Physics and Chemistry: Units, Models and Checks
What is assessed
Physics questions cover mechanics (forces, Newton's laws, work and energy, momentum), electricity (Ohm's law, series and parallel circuits, power), waves and optics, and basic thermodynamics. Chemistry questions cover atomic structure, the periodic table, bonding, mole calculations, solutions, and simple reaction types including acid–base and redox.
Traps that cost marks
- Units: convert to SI before calculating. A mass in grams and a distance in centimetres will produce a wrong answer silently.
- Moles: n = m/M, and molar mass must use the correct formula, including subscripts.
- Series versus parallel: current is the same through series components; voltage is the same across parallel branches.
- Sign conventions: define a positive direction and keep it throughout.
- Balancing equations: atoms and charge must both balance in redox reactions.
Worked example
How many moles are in 9.8 g of sulfuric acid, H₂SO₄? Molar mass: H 1.0 × 2 = 2.0; S 32.1; O 16.0 × 4 = 64.0. Total = 98.1 g/mol. n = 9.8 / 98.1 ≈ 0.10 mol. A quick check: 9.8 g is about one tenth of the molar mass, so the answer is plausible. Always perform this order-of-magnitude check.
Rule of thumb
Before writing the final answer, check units, check the sign, and check the order of magnitude. Three seconds of checking catches most lost marks.