What are Physical Quantities in Physics, Definition, Types & Examples

Physical Quantities in Physics | Definition, Types & Examples

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Physical quantities are the measurable properties of a system that form the basis of all calculations in physics. They are expressed using numerical values along with standard units, ensuring consistency and accuracy in scientific communication. From fundamental quantities like length, mass, and time to derived quantities such as velocity and force, understanding physical quantities is essential for solving problems and interpreting real-world phenomena. Practice JEE Main Units and Measurements PYQs with detailed solutions by clicking the link.

Table of Contents

What are Physical Quantities ?

Physical quantities are those quantities that can be measured and used to explain various physical phenomena in the form of laws. A physical quantity is a measurable property of a physical system or material, expressed by a numerical value and a unit (e.g. 10m, 4 kg). It enables the quantification of physical reality, describing laws of physics, including mass, time, length, temperature, and force. Non-measurable quantities such as happiness or sorrow are not physical quantities.

Define Physical Quantity
Anything that can be measured, such as length, weight, or speed.

What is Measurement ?

Measurement is a technique in which the properties of an object are determined by comparing them to a standard. Before a measurement can be made, a standard or unit must be chosen. The size of the quantity to be measured is then found with an instrument having a scale marked in the unit. Measurement requires tools to provide scientists with a quantity.

Why is Measurement Important?

Measurement helps determine the magnitude of a physical quantity, compare similar physical quantities, and verify physical laws or equations.

What is a unit ?

A unit can be defined as an internationally accepted standard for measuring quantities. The result of a measurement is expressed by a number which is accompanied by a unit.

How is a Physical Quantity Represented?

A physical quantity consists of a numerical magnitude and a unit

Example:
10 meters (where 10 is the numerical value and meter is the unit).

Formula: 
Physical Quantity (Q) = Magnitude × Unit = n × u = constant

If the unit (u) changes, the numerical value (n) changes inversely, but their product remains constant.

If n1 is the numerical value of the physical quantity for a unit u1 and n2 for a unit u2, then n1 u1 = n2 u2

What are the Types of Physical Quantities ?

1. Ratio (Numerical Value Only)

These physical quantities are expressed as the ratio of two similar quantities and have no unit.

  • Relative density = Density of object / Density of water at 4°C
  • Refractive index = Velocity of light in air / Velocity of light in medium
  • Strain = Change in dimension / Original dimension

**Note: Angle is an exception as it requires a unit (degrees or radians) despite being a ratio.

2. Scalar Quantities (Magnitude Only)

These quantities have only magnitude and no direction. When you measure them, you get a numerical value with a unit, like 5 kg or 25°C.

  • Examples: Length, time, work, energy
  • Scalars can be negative; the negative sign indicates a decrease but does not specify direction.
  • Scalars follow the normal laws of addition and subtraction.

3. Vector Quantities (Magnitude and Direction)

These quantities have both magnitude and direction. When you measure vector quantities, you get a numerical value along with a unit and a specific direction, such as 20 m/s east or 10 N north.

  • Examples: Displacement, velocity, acceleration, force
  • Vectors follow special vector laws of addition and subtraction, different from scalar addition.

4. Tensor Quantities

Tensors are used to represent physical quantities that have a directional dependence. Some quantities, like moment of inertia, are neither scalar nor vector. These are called Tensors because their values change with direction but do not behave like vectors.

Frequently Asked Questions (FAQs)

Q1: Why is measurement important in science?

A: Measurement allows us to define physical quantities accurately, compare values, and formulate scientific laws.

Q2: What is the difference between scalar and vector quantities?

A: Scalars have only magnitude, while vectors have both magnitude and direction.

Q3: Why does moment of inertia not behave as a vector?

A: It does not change when the sense of rotation is reversed and has different values along different axes, making it a tensor quantity.

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