Measurement — Class 8 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Science, Physics — Chapter 1. Measuring physical quantities with the correct units and care.
1. About this chapter
This chapter covers physical quantities, the seven base quantities and their SI units, the systems of units, derived quantities, accuracy and precision, and measuring area and volume.
2. Physical quantities and systems of units
- A physical quantity can be measured (length, mass, time…).
- Systems of units: FPS (British: Foot-Pound-Second), CGS (centimetre-gram-second), MKS (metre-kilogram-second) and the SI (International System of Units).
3. The seven base (fundamental) quantities
| Base quantity | SI unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
| Luminous intensity | candela | cd |
| Amount of substance | mole | mol |
- Electric current is a base quantity (1 coulomb of charge flowing per second = 1 ampere).
- Amount of substance is directly proportional to the number of atoms/particles.
- Derived quantities are formed from base quantities — area (m²), volume (m³), speed (m/s), density (kg/m³).
4. Accuracy, precision and measuring
- Accuracy = how close a measured value is to the true value.
- Precision = how close two or more measured values are to each other.
- Least count = the smallest value an instrument can measure.
- The volume of an irregular solid is found by the displacement of water.
5. Worked examples
Example 1. Convert 2.5 km into metres. 2.5 × 1000 = 2500 m.
Example 2. Name the SI unit of electric current and temperature. Ampere (A) and kelvin (K).
Example 3. How is the volume of a stone measured? By the displacement method — the rise in water level gives its volume.
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The British system of units is — (a) SI / (b) FPS / (c) CGS. Ans: (b) FPS.
- Electric current belongs to — (a) derived quantities / (b) base quantities. Ans: (b) base quantities.
- The SI unit of temperature is — (a) celsius / (b) kelvin / (c) fahrenheit. Ans: (b) kelvin.
- The closeness of two or more measured values is called — (a) accuracy / (b) precision. Ans: (b) precision.
- "SI" stands for — International System of Units.
II. Fill in the blanks 6. Quantities other than base quantities are called derived quantities. 7. One coulomb of charge flowing per second is called one ampere. 8. Luminous intensity is the intensity of visible light.
III. True or False 9. SI units are a metric system of units. — True. 10. Amount of substance gives the number of particles present in the substance. — True.
7. Common mistakes
- Mistake: Listing only four base quantities. Fix: There are seven — add electric current (A), luminous intensity (cd) and amount of substance (mol).
- Mistake: Treating accuracy and precision as the same. Fix: Accuracy = closeness to the true value; precision = closeness of repeated readings.
- Mistake: Using length units for area/volume. Fix: Area is in m², volume in m³.
8. Quick revision
- Physics Ch 1 · base quantities, units, accuracy/precision.
- Seven base quantities: length (m), mass (kg), time (s), temperature (K), current (A), luminous intensity (cd), amount of substance (mol).
- Systems: FPS, CGS, MKS, SI; SI = International System of Units.
- Accuracy = close to true value; precision = close repeated readings.
- Irregular-solid volume by water displacement; 1 C/s = 1 ampere.
