a unit named after a person the symbol is a capital letter e.g. for joules we write ‘J’ and the rest of them are in lowercase letters e.g. seconds is written as ‘s’.
The symbols of units do not have plural form i.e. 70 m not 70 ms or 10 N not 10Ns.
Not more than one solid’s is used i.e. all units of numerator written together before the ‘/’ sign and all in the denominator written after that.
i.e. It is 1 ms^{-2} or 1 m/s^{-2} not 1m/s/s.
Punctuation marks are not written after the unit
e.g. 1 litre = 1000 cc not 1000 c.c.
It has to be borne in mind that SI system of units is not the only system of units that is followed all over the world. There are some countries (though they are very few in number) which use different system of units. For example: the FPS (Foot Pound Second) system or the CGS (Centimeter Gram Second) system.
To express dimensions, each fundamental unit is represented by a capital letter. Thus the unit of length is denoted by L, unit of mass by M. Unit of time by T, unit of electric current by I, unit of temperature by K and unit of luminous intensity by C.
Remember that speed will always remain distance covered per unit of time, whatever is the system of units, so the complex quantity speed can be expressed in terms of length L and time T. Now, we say that dimensional formula of speed is LT^{-2}. We can relate the physical quantities to each other (usually we express complex quantities in terms of base quantities) by a system of dimensions.
Density of a substance is defined to be the mass contained in unit volume of the substance.
Hence, [density] = ([mass])/([volume]) = M/L^{3} = ML^{-3}
So, the dimensions of density are 1 in mass, -3 in length and 0 in time.
Hence the dimensional formula of density is written as [ρ]= ML^{-3}T^{0}
It is to be noted that constants such as ½ π, or trigonometric functions such as “sin wt” have no units or dimensions because they are numbers, ratios which are also numbers.
While stating the dimensions of a body, only the powers of fundamental units should be mentioned.
While starting the dimensional formula the fundamental units have to be raised through certain powers.
The dimensional formula of a quantity represents its qualitative nature only. Therefore, it must be enclosed inside brackets [M^{a}L^{b}T^{c}].
Dimensions of a physical quantity are independent of the system of units.
Quantities having similar dimensions can be added to or subtracted from each other.
Multiplication/division of dimensions of two physical quantities (may be same or different) results in production of dimensions of a third quantity.
Dimensions of a physical quantity can be obtained from its units and vice-versa.
Two different physical quantities may have same dimensions.
Dimensional analysis permits conversion of quantities from one system to another only in their fundamental values. Therefore, values of n1 must be noted when the unit is fundamental.
In case a relation contains more than one terms on either side of equation, the relation will be correct only if all the terms involved in that relation have same dimensions.
Owing to the process of its development, the process of dimensional analysis is subjected to the following limitations:
(a) It gives no information regarding the constant of proportionality involved in the equation.
(b) It cannot be used to derive an expression involving trigonometric functions.
(c) It cannot be used to derive an expression involving exponential functions.
(e) It cannot be used to derive an expression for a physical quantity which depends upon factors more than three.
Question 1:-
Which of the following is the dimensionless quantity:
(a) force of surface tension (b) strain
(c) stress (d) co-efficient of viscosity
Question 2:-
Dimensions of velocity gradient are same as that of
(a) time period (b) frequency
(c) angular acceleration (d) acceleration
Question 3:-
Out of the following pairs, only one pair does not have identical dimensions. It is
(a) angular momentum and Planck’s Constant
(b) moment of Inertia and moment of force
(c) work and torque
(d) impulse and momentum
Question 4:-
The dimension of Planck’s constant is same as that of:
(a) angular momentum (b) linear momentum
(c) work (d) coefficient of viscosity
Question 5:-
If K represents kinetic energy, V velocity and T time, and these are chosen as the fundamental units then, the units of surface tension will be:
(a) KV^{-2}T^{-2} (b) K^{-1}V^{-2}T^{-1} (c) KV^{-1}T^{-1} (d) KV^{-2}T^{2}
Q.1
Q.2
Q.3
Q.4
Q.5
b
a
You might like to refer Scalars and Vectors.
For getting an idea of the type of questions asked, refer the Previous Year Question Papers.
Click here to refer the most Useful Books of Physics.
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