CBSE · Class 8
Class 8 Science Formulas and Key Concepts
Chapter-wise important formulas, definitions, and relations for CBSE Class 8 Science
Ch 1.Force and Pressure
| Formula / Relation | Expression |
|---|---|
| Pressure | Pressure = Force / Area |
| Pascal unit | 1 Pa = 1 N/m² |
| Atmospheric pressure (approx.) | P_atm ≈ 10⁵ Pa = 1 atm |
| Liquid pressure | P = ρgh |
Ch 2.Friction
| Formula / Relation | Expression |
|---|---|
| Frictional force | f = μN |
| Types of friction (order) | Static > Sliding > Rolling |
| Fluid friction (drag) | Drag depends on speed, surface area, and fluid density |
Ch 3.Sound
| Formula / Relation | Expression |
|---|---|
| Speed of sound in air | v ≈ 343 m/s (at 20°C) |
| Frequency | Frequency = Number of vibrations / Time |
| Time period | T = 1 / f |
| Wavelength–frequency relation | v = fλ |
| Human hearing range | 20 Hz to 20,000 Hz (20 kHz) |
| Echo condition | Minimum distance = 17.2 m (for echo heard separately) |
Ch 4.Chemical Effects of Electric Current
| Formula / Relation | Expression |
|---|---|
| Electrolysis of water | 2H₂O → 2H₂ + O₂ |
| Electroplating — basic reaction | Cu²⁺ + 2e⁻ → Cu (at cathode) |
Ch 5.Combustion and Flame
| Formula / Relation | Expression |
|---|---|
| Complete combustion of methane | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Calorific value | Energy released per unit mass of fuel (kJ/kg) |
Ch 6.Crop Production and Management
| Formula / Relation | Expression |
|---|---|
| NPK ratio | N (Nitrogen) : P (Phosphorus) : K (Potassium) |
| Irrigation efficiency | Efficiency = (Water used by crops / Total water applied) × 100% |
Ch 7.Light
| Formula / Relation | Expression |
|---|---|
| Law of reflection | ∠i = ∠r |
| Braille system (reference) | 6 dots in 2 columns × 3 rows = 64 possible patterns |
Ch 8.Stars and Solar System
| Formula / Relation | Expression |
|---|---|
| Light year (distance) | 1 ly = 9.46 × 10¹⁵ m ≈ 9.5 × 10¹² km |
| Speed of light | c = 3 × 10⁸ m/s |
| Earth–Sun distance (AU) | 1 AU ≈ 1.5 × 10¹¹ m |
Frequently Asked Questions
What are the most important formulas in Class 8 Science?
The most exam-important formulas in Class 8 Science are: Pressure = Force/Area, v = fλ (sound), Speed of sound = 343 m/s, Echo condition (17.2 m), and the photosynthesis/respiration equations from Microorganisms chapter. These appear in every annual exam.
What is the formula for pressure in Class 8?
Pressure = Force / Area (P = F/A). The SI unit of pressure is Pascal (Pa), where 1 Pa = 1 N/m². Pressure increases with force and decreases with area — this is why knife blades are sharp (small area = high pressure) and snowshoes are wide (large area = low pressure).
What is the speed of sound in Class 8 Science?
The speed of sound in air is approximately 343 m/s at 20°C. Sound travels faster in solids and liquids than in air. Sound requires a medium to travel and cannot travel through vacuum. For an echo to be heard separately, the reflecting surface must be at least 17.2 m away.
How is Class 8 Science connected to Class 9?
Class 8 Science builds the foundation for Class 9. Force and Pressure leads to Class 9 Laws of Motion. Sound leads to Class 9 Sound chapter with detailed wave equations. Chemical Effects and Combustion lead to Class 9 Chemistry (Atoms, Matter). Stars and Solar System introduces concepts expanded in Class 9 Gravitation.
What is the difference between infrasound and ultrasound?
Infrasound: frequency below 20 Hz (humans cannot hear; some animals like elephants use it). Ultrasound: frequency above 20,000 Hz (20 kHz) — humans cannot hear; used in medical imaging (sonography), cleaning, and bat navigation. Human hearing range: 20 Hz to 20,000 Hz.
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