What Are Physical Quantities? Easy Explanation, Types, SI Units & Formulas

Physics Essentials

Physical Quantities: Types, SI Units, Dimensions & Simple Examples

A beginner-friendly, complete guide to understanding how we measure everything in the physical universe.

Have you ever wondered how scientists calculate the speed of a rocket, measure the weight of an apple, or quantify the heat of an engine? All of science starts with one simple idea: measurement.

In this guide, we break down what physical quantities are, how they work, and why they matter—using clear, everyday language and visual breakdowns.

1. What Exactly is a Physical Quantity?

A physical quantity is any characteristic of an object or natural phenomenon that can be measured and expressed using a number and a unit.

The Golden Formula of Measurement

Physical Quantity = Numerical Value (n) × Unit (u)

Example: Distance = 5 (how many times) × meters (standard chosen)

✅ Physical Quantities (Measurable)

• Length (e.g., 1.8 meters)
• Mass (e.g., 65 kilograms)
• Time (e.g., 10 seconds)
• Temperature (e.g., 300 Kelvin)

❌ Non-Physical Quantities (Not Measurable)

• Happiness or Sadness
• Love or Friendship
• Pain intensity (no exact standard unit)
• Anger or Excitement

2. Fundamental vs. Derived Quantities

Physics organizes all quantities into two major groups: the building blocks (fundamental) and the constructions built from them (derived).

A. The 7 Fundamental (Base) Quantities

These are pure, independent quantities. You cannot break them down into anything simpler. In the worldwide International System of Units (SI), there are strictly seven:

Base Quantity	SI Unit	Symbol	Everyday Meaning
Length	Meter	m	Distance between two points
Mass	Kilogram	kg	Amount of matter inside an object
Time	Second	s	Duration of an event or process
Electric Current	Ampere	A	Flow of electric charges in a circuit
Temperature	Kelvin	K	Average thermal energy of particles
Amount of Substance	Mole	mol	Count of atoms, molecules, or ions
Luminous Intensity	Candela	cd	Brightness of a light source

B. Derived Quantities (Formulas at Work)

Derived quantities are formed by combining two or more base quantities using multiplication or division. Think of them like recipes combining base ingredients.

How Speed is Derived:

Formula: Speed = Distance ÷ Time

Base units used: Length (meter) and Time (second)

Resulting Unit: Meter per second (m/s)

Derived Quantity	Formula	Base Unit Expression	Special Name
Area	Length × Width	m × m = m²	Square meter
Density	Mass ÷ Volume	kg ÷ m³	kg/m³
Force	Mass × Acceleration	kg × m/s²	Newton (N)
Work / Energy	Force × Distance	N × m	Joule (J)
Pressure	Force ÷ Area	N ÷ m²	Pascal (Pa)

3. Scalars vs. Vectors: Why Direction Matters

Another crucial classification depends on whether you need a compass direction to describe what is happening.

1. Scalar Quantities

Only Magnitude (Size + Unit)

Scalars do not care about direction. They add up just like ordinary numbers.

Common Examples:
• Mass (5 kg)
• Speed (60 km/h)
• Time (45 minutes)
• Temperature (25°C)
• Work / Energy (200 J)

2. Vector Quantities

Magnitude + Direction

A vector is meaningless without stating where it is pointed. Changing direction changes the vector completely.

Common Examples:
• Displacement (5 km North)
• Velocity (60 km/h due East)
• Force (100 N downward)
• Acceleration (9.8 m/s² toward Earth)
• Momentum (Mass × Velocity)

 Pro Exam Tip: Is Electric Current a Vector?

Electric current has a direction (from positive to negative terminal), but it is not a vector. Why? Because it adds up using normal arithmetic ($2\text{ A} + 3\text{ A} = 5\text{ A}$), rather than the geometric laws of vector addition.

4. Dimensional Analysis Made Easy

In physics, every fundamental quantity is given a capital dimension letter wrapped in square brackets:

[M]

Mass

[L]

Length

[T]

Time

Example: Finding the Dimension of Force

1. Start with the formula: $\text{Force} = \text{Mass} \times \text{Acceleration}$
2. Break down acceleration: $\text{Acceleration} = \frac{\text{Velocity}}{\text{Time}} = \frac{\text{Distance}}{\text{Time} \times \text{Time}}$
3. Insert dimensional symbols: $\text{Force} = [M] \times \frac{[L]}{[T]^2}$
4. Write as one expression: [M¹ L¹ T⁻²]

Quick Summary Checklist

• Physical Quantities = Numerical value × Unit.
• 7 Base Quantities: Length (m), Mass (kg), Time (s), Current (A), Temperature (K), Amount (mol), Luminous Intensity (cd).
• Derived Quantities: Created by multiplying or dividing base quantities (e.g., Velocity, Force, Work).
• Scalars: Have magnitude only (Speed, Distance, Mass).
• Vectors: Have magnitude and direction (Velocity, Displacement, Force).

FAQ

Frequently Asked Questions

What is a physical quantity?

A physical quantity is any property of an object, system, or event that can be measured and expressed as a product of a numerical value and an internationally accepted unit (e.g., $5\text{ kg}$, $10\text{ m/s}$).

What are the 7 base SI physical quantities?

The seven base quantities are:

1. Length: Meter (m)
2. Mass: Kilogram (kg)
3. Time: Second (s)
4. Electric Current: Ampere (A)
5. Thermodynamic Temperature: Kelvin (K)
6. Amount of Substance: Mole (mol)
7. Luminous Intensity: Candela (cd)

What is the difference between a scalar and a vector?

Scalars have only magnitude (size and unit) and combine using basic arithmetic (e.g., mass, distance, speed, energy).
Vectors possess both magnitude and a specific direction in space, requiring vector algebra to add or subtract (e.g., displacement, velocity, acceleration, force).

Why is electric current treated as a scalar if it flows in a direction?

Even though electric current has a specified direction along a wire, it does not obey vector algebra (like the triangle law of vector addition). Current simply adds arithmetically at junctions ($I_{\text{total}} = I_1 + I_2$), so it is treated as a scalar.

What are supplementary units in the SI system?

Supplementary units are purely geometric, dimensionless units:

• Radian (rad): Used for measuring plane angles.
• Steradian (sr): Used for measuring solid angles (3D cone angles).