Electricity
Current, voltage, resistance, and circuit calculations
Current (I) = flow of charge (measured in Amperes, A). Voltage/Potential difference (V) = energy transferred per unit charge (Volts, V). Resistance (R) = opposition to current (Ohms, Ω).
Ohm's Law: **V = IR** (Voltage = Current × Resistance)
Circuit types:
Power: P = IV = I²R = V²/R (measured in Watts, W)
Energy: E = Pt = IVt (measured in Joules, J)
Domestic electricity:
Fuses and circuit breakers protect against overloading. Fuse melts if current exceeds rating. RCDs detect earth faults.
Charge: Q = It (Charge = Current × Time, measured in Coulombs, C)
Key Points to Remember
- 1V = IR (Ohm's Law)
- 2Series: same current, voltages add; Parallel: same voltage, currents add
- 3Power P = IV = I²R
- 4Q = It (Charge = Current x Time)
Pakistan Example
LESCO and K-Electric — Electricity Bills as Physics Lessons
Every LESCO (Lahore) and K-Electric (Karachi) bill shows kWh units. A 100W bulb running 10 hours uses 1 kWh = 1000 Wh. At Rs. 30/kWh, that's Rs. 30. Using P = IV: at 230V, a 100W bulb draws I = P/V = 100/230 = 0.43A. A 13A fuse would protect the appliance. Load shedding areas with fewer appliances per circuit have less current demand — parallel circuit physics.
Quick Revision Infographic
Physics — Quick Revision
Electricity
Key Concepts
Formulas to Know
V = IR (Ohm's Law)P = IV = I²RIt (Charge = Current x Time)LESCO and K-Electric — Electricity Bills as Physics Lessons
Every LESCO (Lahore) and K-Electric (Karachi) bill shows kWh units. A 100W bulb running 10 hours uses 1 kWh = 1000 Wh. At Rs. 30/kWh, that's Rs. 30. Using P = IV: at 230V, a 100W bulb draws I = P/V = 100/230 = 0.43A. A 13A fuse would protect the appliance. Load shedding areas with fewer appliances per circuit have less current demand — parallel circuit physics.