Circuits and Power Consumption

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Power: P = VI = I²R = V²/R. SI unit: watt (W).
Energy: E = Pt. Domestic bill in kWh. 1 kWh = 3.6×10⁶ J = 1 unit.
Domestic supply: India: 220-240 V, 50 Hz, single-phase AC.

Electric circuits convert energy from one form to another. CDS/OTA tests power, energy in kWh, household wiring conventions and domestic safety.

Electric Power

P = VI = I²R = V²/R.
SI unit: watt (W) = J/s. 1 kW = 1000 W.
A 100 W bulb at 220 V draws I = P/V = 100/220 ≈ 0.45 A.
Higher wattage at the same voltage means more current and brighter output.
Resistance of a bulb: R = V²/P. Two bulbs of same V — lower wattage has higher resistance.

Electrical Energy in kWh

E = Pt. If P is in kW and t in hours, E is in kWh.
Domestic electricity bill measured in kilowatt-hour (kWh), also called units.
1 kWh = 3.6 × 10⁶ J.
Example: a 1500 W heater run for 2 hours uses 1.5 × 2 = 3 kWh = 3 units.
Star-rated appliances consume less for the same output, reducing the bill.

Domestic Wiring

Indian domestic supply — 220-240 V, 50 Hz, single-phase AC.
Three wires: Live (red/brown, +ve potential), Neutral (black/blue, near zero), Earth (green/yellow, to ground).
All appliances in parallel — each gets full voltage; can be operated independently.
Switch placed in live wire so the appliance is fully disconnected when off.
Three-pin plug: top pin (earth) is longer and thicker than the live and neutral pins.

CDS/OTA PYQ Examples

Q: 1 kWh equals:

(a) 3.6 × 10³ J (b) 3.6 × 10⁶ J (c) 3.6 × 10⁹ J (d) 1000 J

Answer: (b) 3.6 × 10⁶ J.

Q: A 100 W bulb is used for 10 hours daily. Energy consumed in a 30-day month is:

(a) 10 kWh (b) 30 kWh (c) 100 kWh (d) 300 kWh

Answer: (b) 30 kWh. (0.1 kW × 10 h × 30 days = 30 kWh.)

Q: Indian household supply voltage and frequency are:

(a) 110 V, 60 Hz (b) 220 V, 50 Hz (c) 110 V, 50 Hz (d) 230 V, 60 Hz

Answer: (b) 220-240 V at 50 Hz.

Q: SI unit of electric power is:

(a) Joule (b) Watt (c) Volt (d) Coulomb

Answer: (b) Watt = J/s.

Drill Circuits and Power Consumption for CDS/OTA

CDS/OTA-pattern items on Circuits and Power Consumption with answer keys and explanations.

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Frequently Asked Questions

Is a higher wattage bulb always brighter?

At the same supply voltage, yes. But at different voltages a 100 W (220 V) bulb is much brighter than a 100 W (12 V) bulb only because the latter is rarely run at rated voltage.

Why are switches placed in the live wire?

If placed in the neutral, the appliance would still be at live potential when switched off — anyone touching it could get a shock.

What is the difference between AC and DC?

DC — current flows in one direction (battery). AC — current direction alternates 50 times a second (mains). AC is easier to transform and transmit over long distances.