Coaching Centre
Energy review
A summary of the whole Energy chapter, key words and formulas, and a mixed test covering forms of energy, transformations, Sankey diagrams, heat transfer, circuits, generating electricity, efficiency and renewable energy.
Use this page to check the whole chapter. Read the summary, make sure you can use every formula and explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 3A Forms of energy | Energy in joules (J). Kinetic energy (motion; heat, sound and electrical energy involve moving particles). Potential energy: gravitational, elastic, chemical; also nuclear. Light is electromagnetic radiation. Conservation of energy: never created or destroyed. |
| 3B Energy transformations | Transfer (moves) vs transformation (changes form). Energy chains. Useful vs wasted energy (usually heat). Sankey diagrams with arrow widths in proportion. Food calorimetry: E = mL × 4.2 × ΔT. |
| 3C Heat transfer | Heat flows hot → cold. Conduction (particle to particle; metals conduct, trapped air insulates). Convection (hot fluid rises, cool sinks). Radiation (infrared, crosses a vacuum; dark dull surfaces absorb and emit best). Vacuum flask. |
| 3D Electric circuits | Complete loop: source, wires, load, switch. Current (A, ammeter in series), voltage (V, voltmeter in parallel), resistance (Ω). Symbols. Series: one path, voltage shared, one break stops all. Parallel: full voltage to each branch, independent. Conductors, insulators, safety switches. |
| 3E Generating and using electricity | Generator: moving magnet and coil, kinetic → electrical, AC. Faster, more turns, stronger magnet = more voltage. Turbines. Grid: step-up transformer, high-voltage lines (less heat loss), substations, 230 V homes. Batteries give DC. Incandescent → CFL → LED. Transistors and integrated circuits. |
| 3F Energy efficiency | Efficiency = useful out ÷ total in × 100. Power in watts (J/s). kWh = kW × h; cost = kWh × price. Lifetime cost of globes. Energy Rating Labels. Efficiency vs conservation. |
| 3G Renewable energy | Renewable vs non-renewable. Fossil fuels release CO₂. Photovoltaic cells (about 20–23% efficient), inverters, wind, hydro. Storage: batteries, pumped hydro. Australia's shift from coal to renewables; trade-offs of each source. |
Formulas
| What | Formula | Units |
|---|---|---|
| Gravitational potential energy | GPE = mass × 9.8 × height | J, kg, m |
| Energy to heat water | energy = volume × 4.2 × temperature rise | J, mL, °C |
| Efficiency | efficiency = useful energy out ÷ total energy in × 100 | % |
| Electrical energy used | energy = power × time | kWh, kW, h |
| Running cost | cost = energy × price per kWh | $ or c |
Following energy all the way
A good test of your understanding is to follow one bit of energy as far as you can. Take the light from your desk lamp. It started as nuclear energy in the sun, which reached Earth as light (radiation across empty space). Perhaps it was captured by a solar panel (light → electrical, about 22% efficient) or by ancient plants that became coal (chemical energy, burned to heat water, spin a turbine and turn a generator). The electricity was stepped up by a transformer, sent along high-voltage lines, stepped down to 230 V and carried to your lamp through a parallel circuit. The LED transformed it into light, with some heat. And the light itself? It was absorbed by the walls and your eyes and ended up as a tiny amount of heat in the room. Every step conserved energy; every step wasted some as heat.
Key words
alternating current · ammeter · chemical potential energy · conduction · conductor · conservation of energy · convection · current · direct current · efficiency · elastic potential energy · electromagnetic radiation · energy chain · fossil fuel · generator · gravitational potential energy · grid · infrared · insulator · inverter · joule · kilowatt-hour · kinetic energy · load · non-renewable · parallel circuit · photovoltaic cell · power · pumped hydro · radiation · renewable · Sankey diagram · series circuit · short circuit · thermal energy · transformation · transformer · transfer · turbine · voltage · voltmeter · wasted energy · watt
Test yourself
A rollercoaster car at the top of the first hill has mostly
It is high up and moving slowly.
What is the main energy transformation in a torch?
The battery stores chemical energy.
A motor is supplied with 800 J and produces 600 J of kinetic energy. Its efficiency is
600 ÷ 800 × 100 = 75%.
Double glazing has a layer of air or gas between two panes of glass. It mainly reduces heat transfer by
Still air is a poor conductor (and the narrow gap limits convection).
Which is the best reason why cooking pots often have copper bases?
Metals, especially copper, are excellent conductors.
An ammeter must be connected
All the current must flow through it.
In a parallel circuit with two identical lamps, if one lamp is unscrewed the other lamp
Its branch is still a complete path with the full voltage.
A 2 kW heater runs for 3 hours at 30 c per kWh. The cost is
2 × 3 = 6 kWh; 6 × 30 c = 180 c.
Transformers are used in the electricity grid to
Step-up for transmission, step-down for homes.
Which source of electricity transforms light directly into electrical energy?
Solar cells have no turbine or generator.
Label how heat is moving.
Match each word to its topic.
Put the steps of getting electricity to your home in order.
Energy cannot be , only transformed. In most transformations energy is wasted as . Efficiency is useful energy out divided by total energy . Mains electricity is . Power is measured in .
Jordan says, "LED globes are a waste of money because they cost more than the old ones." Using your knowledge of energy transformations, efficiency and running costs, explain whether Jordan is right. Include a Sankey-style comparison in words and a cost calculation.
Jordan is wrong. An old incandescent globe transforms only about 5% of its electrical energy into light; the other 95% becomes heat. In a Sankey diagram, the light arrow would be a thin sliver and almost the whole input would bend away as wasted heat. An LED transforms about 40% into light, so its light arrow is much wider and its heat arrow much narrower. This is why a 9 W LED gives as much light as a 60 W incandescent globe.
Over 1000 hours (about a year of normal use), the incandescent globe uses 0.06 kW × 1000 h = 60 kWh, costing 60 × $0.30 = $18. The LED uses 0.009 × 1000 = 9 kWh, costing $2.70. That saves over $15 every year, which pays back the LED's extra purchase price within the first year. The LED also lasts about 25 times longer, so you would need to buy around 25 incandescent globes to match one LED.
So although an LED costs more to buy, it is far cheaper over its life. It also means less electricity needs to be generated, reducing greenhouse gas emissions from power stations.