How to approach
These need a rearrangement or two steps, often finding one quantity before the one asked for.
The method
- List what is given, with units.
- Choose the formula and rearrange for the unknown.
- Substitute with units on every line.
- Round to the correct significant figures.
Worked examples
Where marks are lost
Practise these
Try each formula above with your own numbers, showing every line. Bring anything you get stuck on to a one-to-one lesson.
Formulas you use for Electricity calculations
These mid-level problems draw on the 8 relationships in Electricity. None is printed in the exam, so recall each and write its symbols with units before substituting:
- Q = It, Electric charge
- V = W / Q, Potential difference
- V = IR (R = V / I), Ohm's law
- R = R₁ + R₂ + …, Resistors in series
- 1/R = 1/R₁ + 1/R₂ + …, Resistors in parallel
- ε = IR + Ir = V + Ir, Electromotive force
- E = VIt, Electrical energy
- P = VI = I²R = V²/R, Electrical power
A worked example, with units on every line
Using 1/R = 1/R₁ + 1/R₂ + … (R = total resistance (Ω); R₁ = first resistance (Ω); R₂ = second resistance (Ω)):
Two resistors R₁ = 6 Ω and R₂ = 3 Ω are in parallel.
1/R = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R = 2 Ω.
Where the marks are, and the common mistakes
In Paper 2, calculation marks are awarded line by line: the formula, the substitution with units, and the final answer with its unit and sensible significant figures. Watch this trap: Take the reciprocal at the end to get R (not 1/R).
The total is smaller than the smallest resistor. For mid-level items, expect one rearrangement and a unit conversion.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)