Physics Special Units Guide: Length, Mass & Time | CBSE & NEET PrepPhysics Special Units Guide: Length, Mass & Time | CBSE & NEET Prep

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 (Å): 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?

A) Astronomical Unit B) Light Year C) Parsec D) Mile

Correct: C (Parsec). 1 Parsec = 3.26 light years ≈ 3.08 × 10¹⁶ m.

NEET Q2. The value of 1 Bohr radius is approximately:

A) 0.5 Å B) 1.0 nm C) 10^-15 m D) 1.6 km

Correct: A. Bohr radius is 0.5 × 10^-10 m, which equals 0.5 Angstrom (Å).

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).

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.

Dr.Sanjaykumar pawar

-Wed 25 Mar 2026

-4:02p.m