Overview
This lecture introduces the foundational concepts of physical quantities, SI units, and prefixes in A-level Physics, with emphasis on measurement, estimation, and converting between units.
Physical Quantities
- Physics studies measurable aspects of the world, called physical quantities.
- Every physical quantity consists of a numerical value and a unit.
- Common physical quantities: energy, power, density, area, momentum, time, weight, mass, resistance, angle.
Types of Quantities
- There are two types: base quantities and derived quantities.
- Base quantities use agreed-upon, fundamental units (SI units).
- Derived quantities are derived from base quantities through mathematical relationships.
SI Base Quantities and Units
- Length: meter (m)
- Mass: kilogram (kg)
- Time: second (s)
- Electric current: ampere (A)
- Temperature: kelvin (K)
- Amount of substance: mole (mol)
- Luminous intensity: candela (cd)
- These seven SI units must be memorized; focus on the first five for AS Level.
Using Units and Prefixes
- Prefixes simplify writing large or small numbers (e.g., kilo = 10³, milli = 10⁻³).
- Common prefixes: tera (T, 10¹²), giga (G, 10⁹), mega (M, 10⁶), kilo (k, 10³), deci (d, 10⁻¹), centi (c, 10⁻²), milli (m, 10⁻³), micro (μ, 10⁻⁶), nano (n, 10⁻⁹), pico (p, 10⁻¹²).
- To express a quantity: Symbol (e.g., L for length), numerical value, and unit (with or without prefix).
Estimation & Standard Form
- Estimation is important; e.g., fastest human runs 100m in ~10s (10 m/s), average mass ~80kg.
- Kinetic energy: calculated using KE = ½mv² and estimated values.
- Standard form (e.g., 5 × 10⁻⁷) is used to compare and convert quantities.
- Use calculators to convert prefixes and values efficiently.
Order of Magnitude & Real-World Examples
- Estimating realistic values is key (e.g., apple ~100g, room height ~2-3m, pencil diameter ~0.5-1cm).
- Incorrect prefixes can signal unrealistic measurements (e.g., human mass in micrograms is incorrect).
Key Terms & Definitions
- Physical Quantity — Any measurable property with both a numerical value and a unit.
- Base Quantity — Fundamental physical quantity with an SI unit.
- SI Unit — International System of Units standard for measurement.
- Prefix — Symbol that denotes a power of 10 to simplify numerical values.
- Standard Form — Scientific notation to express large/small numbers (e.g., 1.2 × 10³).
Action Items / Next Steps
- Memorize the seven SI base quantities, their units, and symbols.
- Become familiar with common prefixes and their multiplying factors.
- Practice converting between units and using standard form on your calculator.
- Next lesson: uncertainties, units, scalars, and vectors.