In this article, you will learn about dimensions and the dimensional formula of physical quantities with the help of examples.
Let’s start with the definitions first.
What are the dimensions?
The dimension of a physical quantity is defined as the power to which the fundamental quantities are raised to express the physical quantity. For example, the dimension of velocity is 0, 1, -1 in Mass, Length, Time, respectively.
If this feels confusing at first, don’t worry, we will look into the calculations of dimensions later in this article.
What is the dimensional formula?
Dimensional Formula is defined as the expression of the physical quantity in terms of its base unit (fundamental quantity) with proper dimensions. For example,
Dimensional Formula of Velocity is [ M0 L1 T-1 ]
Here,
- M, L, T are fundamental quantities Mass, Length and Time, respectively
- 0, 1, -1 are the dimensions
We generally use 3 fundamental quantities: Mass, Length, and Time to derive dimensional formulas of different physical quantities.
First, let’s look into the dimensional symbol of these fundamental quantities.
Dimensional Formula of Fundamental Quantities
- Mass is [M]
- Length is [L]
- Time is [T]
How to calculate the dimensional formula of physical quantities?
Now that we know what the dimensional formula is, let’s see how we can compute the dimensional formula of a physical quantity.
We can calculate the dimension formula of a physical quantity when its relation with other physical quantities is known. For example, suppose we want to calculate the dimensional formula of density.
We know that density is related to mass and volume, that is, density = mass / volume. Now, let’s use this relation to calculate the dimensional formula of density:
Density = mass / volume
= mass / (length * breadth * volume)
We can use the fundamental quantity
- [L] (Length) – for length, breadth, and volume
- [M] (Mass) – for mass
Hence, our formula becomes
Density = [M] / ([L] * [L] * [L]
= [M] / [ L3 ]
= [ M1 ] * [ L-3 ]
And, since there is no mention of time in density, we can write
Dimensional Formula of Density = [ M1 L-3 T0 ]
In this way, we can calculate the dimensional formula of physical quantities.
Dimensional Formula and SI Unit
We can also derive SI units by using the dimensional formula and vice versa. Let’s see an example.
1. Dimensional formula to SI unit conversion
Dimensional Formula of density = [ M1 L-3 T0 ].
We know the SI unit of
- [M] is kg (kilogram)
- [L] is m (meter)
- [T] is s (second)
Now, if we convert the formula to SI units, it will be [ kg1 m-3 s0 ]. Hence,
SI unit of density is kg m-3.
2. SI unit to dimensional formula conversion
SI unit of density is kg m-3
Here, we know that
- kg is the SI unit of mass
- m is the SI unit of length
So our formula will look like this [M L-3].
Since there is no mention of time here, we can use 0 for time.
Dimensional formula of density = [M1 L-3 T0 ]
Dimensional Formula of Physical Quantity
Now that we know the dimensional formula, let’s see the dimensional formula of various physical quantities.
| Physical Quantity | Dimensional Formula | SI Unit |
|---|---|---|
| Area | [ M0 L2 T0 ] | m2 |
| Volume | [ M0 L3 T0 ] | m3 |
| Speed/Velocity | [ M0 L1 T-1 ] | ms-1 |
| Acceleration | [ M0 L1 T-2 ] | ms-2 |
| Momentum | [ M1 L1 T-1 ] | kg m s-1 |
| Force | [ M1 L1 T-2 ] | kg m s-2 |
| Pressure | [ M1 L-1 T-2 ] | kg m-1 s-2 |
| Work | [ M1 L2 T-2 ] | kg m2 s-2 |
| Energy | ||
| Power | [ M1 L2 T-3 ] | kg m2 s-3 |
| Gravitational Constant | [ M-1 L3 T-2 ] | kg-1 m3 s-2 |
| Impulse | [ M1 L1 T-1 ] | kg m s-1 |
| Surface Tension | [ M1 L0 T-2 ] | kg s-2 |
| Coefficient of Viscosity | [ M1 L-1 T-1 ] | kg m-1 s-1 |
| Momentum of Inertia | [ M1 L2 T0 ] | kg m2 |
| Angular Momentum | [ M1 L2 T-1 ] | kg m2 s-1 |
| Torque | [ M1 L2 T-2 ] | kg m2 s-2 |
| Frequency | [ M0 L0 T-1 ] | s-1 |
| Magnetic Flux | [ M1 L2 T-2 A-1] | kg m2 s-2 A-1 |
| Stress | [ M1 L-1 T-2 ] | kg m-1 s-2 |
| Capacitance | [ M-1 L-2 T2 A2 ] | kg-1m-2 s2 A2 |
| Charge | [ M0 L0 T1 A1 ] | s A |
| Resistance | [ M1 L2 T-3 A-2 ] | kg1m2 s-3 A-2 |
| Planck’s Constant | [ M1 L2 T-1 ] | kg m1 s-1 |
| Surface Tension | [ M1 L0 T-2 ] | kg s-2 |
Dimensional Equation
An equation containing physical quantities with dimensional formula is known as the dimensional equation. A dimensional equation is obtained by equating the dimensional formula on the right-hand and left-hand sides of an equation. For example,
Dimensional equation of v = u + at is
[ M0 L1 T-1 ] = [ M0 L1 T-1 ] + [ M0 L1 T-1 ] * [ M0 L0 T1 ]
Principle of Homogeneity of Dimensional Equation
According to this principle, the dimensions of fundamental quantities on the left-hand side of an equation must be equal to the dimensions of the fundamental quantities on the right-hand side of that equation.
Let us consider three quantities A, B, and C, such that C = A + B. Therefore, according to this principle, the dimensions of C are equal to the dimensions of A and B.
Let’s see an example. Here we have our earlier dimensional equation of v = u + at:
[ M0 L1 T-1 ] = [ M0 L1 T-1 ] + [ M0 L1 T-1 ] * [ M0 L0 T1 ]
1. Solving Right Hand Side
RHS = [M0 L1 T-1] + [M0 L T-1] X [M0 L0 T]
= [M0 L1 T-1] + [M0 L1 T0 ]
RHS = [M0 L1 T-1 ] = LHS
Here, the right-hand side equation is now the same as the left-hand side equation. Hence, the principle of homogeneity of dimensional equations is proved.
What are the uses of dimensional equations?
Here are some of the uses of the dimensional equation:
- We can use dimensional equations to check the correctness of physical relations.
- It also helps us to derive the relation between various physical quantities.
- We can convert the value of physical quantity from one system of units to another system using the dimensional equation.
- It finds the dimension of constants in a given relation.
What are the limitations of dimensional analysis?
Following are the limitations of dimensional analysis.
- We cannot get the information about the dimensional constant using the dimensional analysis.
- If a quantity depends on more than three factors having dimension, We cannot derive the dimensional formula.
- We cannot derive the formula containing trigonometric, exponential, logarithmic functions, etc.
- We cannot develop the exact form of a relationship when there is more than one part in any relationship.
- It gives no information on whether a physical quantity is a scalar or vector.