Study Notes
Physical Quantities
Core Idea
Any physical phenomenon that can be measured quantitatively is called a physical quantity.
If you can assign:
- A number
- A unit
→ It is a physical quantity.
Examples
Physical Quantities:
- Length
- Time
- Speed
- Magnetic field
- Force
Not Physical Quantities:
- Smell
- Taste
- Emotion
Why?
Because they cannot be measured universally with a standard unit.
IOE Insight:
If a question asks:
“Which of the following is not a physical quantity?”
Immediately eliminate anything that:
- Cannot be expressed numerically
- Has no standard unit
Types of Physical Quantities
There are two main categories:
- Fundamental Quantities
- Derived Quantities
1. Fundamental Quantities
Definition
Quantities that are independent and cannot be expressed in terms of other physical quantities.
They form the base of measurement system.
There are 7 fundamental quantities:
| Fundamental Quantity | SI Unit |
|---|---|
| Length | meter (m) |
| Mass | kilogram (kg) |
| Time | second (s) |
| Temperature | kelvin (K) |
| Electric Current | ampere (A) |
| Luminous Intensity | candela (Cd) |
| Amount of Substance | mole (mol) |
Key Concept
All other quantities in physics can ultimately be expressed using combinations of these seven.
Exam Trap
- Speed is NOT fundamental.
- Force is NOT fundamental.
- Temperature is fundamental.
- Electric current is fundamental.
Students often forget electric current and luminous intensity.
2. Derived Quantities
Definition
Quantities that depend on fundamental quantities are called derived quantities.
They are formed by mathematical combination (multiplication/division) of fundamental quantities.
Examples
Area
\[ \text{Area} = \text{Length} \times \text{Length} \]
Depends only on length → derived quantity.
Speed
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
Depends on:
- Length
- Time
Hence derived.
Other Important Derived Quantities
- Volume
- Force
- Power
- Sound intensity
- Magnetic flux
- Density
- Momentum
- Pressure
IOE Trick:
If quantity involves a formula, it is almost always derived.
Deep Conceptual Clarity
Fundamental quantities are like primary colors.
Derived quantities are like mixed colors formed from them.
In dimensional form:
Derived quantities reduce to combinations of:
\[ [M^a L^b T^c I^d \Theta^e N^f J^g] \]
Where each symbol represents one fundamental quantity.
Quick Elimination Strategy (MCQ Mode)
If options include:
- Length → Fundamental
- Mass → Fundamental
- Speed → Derived
- Force → Derived
- Volume → Derived
- Temperature → Fundamental
Always count 7. No more, no less.
Common IOE Confusions
- Intensity of sound → Derived
- Magnetic field → Derived
- Amount of substance → Fundamental
- Luminous intensity → Fundamental
Remember:
If it can be written in terms of M, L, T (and other base quantities), it is derived.
Practice MCQs
1. Which of the following is a fundamental quantity?
A. Speed
B. Force
C. Temperature
D. Pressure
Answer
C. Temperature
2. Which of the following is NOT a physical quantity?
A. Mass
B. Magnetic Field
C. Taste
D. Time
Answer
C. Taste
3. Area is a:
A. Fundamental quantity
B. Derived quantity
C. Non-physical quantity
D. Constant
Answer
B. Derived quantity
Summary
- Physical quantity → measurable phenomenon with unit.
- Two types: Fundamental and Derived.
- There are exactly 7 fundamental quantities.
- All other measurable quantities are derived.
- IOE often tests classification and elimination.
Master this classification clearly — many easy marks come from simple recognition.