Complete Study Material for JEE, NEET, CBSE Board Class 11 Exams
What is a System of Units?
A: A system of units is a complete set of fundamental and derived units used to measure all physical quantities. Different systems have been developed based on fundamental quantities.
Types of Systems of Units
1. CGS System (Gaussian System)
- Fundamental Quantities: Length, Mass, Time
- Fundamental Units: Centimetre (cm), Gram (g), Second (s)
2. MKS System (Giorgi System)
- Fundamental Quantities: Length, Mass, Time
- Fundamental Units: Metre (m), Kilogram (kg), Second (s)
3. FPS System
- Fundamental Quantities: Length, Mass, Time
- Fundamental Units: Foot (ft), Pound (lb), Second (s)
- In this system, force is a derived quantity with the unit poundal.
4. SI System (International System of Units)
- The most widely used system, applicable to all branches of physics.
- It includes seven fundamental quantities and their corresponding units:
| Quantity | Name of Unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Electric Current | Ampere | A |
| Temperature | Kelvin | K |
| Amount of Substance | Mole | mol |
| Luminous Intensity | Candela | cd |
- Additionally, two supplementary units are defined:
- Radian (rad) for plane angle
- Steradian (sr) for solid angle
Practical Units in Measurement
- Practical units are frequently used fundamental or derived units that may not always belong to a standard system but can be converted into standard units.
- Examples:
- Light year (practical fundamental unit) → Distance measurement
- Horsepower (practical derived unit) → Power measurement
- 1 mile = 1.6 km = 1.6 × 10³ m
Multiple Choice Questions (MCQs) with Answers & Explanations
Q1: Which system of units uses centimetre, gram, and second as fundamental units?
A) MKS
B) SI
C) CGS
D) FPS
Answer: C) CGS
Explanation: The CGS system is also called the Gaussian system and uses cm, g, and s as fundamental units.
Q2: The SI unit of luminous intensity is:
A) Radian
B) Steradian
C) Candela
D) Mole
Answer: C) Candela
Explanation: Candela (cd) is the SI unit of luminous intensity.
Q3: Which system of units is also called the Giorgi system?
A) SI
B) CGS
C) FPS
D) MKS
Answer: D) MKS
Explanation: The MKS system, also known as the Giorgi system, uses metre, kilogram, and second as fundamental units.
Q4: How many fundamental quantities are there in the SI system?
A) 5
B) 6
C) 7
D) 8
Answer: C) 7
Explanation: The SI system defines seven fundamental quantities.
Q5: The practical unit of power is:
A) Newton
B) Joule
C) Horsepower
D) Watt
Answer: C) Horsepower
Explanation: Horsepower is a practical derived unit of power, commonly used for engine performance.
Worksheet on System of Units
Q1: Define a system of units and list the different systems used in physics.
Q2: What is the difference between the CGS and MKS systems?
Q3: How is force treated as a derived quantity in the FPS system?
Q4: Convert the following units:
- 1 mile to metres
- 5 light years to kilometres
- 10 horsepower to watts
Frequently Asked Questions (FAQs)
Q1: Why is the SI system preferred over other systems?
A: The SI system is internationally recognized, standardized, and universally accepted across all scientific disciplines.
Q2: What is the unit of solid angle in the SI system?
A: The unit of solid angle is Steradian (sr).
Q3: Can practical units be converted into SI units?
A: Yes, practical units can be expressed in terms of SI units. Example: 1 horsepower = 746 watts.
Q4: Why are supplementary units like Radian and Steradian used?
A: They are necessary for measuring angles in physics, which cannot be directly defined using fundamental quantities.
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