Physics and Chemistry
What the exam tests
Physics and Chemistry tests heat transfer, states of matter, combustion chemistry, gas laws, electricity and the physical properties that affect fire behaviour.
Core rules
- Heat transfer: conduction in solids, convection in fluids, radiation by electromagnetic waves along line of sight. Radiant heat rises sharply with absolute temperature.
- Combustion of methane: CH4 + 2O2 -> CO2 + 2H2O. One volume of methane needs two volumes of oxygen; air is about 21% oxygen, so about 9.5 volumes of air.
- Specific gravity: liquids compared with water; vapours compared with air. Petrol (about 0.75) floats on water; propane (about 1.5) sinks.
- Electricity: V = IR; a 12 V circuit with 3 ohm resistance draws 4 A. Water and foam conduct; CO2 does not.
- Pressure: 1 bar is about 100 kPa or 10 m of water column. Boiling point falls as pressure falls.
Worked example
How much air is theoretically needed to burn 2 m³ of methane? CH4 + 2O2 -> CO2 + 2H2O, so 2 m³ methane needs 4 m³ oxygen. Air is 21% oxygen, so air = 4 / 0.21, approximately 19 m³. Reasoning: balance the equation, scale by volume, then divide by the oxygen fraction. Real combustion needs excess air, so treat 19 m³ as a minimum.
Traps
Forgetting to balance water in combustion equations; using 21% for oxygen in air but 23% by mass; confusing specific gravity with vapour density; assuming water conducts only at high voltage.