IGCSE Physics 0625 · Common mistakes
Common mistakes in IGCSE Physics electricity
Updated · by the Fahmazing team
Electricity and magnetism carry about a quarter of the marks. Most lost marks come from equations used the wrong way round and missed unit conversions.
Charge and current: two conversions
A current of 200 mA flows for 2 minutes. How much charge flows?
✗ Wrong: Q = It = 200 × 2 = 400 C
✓ Right: Q = It = 0.200 A × 120 s = 24 C
Tip: Current is charge per second. Change mA to A (÷ 1000) and minutes to s (× 60).
Free lesson: Current, e.m.f. and potential difference
Resistance the wrong way up
A p.d. of 6.0 V gives a current of 0.50 A. Find the resistance.
✗ Wrong: R = 0.50 ÷ 6.0 = 0.083 Ω
✓ Right: R = V ÷ I = 6.0 ÷ 0.50 = 12 Ω
Free lesson: Resistance
Forgetting to flip 1/R in parallel (Extended)
Find the combined resistance of 10 Ω and 20 Ω in parallel.
✗ Wrong: 1/R = 1/10 + 1/20 = 0.15, so R = 0.15 Ω
✓ Right: 1/R = 0.15, so R = 1 ÷ 0.15 = 6.7 Ω. It is less than the smaller resistor, as it should be.
Tip: In series, resistances simply add: 10 Ω + 20 Ω = 30 Ω.
Free lesson: Series and parallel circuits
Power: multiply, do not divide
A heater takes 2.0 A from a 12 V supply. Find its power.
✗ Wrong: P = 12 ÷ 2.0 = 6.0 W
✓ Right: P = IV = 2.0 × 12 = 24 W
Free lesson: Electrical energy and electrical power
Transformer ratio upside down
A transformer has 100 primary turns and 200 secondary turns. The input is 230 V. Find the output.
✗ Wrong: Vₛ = 230 × 100 ÷ 200 = 115 V
✓ Right: Vₛ = Vₚ × Nₛ ÷ Nₚ = 230 × 200 ÷ 100 = 460 V. More secondary turns gives a bigger output p.d.
Free lesson: The transformer
Induction needs change
A strong magnet is held still inside a coil. What e.m.f. is induced?
✗ Wrong: A large e.m.f., because the magnet is strong.
✓ Right: None. An e.m.f. is induced only while the magnetic field through the coil is changing.
Free lesson: Electromagnetic effects