Physics (5054)
Topic 8 of 8Cambridge O Levels

Electricity

Circuits, current, voltage, and why load shedding happens

Electric Current: The flow of electric charge (electrons) through a conductor.

Current (I) is measured in Amperes (A).


Voltage (Potential Difference): The energy given to each coulomb of charge.

Voltage (V) is measured in Volts (V). It's like the "push" that drives current.


Resistance: How much a component opposes current flow.

Resistance (R) is measured in Ohms (Ω).


Ohm's Law: V = IR (Voltage = Current × Resistance)


Series Circuits:

  • One path for current. If one component breaks, everything stops
  • Current is the same everywhere
  • Voltage splits across components: V_total = V₁ + V₂ + V₃
  • Total resistance = R₁ + R₂ + R₃

    • Parallel Circuits:

  • Multiple paths for current. If one path breaks, others still work
  • Voltage is the same across each branch
  • Current splits: I_total = I₁ + I₂ + I₃
  • Total resistance is less than the smallest individual resistance

    • Electrical Power & Energy:

  • Power = Voltage × Current (P = VI)
  • Energy = Power × Time (E = Pt)
  • Your electricity bill is based on kilowatt-hours (kWh): E = P(kW) × t(hours)

    • Key Points to Remember

    • 1Ohm's Law: V = IR
    • 2Series: same current, voltage splits, R_total = R₁ + R₂
    • 3Parallel: same voltage, current splits
    • 4Power = VI, Energy = Pt, Bills in kWh

    Pakistan Example

    Load Shedding & Generator Backup

    When WAPDA cuts power (load shedding), many homes switch to generators. A typical home generator produces about 2-3 kW. If you run a 1500W AC, a 100W fan, and a 200W fridge (total 1800W), you're using 1.8 kW. Running this for 4 hours uses 1.8 × 4 = 7.2 kWh of energy. At K-Electric rates of about Rs 35/kWh, that costs Rs 252. But generator diesel for 4 hours might cost Rs 600+ — that's why load shedding is so expensive for families!

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