Overview
This lecture introduces the foundational concepts of physical quantities and units in A-level Physics, covering SI base units, derived quantities, and the use of prefixes for measurement and estimation.
Physical Quantities and Their Importance
- Physics studies measurable aspects of the world called physical quantities.
- Every physical quantity has a numerical value and a unit.
- Examples include energy, power, density, area, momentum, time, weight, mass, resistance, and angle.
Types of Physical Quantities
- There are two main types: base quantities and derived quantities.
- Base quantities use agreed-upon SI (International System of Units) base units.
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)
- Knowing these seven is essential for exams.
Prefixes and Scientific Notation
- Prefixes simplify 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).
- Numbers can be written using prefixes, standard form, or multiplying factors.
Estimation and Calculation Practice
- Kinetic energy can be estimated using KE = ½mv², with typical human values for mass and speed.
- Converting between units/prefixes often requires using indices and the appropriate multiplying factors.
- Use calculators' engineering and standard form functions to aid conversion between prefixes.
Measurement Estimation Examples
- Mass of an apple ~100g; adult humans ~60-80kg.
- Height of a room ~2-3m; diameter of a pencil ~0.5-1.0cm; lamp post ~3m.
- Understanding everyday measurements helps in making reasonable scientific estimates.
Key Terms & Definitions
- Physical quantity — anything that can be measured and given a numerical value with a unit.
- SI Unit (International System of Units) — globally agreed units for scientific measurement.
- Prefix — a shorthand multiplier used with SI units to denote scale (e.g., kilo-, milli-).
- Base quantity — fundamental measurable quantity (e.g., mass, length).
- Derived quantity — quantity calculated from base quantities (e.g., speed = distance/time).
Action Items / Next Steps
- Memorize the seven SI base quantities, their units, and symbols.
- Practice converting numbers to and from standard form and using prefixes.
- Review estimation examples and practice measuring everyday objects.
- Prepare for next lesson: uncertainties, units, scalars, and vectors.