Series circuit: components in one path; the same current flows; voltages add. Parallel circuit: components on separate branches; same voltage across each; currents add.
Electric power: rate of using electrical energy, unit watt. Electrical energy: unit joule or kilowatt hour (1 kWh = 3.6 × 10⁶ J, also called one unit).
Potential divider: series resistors that split a voltage into smaller fractions.
Thermistor: resistor whose resistance changes with temperature. NTC: resistance falls as temperature rises. PTC: resistance rises as temperature rises.
Cells: in series emf adds (Veq = V1 + V2); in parallel the emf is the same as one cell (but it lasts longer).
P = VI = I²R = V²/R. E = Pt. W = I²Rt. Bill = kWh × rate per unit.
Colour code: first two bands digits, third band multiplier (brown, black, red = 10 × 10² = 1 kΩ).
Must-know points
Derivation for resistors in parallel: same V across each; total I = I1 + I2 = V/R1 + V/R2; since I = V/Req, 1/Req = 1/R1 + 1/R2. In series: the same I, V = V1 + V2 = IR1 + IR2 = I(R1 + R2), so Req = R1 + R2. Draw both circuits.
Homes are wired in parallel so each device gets full voltage and works independently; in series one fault would switch everything off.
Safety devices (name three): fuse, circuit breaker, earth (ground) wire, switch on the live wire, insulation, RCD. Circuit breaker cuts the supply when current is too high. Earthing sends fault current to ground. Do not touch switches with wet hands because water conducts and the body resistance drops.
Brighter bulb in parallel has the lower resistance (higher power at same voltage).
Past-paper questions with model answers
Cost of a 1000 W heater for 5 h at Rs 30/unit: energy 5 kWh, cost Rs 150. 2 kW iron for 3 h at Rs 25: 6 kWh, Rs 150.
Energy of a 100 W bulb for 10 h: 1 kWh = 3.6 × 10⁶ J.
Power at 220 V and 2 A: P = 440 W. 220 V and 0.5 A: 110 W. Energy of 220 V, 1 A for 5 s: E = VIt = 1100 J.
Total emf of three 1.5 V cells: series 4.5 V; parallel 1.5 V. Four 2 V cells: 8 V and 2 V. Two 3 V cells: 6 V and 3 V.
Req of R1 and R2 in parallel with sketch (2+1).
200 W bulb on 220 V (Section C): I = P/V = 0.91 A, R = V/I = 242 Ω, energy in 10 h = 2 kWh, cost at Rs 40 = Rs 80.
Name any three safety devices used in household circuits.