Electricity: Common Mistakes (Form 5 SPM Physics)

The mistakes, and the fix

1. Q = It, Time must be in seconds (convert minutes). Charge is in coulombs.
2. V = W / Q, W is energy in joules, Q is charge in coulombs. Recall that 1 V = 1 J C⁻¹.
3. V = IR (R = V / I), Ohm's law holds only for ohmic conductors at constant temperature. Resistance R = V / I.
4. R = R₁ + R₂ + …, In series, just add the resistances. The total is larger than any single resistor.
5. 1/R = 1/R₁ + 1/R₂ + …, Take the reciprocal at the end to get R (not 1/R). The total is smaller than the smallest resistor.
6. ε = IR + Ir = V + Ir, The terminal p.d. V is always less than the e.m.f. because of the 'lost volts' Ir across the internal resistance.
7. E = VIt, Time must be in seconds; energy is in joules (1 J = 1 W s). For kWh, use E = Pt with P in kW and t in hours.
8. P = VI = I²R = V²/R, Pick the form that matches the known quantities. In I²R square the current; in V²/R square the voltage.
9. In the Comparing ohmic and non-ohmic conductors practical, skipping this: Switch off between readings so the ohmic wire does not heat up and change resistance.
10. In the Comparing ohmic and non-ohmic conductors practical, skipping this: Check the meters for zero error before starting.
11. In the How the length of a wire affects its resistance practical, skipping this: Use the same wire throughout so the area and material stay the same.
12. In the How the length of a wire affects its resistance practical, skipping this: Take each reading quickly and switch off between readings to avoid heating.
13. In the Determining the e.m.f. and internal resistance of a dry cell practical, skipping this: Switch off between readings to avoid draining the cell.
14. In the Determining the e.m.f. and internal resistance of a dry cell practical, skipping this: Avoid drawing very large currents, which would heat the cell and change its behaviour.
15. Dropping the unit on the final line, an answer without its SI unit is incomplete.
16. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Electricity resources

• Chapter overview: Electricity
• Revision Notes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)