Topic 6 of 8 · Form 4
Electricity: charge, circuits and practical electricity
Charging by friction (electron transfer); electric fields; current I = Q/t; conventional current flows from + to −.
Notes for this topic 1 file, in order
Create a free account to download PDFs.
Charge and current
Charging by friction (electron transfer); electric fields; current I = Q/t; conventional current flows from + to −.
Potential difference and e.m.f.
p.d. = energy transferred per unit charge; measured with a voltmeter in parallel. Resistance R = V/I; I–V graphs for resistors, filament lamps and diodes.
Resistance of a wire
R increases with length and decreases with cross-sectional area (R ∝ L/A).
Circuits
- Series: same current; voltages add; R = R₁ + R₂.
- Parallel: same p.d.; currents add; 1/R = 1/R₁ + 1/R₂.
- Potential dividers with LDRs and thermistors.
Worked example: Find the total resistance of 3 Ω in series with a parallel pair of 6 Ω and 6 Ω.
Parallel pair = 3 Ω → total = 3 + 3 = 6 Ω
Practical electricity
P = IV = I²R; energy = Pt; cost of electricity in kWh. Fuses and circuit breakers; earthing; double insulation; dangers of damaged insulation, overloading and damp conditions.
Worked example: A 2.5 kW geyser runs for 3 hours a day for 30 days at US$0.10 per kWh. Find the cost.
2.5 × 3 × 30 = 225 kWh → US$22.50
Check yourself
- A charge of 60 C passes in 20 s. Find the current.
- A 12 V supply drives 0.5 A through a lamp. Find its resistance and power.
- Why is earthing used for metal-cased appliances?
Show answers
- 3 A
- 24 Ω; 6 W
- If a live wire touches the case, a large current flows to earth and the fuse blows, preventing electric shock.
Questions about this topic (0)
No questions about this topic yet.
Sign in or create a free account to join in.