Notes For All Chapters – General Science Class 7
Measurement of Physical Quantities
1. Introduction
- Measurement is the process of finding the size, length, mass, time, or other properties of an object.
- We measure different physical quantities like length, mass, time, temperature, and volume using standard units.
2. What is a Physical Quantity?
- A physical quantity is a property that can be measured.
- Example: Length, mass, weight, time, speed, and temperature.
Types of Physical Quantities
1. Fundamental Quantities – Cannot be broken down into simpler units.
- Example: Length, mass, time, temperature.
2. Derived Quantities – Made by combining fundamental quantities.
- Example: Speed, area, density, volume.
3. Standard Units of Measurement
- To ensure accuracy and consistency, scientists use standard units called SI units (International System of Units).
Important SI Units
| Physical Quantity | Unit Name | Symbol |
|---|---|---|
| Length | Meter | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Temperature | Kelvin | K |
| Speed | Meter per second | m/s |
| Volume | Cubic meter | m³ |
4. Measurement of Length
Length is the distance between two points. Devices used for measuring length:
- Ruler/scale – Used for small objects like books.
- Measuring tape – Used for larger objects like rooms.
- Vernier calipers – Used for very small objects like wires.
Precautions while measuring length:
- Place the ruler correctly along the object.
- Avoid zero error (make sure the ruler starts at 0).
- Look straight at the scale to avoid parallax error.
5. Measurement of Mass
- Mass is the amount of matter in an object.
- It is measured using a beam balance, digital balance, or spring balance.
- Mass remains constant everywhere, unlike weight.
Differences between Mass and Weight
| Mass | Weight |
|---|---|
| Measured in kg | Measured in Newton (N) |
| Remains constant everywhere | Changes with gravity |
| Measured using balance | Measured using spring balance |
6. Measurement of Time
- Time is measured using clocks and stopwatches.
- The most accurate clock is the atomic clock.
- SI unit of time is seconds (s).
Smaller & Larger Units of Time
- Millisecond (ms) → 1/1000 of a second
- Minute (min) → 60 seconds
- Hour (h) → 60 minutes
7. Measurement of Speed
- Speed is how fast an object moves.
- Formula: Speed = Distance/Time
- SI unit of speed: Meter per second (m/s).
Example Calculation:
- If a car travels 100 meters in 20 seconds, its speed is: 100÷20=5 m/s100 \div 20 = 5 \text{ m/s}100÷20=5 m/s
8. Measurement of Temperature
Temperature tells us how hot or cold something is.
Measured using a thermometer in units:
- Celsius (°C)
- Fahrenheit (°F)
- Kelvin (K) – SI unit
Comparison of Temperature Scales
| Scale | Boiling Point of Water | Freezing Point of Water |
|---|---|---|
| Celsius (°C) | 100°C | 0°C |
| Fahrenheit (°F) | 212°F | 32°F |
| Kelvin (K) | 373 K | 273 K |
9. Scalar and Vector Quantities
(i) Scalar Quantities
- Quantities that have only magnitude (size) but no direction.
- Example: Mass, temperature, time, speed.
(ii) Vector Quantities
- Quantities that have both magnitude and direction.
- Example: Velocity, force, weight.
10. Importance of Accurate Measurement
- Measurements are needed in daily life, science, and industries.
- Example:
- Cooking – Correct measurement of ingredients.
- Construction – Measuring materials accurately.
- Science Experiments – Precise calculations for correct results.
11. Precautions While Measuring Physical Quantities
- Always use the correct measuring instrument.
- Avoid parallax error (wrong reading due to incorrect eye position).
- Ensure that devices like scales and balances are properly calibrated.
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