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VCE Units 1–4 · Physics

VCE Physics — Electric Power

Electric power covers circuits, energy and the infrastructure that transmits electricity across Victoria. Transformers, power loss in transmission lines and AC vs DC circuits all appear in VCE Physics exams. The key skill is applying the right form of the power equation and understanding why high-voltage transmission reduces power loss.

Key Concepts & Formulas

  • Ohm's Law: V = IR (volts, amperes, ohms)

  • Power: P = VI = I²R = V²/R (watts)

  • Energy: E = Pt (joules)

  • Series circuit: same current; voltages add; Rₜₒₜₐₗ = R₁ + R₂ + ...

  • Parallel circuit: same voltage; currents add; 1/Rₜₒₜₐₗ = 1/R₁ + 1/R₂ + ...

  • Transformer ratio: Vₚ/Vₛ = Nₚ/Nₛ (ideal transformer conserves power: VₚIₚ = VₛIₛ)

  • Step-up transformer increases voltage; step-down transformer decreases voltage

  • Power loss in transmission line: P_loss = I²R_line — reduce current (increase voltage) to reduce loss

  • AC voltage and current are sinusoidal: V(t) = Vₚₑₐₖ sin(2πft)

  • RMS values: V_rms = Vₚₑₐₖ/√2; I_rms = Iₚₑₐₖ/√2 — use RMS in power calculations for AC

Practice Questions

4 questions

Attempt each question before reading the hint. These are styled to match VCE exam format.

Q1.A 12 V battery is connected to a 4 Ω and 8 Ω resistor in series. Find the current and the voltage across each resistor.

3 marks

Q2.A power station generates 10 kW at 250 V. The electricity is stepped up to 10 000 V for transmission through a line of resistance 2 Ω. Find the power lost in the line.

4 marks

Q3.A transformer has 200 turns on the primary and 50 turns on the secondary. The primary voltage is 240 V AC. Find the secondary voltage and the primary current if the secondary current is 8 A.

3 marks

Q4.An AC supply has a peak voltage of 340 V. Find the RMS voltage and the power dissipated in a 100 Ω load.

3 marks

Common Mistakes to Avoid

These are the errors that VCE students most frequently make in Electric Power — and that examiners are specifically watching for.

  • Using peak voltage instead of RMS values in AC power calculations — always use V_rms for average power

  • Applying series resistance formula to a parallel circuit — check the circuit diagram before writing equations

  • Confusing the direction of the transformer ratio: step-up has more turns on the secondary (Nₛ > Nₚ)

  • Forgetting that power loss in transmission lines depends on I² — doubling the voltage halves the current, reducing loss by a factor of 4

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