Overview
This lecture explains the difference between base and derived SI units, demonstrates how to determine derived units using equations, and practices applying this knowledge to exam-style questions.
Base vs Derived SI Units
- The seven base SI units correspond to seven fundamental physical quantities.
- Any physical quantity not on the base list is not a base SI unit; these are called derived units.
- Derived units are combinations of base SI units through multiplication or division.
Identifying Derived Units
- To find a derived unit, recall an equation involving the quantity (e.g., force, volume, pressure).
- Substitute the SI units of each variable into the equation to express the derived unit in terms of base units.
Examples of Derived Units
- Volume (V): measured in cubic meters (m³), derived from length × width × height.
- Force (F): measured in newtons (N), derived as kg·m·s⁻² using F = m·a.
- Pressure (P): measured in pascals (Pa), derived as kg·m⁻¹·s⁻² using P = F/A.
Exam-Style Questions
- When asked if two expressions have the same SI base units, convert all quantities to base units before comparing.
- Use known equations and break down non-base units to their SI base unit forms for accurate comparison.
- If an unfamiliar quantity appears, use elimination or known relationships; focus on converting everything to base units.
Key Terms & Definitions
- Base SI Unit — One of the seven fundamental SI units for physical quantities.
- Derived Unit — Unit formed by combining base SI units through multiplication or division.
- Newton (N) — SI unit of force; equivalent to kg·m·s⁻².
- Pascal (Pa) — SI unit of pressure; equivalent to kg·m⁻¹·s⁻².
- Prefix — A modifier placed before a unit (e.g., kilo- in kilogram).
Action Items / Next Steps
- Practice breaking down physical quantities into SI base units using equations.
- Complete past year exam questions related to SI units and conversions.
- Review the seven base SI units and their associated physical quantities.