Physics • Year 12 • Module 6 • Lesson 20
Power Transmission and Distribution
Lock in the core vocabulary, the Ploss = I²R formula, and the roles of step-up and step-down transformers before tackling harder calculations.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: transmission lines, power loss, substation, grid, step-up transformer, step-down transformer, alternating current (AC), direct current (DC), Ploss = I²R, transformer efficiency. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | High-voltage power lines that carry electrical energy over long distances from power stations to cities. | |
| 1.2 | Energy dissipated as heat in the resistance of transmission wires; calculated using the formula P = I²R. | |
| 1.3 | A facility containing transformers that step voltage up or down at key points in the electrical network. | |
| 1.4 | The interconnected network of power stations, high-voltage lines, substations, and distribution systems that delivers electricity to consumers. | |
| 1.5 | A transformer with more secondary coil turns than primary coil turns; it increases output voltage. | |
| 1.6 | A transformer with fewer secondary coil turns than primary coil turns; it decreases output voltage for safe domestic use. | |
| 1.7 | An electric current that periodically reverses direction; the type used throughout the national electricity grid. | |
| 1.8 | An electric current that flows in one direction only; cannot be transformed directly to different voltages by a simple transformer. | |
| 1.9 | The formula expressing that power wasted as heat in a conductor equals the square of the current multiplied by the resistance. | |
| 1.10 | The ratio of useful power output to total power input in a transformer, often expressed as a percentage. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Power loss in a transmission line depends on the square of the current flowing through the line. T / F
2.2 Doubling the transmission voltage while keeping transmitted power constant will halve the power loss in the line. T / F
2.3 Transformers work equally well with both AC and DC supplies. T / F
2.4 The electrical grid in Australia uses alternating current primarily because AC can be stepped to different voltages using transformers. T / F
2.5 A step-up transformer is used at the power station to decrease the voltage before electricity enters the transmission lines. T / F
2.6 For a fixed power output, using a higher transmission voltage results in a higher current in the transmission lines. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
AC · current · heat · higher · lower · resistance · step-down · step-up
When electricity flows through transmission lines, some energy is lost as ___________ due to the ___________ of the wires. This power loss is given by Ploss = I²R, which shows that loss depends on the square of the ___________ in the line. For a fixed power, using a ___________ transmission voltage reduces the current and dramatically ___________s the power loss. At the power station, a ___________ transformer increases the voltage for long-distance transmission. Near homes, a ___________ transformer reduces voltage to the safe 240 V level. Transformers require ___________ because they rely on a changing magnetic flux to induce a voltage in the secondary coil.
4. Formula and concept recall
Answer each question in 1–2 sentences using precise terms and correct symbols. 8 marks (2 each)
4.1 State the formula for power loss in a transmission line and identify what each symbol represents.
4.2 Explain why increasing the transmission voltage for a fixed power output reduces power loss in the lines.
4.3 Explain why the national electricity grid uses AC rather than DC.
4.4 State the transformer turns ratio formula and explain what the symbols Vp, Vs, np, and ns represent.
5. Sequence the electricity grid
The five descriptions below are out of order. Number them 1–5 to show the correct sequence from generation to consumer, then write one sentence explaining what happens at each stage. 10 marks (1 sequence + 1 explanation each)
| Order (1–5) | Stage description | What happens here |
|---|---|---|
| Pole-top or pad-mounted transformers reduce voltage to 240 V for homes, schools, and small businesses. | ||
| A transformer at the power station increases voltage from ~20 kV to 330 kV or higher. | ||
| High-voltage transmission lines carry power efficiently across hundreds of kilometres. | ||
| Power generators produce AC electricity at 10–25 kV. | ||
| Regional substations step voltage down to 66 kV or 11 kV for distribution within cities. |
Q1 — Term–definition match
1.1 transmission lines • 1.2 power loss • 1.3 substation • 1.4 grid • 1.5 step-up transformer • 1.6 step-down transformer • 1.7 alternating current (AC) • 1.8 direct current (DC) • 1.9 Ploss = I²R • 1.10 transformer efficiency.
Q2 — True / false with correction
2.1 True. Ploss = I²R; power loss is proportional to the square of the current.
2.2 False. Doubling the voltage halves the current (P = VI, fixed P), so power loss ∝ I² decreases by a factor of four (quarters), not halves.
2.3 False. Transformers operate only on AC. A changing current is required to produce a changing magnetic flux in the primary coil, which then induces a voltage in the secondary coil. DC provides a constant (non-changing) flux and cannot induce an EMF in the secondary.
2.4 True. AC is used because transformers require changing flux, which only AC provides, enabling efficient voltage transformation.
2.5 False. A step-up transformer increases the voltage at the power station before electricity enters the transmission lines, in order to reduce transmission current and minimise power losses.
2.6 False. For fixed power P = VI, a higher transmission voltage means a lower current. Since P is constant, I = P/V decreases as V increases.
Q3 — Cloze paragraph
In order: heat / resistance / current / higher / lower / step-up / step-down / AC.
Q4.1 — Power loss formula
Ploss = I²R, where Ploss = power lost as heat in the transmission lines (W), I = current flowing through the lines (A), and R = total resistance of the transmission lines (Ω).
Q4.2 — Why higher voltage reduces loss
For a fixed power P, the current is I = P/V. Increasing V decreases I proportionally. Since Ploss = I²R, halving the current reduces power loss by a factor of four (quarter), making high-voltage transmission far more efficient.
Q4.3 — Why AC is used in the grid
Transformers require a continuously changing magnetic flux to induce a voltage in the secondary coil; only AC provides this changing flux. DC produces a constant (non-changing) field that cannot induce an EMF, so it cannot be stepped up or down by a simple transformer. Without transformers, efficient high-voltage transmission would be impossible.
Q4.4 — Transformer turns ratio
Vp/Vs = np/ns, where Vp = primary (input) voltage, Vs = secondary (output) voltage, np = number of turns on the primary coil, and ns = number of turns on the secondary coil. A step-up transformer has ns > np; a step-down transformer has ns < np.
Q5 — Sequence the grid
1 — Generation (10–25 kV): Power generators produce AC electricity at relatively low voltages.
2 — Step-up at power station (~330 kV): A step-up transformer increases the voltage dramatically, reducing the current and minimising transmission losses.
3 — High-voltage transmission: Electricity travels efficiently along high-voltage lines across long distances with minimal energy loss.
4 — Regional substation step-down (66 kV or 11 kV): Transformers reduce the voltage to safer levels for distribution within suburban areas.
5 — Local step-down to 240 V: Pole-top or pad-mounted transformers reduce voltage to the safe 240 V level suitable for household appliances.