Overview
This lecture explains physical quantities, their classification into base and derived types, the International System of Units (SI), and methods for measuring length accurately.
Physical Quantities
- Physical quantities are measurable properties with both numerical magnitude and units.
- A quantity described without its unit is not a physical quantity.
- Example: "165 cm" is a physical quantity; "165" alone is not.
Classification of Physical Quantities
- Base quantities are fundamental physical quantities used to define other quantities.
- There are seven base quantities: length, mass, time, temperature, electric current, amount of substance, and intensity of light.
- Derived quantities are defined in terms of base quantities, e.g., area, volume, velocity, force.
International System of Units (SI)
- SI is the internationally accepted system for measuring physical quantities.
- SI was adopted in 1960 during the 11th General Conference on Weights and Measures.
- SI ensures universal standards for scientific communication.
Base and Derived Units
- Base units correspond to base quantities: meter (length), second (time), kilogram (mass), ampere (electric current), kelvin (temperature), mole (amount), candela (intensity of light).
- Derived units are combinations of base units: e.g., meter/second (velocity), meter/second² (acceleration), newton (force), pascal (pressure), coulomb (charge), tesla (magnetic flux density).
Measurement of Length
- A meter rule measures distances up to one meter, with a least count of 1 mm; eyes should be directly above the reading point.
- Measuring tape is used for longer distances; least count is 1 cm; read from above for accuracy.
- Vernier caliper provides accuracy up to 0.1 mm; consists of main and vernier scale; requires zero error correction for accuracy.
- Screw gauge (micrometer screw) measures even smaller lengths, with a least count of 0.01 mm; also requires zero error correction.
Key Terms & Definitions
- Physical quantity — A measurable property with both magnitude and unit.
- Base quantity — A fundamental quantity for defining others (e.g., mass, time).
- Derived quantity — A quantity expressed in terms of base quantities.
- SI (International System of Units) — The global standard for measurement units.
- Least count — The smallest value that an instrument can measure accurately.
- Zero error — Correction applied to an instrument’s reading based on its calibration offset.
Action Items / Next Steps
- Review and memorize the seven SI base quantities and units.
- Practice using measuring instruments and identifying their least count and possible zero error.
- Prepare for questions about classifying quantities and units.