Unit 1, Topic 1.3 International System of Units (SI) Questions and Answers – Short & Long Questions

Chapter 1 Physical Quantities and Measurement Physics Notes | Complete Guide with Formulas & Derivations 

1.3 International System of Units (SI) Expected Board Short & Long Questions

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Short Questions

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1. What is meant by the SI system?

Answer:
The International System of Units (SI) is an internationally accepted system of measurement based on a set of defined base units from which other units are derived.

It provides a common and standardized system for measuring physical quantities and is widely used in Physics, Chemistry, Engineering, Medicine, Industry, Technology, and scientific research.

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2. Why is a standard system of units necessary?

Answer:
A standard system of units is necessary so that measurements can be expressed and compared consistently throughout the world. Without standardized units, different people or countries could use different units for the same physical quantity, making scientific communication and comparison difficult.

The SI system provides common standards for scientific experiments, engineering, manufacturing, international trade, and technological applications.

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3. Name the seven SI base units.

Answer:
The seven SI base units are:

1. metre (m) — unit of length
2. kilogram (kg) — unit of mass
3. second (s) — unit of time
4. ampere (A) — unit of electric current
5. kelvin (K) — unit of thermodynamic temperature
6. mole (mol) — unit of amount of substance
7. candela (cd) — unit of luminous intensity

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4. What is the SI unit of length?

Answer:
The SI base unit of length is the metre, represented by the symbol m.

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5. What is the SI unit of mass?

Answer:
The SI base unit of mass is the kilogram, represented by the symbol kg.

6. What is the SI unit of electric current?

Answer:
The SI base unit of electric current is the ampere, represented by the symbol A.

7. What is an SI derived unit?

Answer:
An SI derived unit is a unit obtained by combining SI base units mathematically according to the relationship defining a derived physical quantity.

For example, speed is defined as distance divided by time:

$v=\frac{d}{t}$

Therefore, its SI unit is:

$\frac{\text{m}}{\text{s}}=\text{m/s}$

8. Give three examples of SI derived units.

Answer:
Three examples of SI derived units are:

1. Newton (N) — unit of force
2. Joule (J) — unit of energy
3. Watt (W) — unit of power

Other examples include the pascal (Pa) for pressure, hertz (Hz) for frequency, and coulomb (C) for electric charge.

9. What is the purpose of SI prefixes?

Answer:
SI prefixes are used to represent decimal multiples and submultiples of SI units. They make it easier and more convenient to express extremely large or extremely small physical quantities.

For example:

$1\text{km}=1000\text{m}$

and:

$1\text{mm}=0.001\text{m}$

10. What does the prefix kilo mean?

Answer:
The SI prefix kilo, represented by the symbol k, means a factor of one thousand.

In scientific notation:

$\text{kilo}={10}^3=1000$

For example:

$1\text{km}=1000\text{m}$

Long Questions

1. Explain the International System of Units and discuss its importance.

Answer:
The International System of Units (SI) is an internationally accepted system of measurement based on seven defined base units. These base units provide the foundation for the measurement of physical quantities and the formation of derived units.

The SI system is used throughout the world in Physics, Chemistry, Engineering, Medicine, Industry, Technology, and scientific research.

Importance of the SI system:

• It provides a common international standard for measurement.
• It allows measurements from different countries to be compared reliably.
• It improves communication among scientists and engineers.
• It provides consistency in scientific experiments.
• It supports engineering, manufacturing, and technological development.
• It simplifies the expression and conversion of physical quantities.
• It provides a systematic basis for derived units and SI prefixes.

Thus, the SI system provides a consistent and universal framework for scientific measurement.

2. Describe the seven SI base units with their symbols.

Answer:
The SI system is based on seven base units. Each base unit corresponds to one of the seven fundamental physical quantities.

Physical Quantity	SI Base Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Length — metre (m): The metre is the SI base unit used to measure length.

Mass — kilogram (kg): The kilogram is the SI base unit of mass.

Time — second (s): The second is the SI base unit of time.

Electric current — ampere (A): The ampere is the SI base unit of electric current.

Thermodynamic temperature — kelvin (K): The kelvin is the SI base unit of thermodynamic temperature.

Amount of substance — mole (mol): The mole is the SI base unit for amount of substance.

Luminous intensity — candela (cd): The candela is the SI base unit of luminous intensity.

3. Explain how derived SI units are formed from base units with examples.

Answer:
Derived SI units are formed by combining SI base units mathematically according to the definitions of derived physical quantities.

Speed:

Speed is defined as distance divided by time:

$v=\frac{d}{t}$

Therefore, its SI unit is:

$\frac{\text{m}}{\text{s}}=\text{m/s}$

Acceleration:

Acceleration is the change in velocity per unit time:

$a=\frac{\Delta v}{t}$

Its SI unit is:

$\frac{\text{m}}{{\text{s}}^2}=\text{m/s²}$

Force:

Force is defined by Newton's second law:

$F=ma$

Therefore, its SI unit can be expressed in base units as:

$\text{kg}\times \frac{\text{m}}{{\text{s}}^2}=\text{kg m/s²}$

This derived unit is called the newton (N).

Thus, derived SI units are constructed systematically from the seven SI base units.

4. Discuss common SI prefixes and their applications.

Answer:
SI prefixes are added to unit names to represent decimal multiples or submultiples of SI units. They are especially useful when physical quantities are extremely large or extremely small.

Prefix	Symbol	Factor	Power of Ten
giga	G	1,000,000,000	${10}^9$
mega	M	1,000,000	${10}^6$
kilo	k	1,000	${10}^3$
centi	c	0.01	${10}^{\mathrm{-2}}$
milli	m	0.001	${10}^{\mathrm{-3}}$
micro	μ	0.000001	${10}^{\mathrm{-6}}$
nano	n	0.000000001	${10}^{\mathrm{-9}}$

For example:

$1\text{km}=1000\text{m}$

and:

$1\text{mm}=0.001\text{m}$

Prefixes therefore make measurements shorter, clearer, and more convenient.

5. State and explain important rules for writing SI units and symbols.

Answer:
SI units and symbols follow internationally agreed conventions. Important rules include the following:

1. Unit symbols are written after the numerical value.

For example:

$25\text{ m}$

not simply 25 when a physical measurement is intended.

2. A space is normally left between the numerical value and the unit symbol.

For example:

$5\text{ kg}$

3. Unit symbols are not normally followed by a full stop.

For example, write 5 kg, not 5 kg. when the period is not serving as sentence punctuation.

4. Unit symbols are case-sensitive.

For example, m represents metre, while M is the SI prefix for mega.

5. Symbols for units named after people are written with a capital initial letter.

Examples include:

• newton — N
• joule — J
• watt — W
• pascal — Pa
• ampere — A

6. Unit names are generally written in lowercase.

For example: metre, kilogram, second, newton, joule, and watt.

Following these conventions helps maintain clarity, consistency, and accuracy in scientific communication.

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