 |
| Scaling the Universe: From the subatomic Fermi (10^{-15} \text{ m}) to the vastness of a Parsec (3 \times 10^{16} \text{ m}). |
🔗 Internal Link
* "Basic SI Units and Dimensions": Link this when mentioning the "milli" or "kilo" prefixes to explain the base system.
* "Errors in Measurement": Link to this after the "Scientific Notation" section to explain why precision matters.
* "Atomic Structure": Link the Bohr Radius and amu values to your chemistry or atomic physics chapter.
* "Work, Energy, and Power": Link the Horsepower (HP) and kWh conversions to the energy chapter.
Physics Quick Reference Guide
Special Units for Length
Fermi (f): 10-15 m
Angstrom (Ã…): 10-10 m
Nanometre (nm): 10-9 m
Micron (μ): 10-6 m
X-Ray Unit: ~10-13 m
Bohr Radius: 0.5 × 10-10 m
Mile: 1.609 km
Large Scale (Space):
- Astronomical Unit (AU): 1.496 × 1011 m (Mean Sun-Earth distance)
- Light Year (ly): 9.46 × 1015 m (Distance light travels in 1 year at 3 × 108 m/s)
- Parsec: 3.084 × 1016 m (3.26 light years)
Special Units for Mass & Time
Quintal: 100 kg
Metric Ton: 1000 kg
Atomic Mass Unit (amu): 1.67 × 10-27 kg
Chandrasekhar Limit: 1.4 × Mass of Sun
Shake (Time): 10-8 seconds
Prefixes for Powers of 10
| Macro Prefixes (Multiples) | ||
|---|---|---|
| Multiplier | Symbol | Name |
| 101 | da | Deca |
| 102 | h | Hecto |
| 103 | k | Kilo |
| 106 | M | Mega |
| 109 | G | Giga |
| 1012 | T | Tera |
| 1015 | P | Peta |
| Micro Prefixes (Sub-multiples) | ||
|---|---|---|
| Multiplier | Symbol | Name |
| 10-1 | d | deci |
| 10-2 | c | centi |
| 10-3 | m | milli |
| 10-6 | μ | micro |
| 10-9 | n | nano |
| 10-12 | p | pico |
| 10-15 | f | femto |
Important Conversions
| 1 km/h = 5/18 m/s | 1 Newton = 105 dyne |
| 1 Joule = 107 erg | 1 Calorie = 4.18 Joule |
| 1 eV = 1.6 × 10-19 J | 1 HP = 746 W |
| 1 Tesla = 104 Gauss | 1 kWh = 3.6 × 106 J |
Important Physical Constants
| Constant | Value |
|---|---|
| Avogadro's Number (N) | 6.023 × 1023 mol-1 |
| Planck's Constant (h) | 6.62 × 10-34 J·s |
| Permittivity of Free Space (ε₀) | 8.854 × 10-12 F/m |
| Boltzmann's Constant (k) | 1.38 × 10-23 J/K |
| Universal Gas Constant (R) | 8.314 J·mol-1·K-1 |
| Atmospheric Pressure (1 atm) | 1.013 × 105 Pa |
Units & Measurements: Practice Paper
I. Multiple Choice Questions (NEET Pattern)
NEET Q1. Which of the following is the largest unit of length?
Correct: C (Parsec). 1 Parsec = 3.26 light years ≈ 3.08 × 1016 m.
NEET Q2. The value of 1 Bohr radius is approximately:
Correct: A. Bohr radius is 0.5 × 10-10 m, which equals 0.5 Angstrom (Ã…).
II. Short & Very Short Questions (CBSE Pattern)
1 Mark Q3. Define 'Shake'. What is its value in SI units?
Shake is a non-SI unit of time used in nuclear physics. 1 Shake = 10-8 seconds.
2 Marks Q4. Differentiate between Astronomical Unit (AU) and Light Year (ly).
AU: Mean distance between Earth and Sun (1.496 × 1011 m).
Light Year: Distance traveled by light in vacuum in one year (9.46 × 1015 m).
Light Year: Distance traveled by light in vacuum in one year (9.46 × 1015 m).
III. Assertion and Reason
Options: (A) Both A and R are true and R is correct explanation. (B) Both true but R is NOT correct explanation. (C) A is true, R is false. (D) A is false, R is true.
Assertion (A): Parsec is a unit used to measure time in astronomical observations.
Reason (R): 1 Parsec is equal to 3.26 light years.
Correct: D. Assertion is false because Parsec is a unit of distance, not time. Reason is true.
IV. Long Answer Questions (CBSE)
5 Marks Q5. List the macro and micro prefixes from 1015 to 10-15. Explain why scientific notation is preferred over using full zeros in physics.
Students should list: Peta (1015), Tera, Giga, Mega, Kilo, Hecto, Deca... down to Femto (10-15).
Reasoning: Scientific notation prevents "zero-counting" errors, makes multiplication/division easier (adding/subtracting exponents), and clearly indicates the precision (significant figures) of a measurement.
Reasoning: Scientific notation prevents "zero-counting" errors, makes multiplication/division easier (adding/subtracting exponents), and clearly indicates the precision (significant figures) of a measurement.
-Wed 25 Mar 2026
-4:02p.m
