SI Units & Their Conventions Class 11 Physics | Question-Answer, FAQS, Worksheet, MCQS, Test Paper

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SI Units and Their Conventions

SI (Système International d’Unités) is the internationally accepted system of measurement. Proper use of SI units ensures clarity, accuracy, and consistency in scientific and technical communication. Below are the fundamental conventions for using SI units:

1. Representation of Units: The unit of every physical quantity should be represented by its symbol. Example : Length is measured in meters (m), time in seconds (s).
2. Capitalization Rules:
   • The full name of a unit always starts with a lowercase letter, even if named after a scientist. Example: newton, joule.
   • However, the symbol for such units is capitalized. Example: Newton (N), Joule (J).
3. Plural Form: Symbols for units do not take a plural form. Example: Force is 20 N, not 20 newtons or 20 Ns.
4. Punctuation: Symbols for units should not contain any full stops. Example: 25 kg, not 25 kg..
5. Writing Units in Numerator and Denominator:
   • The SI unit of acceleration is written as m/s² or m s⁻², but not m/s/s.
6. Combination of Units:
   • Avoid mixing full unit names with symbols. Example: The correct notation for specific heat capacity is J/kg K, not joule/kg K.
7. Prefix Symbols:
   • Prefixes are used before unit symbols to denote powers of 10. Example:
     • 1 ms = 1 millisecond = 10⁻³ s
     • 1 μs = 1 microsecond = 10⁻⁶ s
     • 1 ns = 1 nanosecond = 10⁻⁹ s
   • Double prefixes should be avoided when a single one is available. Example:
     • 10⁻⁶ s = 1 μs (not 1mms).
     • 10⁻⁹ s = 1 ns (not 1 mμs).
8. Spacing Rules:
   • A space or hyphen should be used when indicating multiplication of two units. Example:
     • Correct: m s⁻¹ or m-s⁻¹.
     • Incorrect: ms⁻¹ (ms can be misinterpreted as milliseconds).

Exam-Oriented Questions and Answers

MCQs with Answers

1. Which of the following is correct regarding SI unit symbols?
   • (a) The full name of a unit named after a scientist is written with a capital letter.
   • (b) The unit symbol is always written in lowercase.
   • (c) The unit symbol named after a person is capitalized.
   • (d) Symbols for units take plural forms.
   Answer: (c) The unit symbol named after a person is capitalized. Explanation: While the full name of the unit (e.g., newton, joule) starts with a lowercase letter, the symbol (N, J) is written in uppercase.
2. Which of the following is the correct way to express acceleration in SI units?
   • (a) m/s/s
   • (b) m/s²
   • (c) ms²
   • (d) m-s¹
   Answer: (b) m/s² Explanation: The correct SI notation for acceleration is meters per second squared (m/s² or m s⁻²).

Conceptual Questions with Answers

1. Why should unit symbols not have a full stop at the end?
   • Answer: A full stop might be misinterpreted as a decimal point, leading to confusion in scientific calculations.
2. Why is “ms⁻¹” incorrect notation for meters per second?
   • Answer: “ms” is already the symbol for milliseconds, so “ms⁻¹” could be misinterpreted as milliseconds inverse. The correct notation is “m s⁻¹”.

Do You Know?

• The SI system was established in 1960 by the General Conference on Weights and Measures (CGPM).
• The seven base units in the SI system are meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).
• The speed of light is exactly 299,792,458 m/s, as per SI definitions.

Test Your Knowledge(Quiz)

Worksheet

1. Fill in the blanks:
   • The SI unit of force is ________.
   • The symbol for microsecond is ________.
   • 10⁻¶ seconds is written as ________.
2. True/False:
   • The symbol for kilogram is Kg.
   • 1 newton is equal to 1 kg m/s².
3. Convert:
   • 500 milliseconds to seconds.
   • 5 km to meters.

Test Paper

1. Choose the correct SI notation:
   • (a) m/s/s
   • (b) ms⁻¹
   • (c) m/s²
   • (d) m-s¹
2. Match the following: Quantity

3. Answer the following:

• Why do we use SI units?
• What is the significance of unit prefixes?

Important Points for Quick Revision

• SI unit names are written in lowercase (e.g., newton), but their symbols are capitalized if named after a person (e.g., N).
• Unit symbols do not take plural forms (e.g., 20 N, not 20 newtons).
• Use space or hyphen in derived units (e.g., m s⁻¹, not ms⁻¹).
• Prefixes like milli (10⁻³), micro (10⁻⁶), and nano (10⁻⁹) are used before unit symbols to represent powers of ten.

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