Where electricity comes from — answer key

1. RQ1 · Practice

Options as numbered on the pupil sheet: 1 — Is pushed round by the steam, so that its shaft spins; 2 — Cools the used steam, carrying wasted energy into the air; 3 — Stores the electricity until the town switches it on; 4 — Is turned by the shaft and generates the electric current; 5 — Heats water until it boils into high-pressure steam.

Model answer. Gas burns under the boiler (A), and the energy released heats the water until it boils into high-pressure steam. The steam rushes along the pipe and pushes on the blades of the turbine (B), so it spins. The turbine’s shaft turns the generator (C), which generates electricity for the cables. The used steam must be cooled back into water before it goes round again; the cooling tower (D) carries this wasted energy away by heating the air. Nothing in the power station stores electricity: it is generated while the town uses it.

A full answer includes

• I matched part A, the boiler, to heating water until it boils into steam.
• I matched part B, the turbine, to being pushed round by the steam.
• I matched part C, the generator, to generating the current.
• I matched part D, the cooling tower, to cooling the used steam.
• I did not match any part to storing electricity: a power station stores none.

Also accept

• A boiler; B turbine; C generator; D cooling tower

Watch for

• “The flame turns the turbine directly.” — The flame heats water in the boiler. It is the steam from the boiling water that pushes the turbine blades round.
• “The generator stores the electricity until it is needed.” — A generator generates electricity only while it is turning. The power station stores no electricity.
• “The cooling tower makes the electricity, because the cloud rises from it.” — The cloud is water vapour carrying wasted energy away. The current comes from the generator, part C.

2. RQ2 · Practice

Model answer. A tidal barrage holds back water at high tide. As the tide goes out, the water flows through tunnels in the barrage and pushes the blades of turbines round; each turbine turns a generator, which generates the current. Nothing is burnt and no water is boiled, so there is no boiler. Row B.

A full answer includes

• I chose row B.
• I did not choose row A: no water is boiled at a tidal barrage.

Also accept

• Row B

Watch for

• “Row A.” — The moving water pushes the turbine round directly. Nothing is burnt, so no water is boiled into steam: there is no boiler.
• “Row C.” — A generator has to be turned. At a barrage the moving water pushes a turbine round, and the turbine turns the generator.
• “Row D.” — A turbine only spins: it makes no current. The generator makes the current, and no water is boiled at a barrage.

3. RQ3 · Practice

Options as numbered on the pupil sheet: 1 — Its atoms’ nuclei release energy that boils water into steam; 2 — Moving air pushes the blades of a turbine round; 3 — Light makes a current directly, with no turbine at all; 4 — Water pumped down to them comes back up as steam; 5 — Moving water pushes a turbine round; no steam is needed; 6 — Burnt in a boiler to boil water into steam for a turbine.

Model answer. Fossil fuels such as coal and biofuels such as wood pellets hold energy in their chemical store; they are burnt in a boiler, and the steam turns a turbine joined to a generator. Nuclear fuel such as uranium releases energy from the nuclei of its atoms, which also boils water. In a geothermal power station, water pumped down to hot rocks deep underground comes back as steam. Water flowing down from behind a hydroelectric dam, or out of an estuary as the tide falls, turns turbines directly: no steam is needed. The wind pushes the blades of a wind turbine round. Solar cells are different from all the rest: light makes a current in them, with no moving parts.

A full answer includes

• I matched coal and wood pellets to being burnt in a boiler.
• I matched uranium to its nuclei releasing energy that boils water.
• I matched the hot rocks to water pumped down and coming back as steam.
• I matched the dam and the tide to moving water pushing a turbine.
• I matched the wind to moving air pushing the blades of a turbine.
• I matched sunlight on solar cells to making a current with no turbine.
• I did not match wood pellets to a different way from coal.

Also accept

• coal and wood pellets burnt in a boiler; uranium nuclei boil water; hot rocks give steam; dam and tide moving water; wind moving air; solar cells current from light

Watch for

• “Biofuels generate electricity in a different way from fossil fuels.” — Both are fuels: they are burnt to boil water, and the steam turns a turbine and generator. The difference is where the fuel comes from.
• “Solar cells use the Sun’s heat to make steam.” — Solar cells turn light straight into electricity. There is no steam, turbine or generator.
• “Uranium is burnt in a boiler, like coal.” — Nothing burns in a nuclear power station. Energy released from the nuclei of the uranium atoms heats the water.

4. RQ4 · Practice

Model answer. The phone charger must give a current to charge the phone: light falling on its solar cells makes that current directly, with no turbine or generator. The shower bag makes no electricity. Its black surface absorbs infrared and other electromagnetic waves from the Sun and heats the water inside, which is what a solar panel does.

A full answer includes

• I chose: the charger uses solar cells; the shower works like a solar panel.
• I did not choose the swapped statement: solar cells make a current, solar panels heat water.

Also accept

• the charger uses solar cells; the shower works like a solar panel

Watch for

• “The charger works like a solar panel; the shower uses solar cells.” — Swapped. The charger must give a current, so it uses solar cells. The shower must give hot water, which is what a solar panel does.
• “Both use solar cells, because both take in energy from the Sun.” — Solar cells make electricity from light. The shower bag makes no electricity: it absorbs the Sun’s radiation and heats the water, like a solar panel.
• “Both work like solar panels, because both get warm in the sunshine.” — Getting warm is not the charger’s job. Light makes a current in its solar cells, and that current charges the phone.

5. RQ5 · Practice

Model answer. (a) The barrage is a low dam across the estuary. As the tide comes in, the estuary behind it fills; then the gates are closed. As the tide goes out, the water held back is let out through tunnels in the barrage and turns turbines, which turn generators. This happens at times that can be worked out years ahead. (b) The waves lift and drop the floating machines, and the movement of their parts drives generators. (c) Both use generators. The turbine or the moving parts only turn; the generator produces the current. A solar cell needs neither: light makes the current directly.

A full answer includes

• (a) I described water held behind the barrage at high tide.
• (a) I described the water flowing out through turbines as the tide goes out.
• (a) I wrote that the turbines turn generators.
• (b) I described the waves moving parts of the machines up and down.
• (b) I wrote that this movement drives generators.
• (c) I wrote: a generator.
• (c) I did not write turbine: a turbine turns, but the generator produces the current.

Also accept

• (a) water held at high tide flows out through turbines that turn generators (b) the waves move the machines up and down, driving generators (c) a generator

Watch for

• “A tidal barrage generates electricity steadily, all day.” — It generates when the water held behind it is let out as the tide goes out: two bursts a day, at times known in advance.
• “Tides and waves are the same thing.” — Tides are the slow rise and fall of the whole sea, twice a day; waves are made by the wind blowing over the surface. Different machines use them.
• “The turbine produces the current.” — The turbine only spins. It turns the generator, and the generator produces the current.

6. RQ6 · Practice

Model answer. (a) Coal, a fossil fuel, and wood pellets, a biofuel, hold energy in their chemical store; burning releases it. (b) Nuclear fuel such as uranium holds energy in its nuclear store, in the nuclei of its atoms. (c) In a coal-fired power station the coal burns, a chemical reaction with oxygen, and the energy released heats the water. In a nuclear power station nothing burns: energy released from the nuclei of the uranium atoms heats the water. After that the two work in the same way: the steam turns a turbine joined to a generator. (d) In some places the rocks a few kilometres down are very hot. Cold water is pumped down to them and comes back up as steam or very hot water; the steam turns a turbine and generator.

A full answer includes

• (a) I wrote: the chemical store.
• (b) I wrote: the nuclear store.
• (c) I wrote that in a coal-fired power station the coal burns, and this heats the water.
• (c) I wrote that in a nuclear power station nothing burns: energy released from the nuclei of the fuel heats the water.
• (c) I did not write that nuclear fuel is burnt.
• (d) I described water pumped down to hot rocks deep underground.
• (d) I wrote that it comes back as steam, which turns a turbine and generator.

Also accept

• (a) chemical (b) nuclear (c) coal burns; in a nuclear power station nothing burns, energy from the nuclei heats the water (d) hot rocks underground heat water into steam that turns a turbine and generator

Watch for

• “Nuclear fuel is burnt, like coal.” — Nothing burns in a nuclear power station. Energy is released from the nuclei of the fuel’s atoms, and it heats the water.
• “Geothermal energy comes from the Sun warming the ground.” — It comes from hot rocks deep inside the Earth, not from sunlight on the surface.

7. RQ7 · Practice

Model answer. (a) Moving air pushes on the blades of the wind turbine and turns them. The blades turn a shaft that turns a generator at the top of the tower, and the generator generates electricity. There is no boiler and no steam. (b) Solar cells turn light into electricity, so their power depends on how much light falls on them. On a winter afternoon the Sun is low in the sky, its light is spread thinly and the sky is often cloudy, so much less light reaches the cells than at noon in summer. It is the light that matters, not the warmth: cold does not stop solar cells. (c) Waves are made by the wind blowing over the surface of the sea. Out in the open sea the wind blows over a long stretch of water and builds big waves; the harbour walls shelter the water inside, so its waves are small. Wave machines are driven by the up-and-down movement of the waves, so bigger waves move their parts more and transfer more energy to the generators.

A full answer includes

• (a) I wrote that the wind pushes the blades round.
• (a) I wrote that the turbine turns a generator, which generates electricity.
• (b) I explained that solar cells make electricity from light.
• (b) I explained that less light reaches them on a winter afternoon (the Sun is low, or the sky is cloudier).
• (b) I did not write that the cells need heat, or that the cold stops them.
• (c) I explained that waves are made by the wind blowing over the sea.
• (c) I explained that the waves are much bigger in the open sea, because the harbour walls shelter the water inside.
• (c) I explained that bigger waves move the machines’ parts more, so more energy is transferred.

Also accept

• (a) wind turns the blades, the turbine turns a generator (b) less light reaches the cells on a winter afternoon (c) waves are made by the wind and are bigger in the open sea; the harbour is sheltered

Watch for

• “The wind turbine uses steam, like a gas-fired power station.” — The wind turns the turbine blades directly; no water is boiled.
• “Solar cells need the Sun’s heat, so they stop working when it is cold.” — Solar cells need light, not heat. They give less power in winter because less light reaches them.
• “Waves come from the tides, so they are the same size everywhere.” — Waves are made by the wind blowing over the sea, so they are biggest where the wind blows over open water. The tides are a separate, slow rise and fall.

8. RQ8 · Practice

Model answer. Coal formed from plants that grew in sunlight millions of years ago; wood pellets come from trees that grew in sunlight a few years ago. The Sun heats the ground and the air unevenly, which makes the winds, and the wind makes waves. The Sun evaporates water that falls as rain on the hills and fills the reservoir behind a dam. Tides are caused mainly by the Moon’s gravity, geothermal energy comes from hot rocks inside the Earth, and nuclear fuel holds energy in the nuclei of atoms that were in the Earth when it formed: none of these three comes from the Sun’s radiation.

A full answer includes

• I selected coal and wood pellets.
• I selected wind and waves.
• I selected water behind a hydroelectric dam.
• I did not select tides, geothermal energy or nuclear fuel.

Also accept

• coal, wind, hydroelectric dam, wood pellets, waves

Watch for

• “Coal is non-renewable, so its energy cannot have come from the Sun.” — Coal formed from ancient plants, which grew using sunlight: its energy came from the Sun long ago.
• “Tides come from the Sun, like everything else.” — Tides are caused mainly by the Moon’s gravity pulling on the oceans, not by the Sun’s radiation.
• “Geothermal energy comes from the Sun warming the ground.” — The heat comes from hot rocks deep inside the Earth, not from sunlight on the surface.
• “Nuclear fuel got its energy from the Sun, like coal.” — Uranium was in the rocks when the Earth formed, and its energy is in the nuclei of its atoms. It did not come from sunlight.

9. RQ9 · Practice

Options as numbered on the pupil sheet: 1 — The water vapour rises, cools and falls as rain on the hills; 2 — Water flows down through the turbines, which turn the generators; 3 — The rain fills the reservoir high up behind the dam; 4 — Nuclear fusion in the Sun releases energy; 5 — Radiation from the Sun heats the sea, and water evaporates.

Model answer. Energy is released by nuclear fusion in the Sun and reaches the Earth as radiation. It heats the sea, and water evaporates. The water vapour rises, cools, forms clouds and falls as rain on high ground. The rain runs into the reservoir behind the dam, where the water has energy in its gravitational potential store. When it flows down through the turbines, they turn the generators.

A full answer includes

• Step 1: nuclear fusion in the Sun releases energy.
• Step 2: radiation from the Sun heats the sea, and water evaporates.
• Step 3: the water vapour rises, cools and falls as rain on the hills.
• Step 4: the rain fills the reservoir behind the dam.
• Step 5: the water flows down through the turbines, which turn the generators.

Also accept

• fusion in the Sun, radiation evaporates water, rain on the hills, reservoir, turbines and generators

Watch for

• “The dam’s energy comes from the river, not from the Sun.” — The river keeps flowing because of rain, and the rain comes from water evaporated by the Sun’s radiation.

10. RQ10 · Practice

Model answer. Energy is released in the Sun by nuclear fusion: in its very hot core, light nuclei join together, and energy is released. It is not burning, which is a chemical reaction with oxygen.

A full answer includes

• I wrote: nuclear fusion.
• I did not write burning.

Also accept

• nuclear fusion
• fusion

Watch for

• “The Sun is a ball of burning gas.” — Burning is a chemical reaction with oxygen. The Sun’s energy is released by nuclear fusion, a change in the nuclei of its atoms.

11. RQ11 · Practice

Model answer. (a) Waves are made by the wind blowing over the surface of the sea. The wind itself blows because the Sun’s radiation heats the land, the sea and the air unevenly: warm air rises, and cooler air moves in to take its place. So the energy of the waves came from the Sun’s radiation, through the wind. The tides are different: they are caused mainly by the Moon’s gravity. (b) Research is being carried out to find out how energy released by nuclear fusion, the process that releases the Sun’s energy, can be used to produce electrical energy on a large scale. No fusion power station supplies electricity to homes yet.

A full answer includes

• (a) I wrote that waves are made by the wind blowing over the sea.
• (a) I wrote that the wind blows because the Sun’s radiation heats the land, the sea and the air unevenly.
• (a) I did not write that waves are caused by the Moon: that is the tides.
• (b) I wrote that the research aims to find how energy released by fusion can be used to produce electrical energy.
• (b) I wrote: on a large scale.

Also accept

• (a) the wind makes the waves, and the Sun heating the air unevenly makes the wind (b) how fusion can be used to produce electrical energy on a large scale

Watch for

• “Waves are caused by the Moon, like the tides.” — The Moon’s gravity causes the tides. Waves are made by the wind, and the wind comes from the Sun heating the air unevenly.
• “Fusion power stations already supply much of our electricity.” — Fusion is still being researched; no power station supplies electricity from fusion yet.

12. RT1 · Apply

Model answer. (a) Cold water is pumped down a deep hole to the hot rocks, heated until it turns to steam, and the steam comes back up and turns a turbine joined to a generator. No fuel is burnt. (b) Solar panels absorb infrared and other radiation from the Sun and heat the water flowing through them. The water that the electric heaters used to heat now arrives already hot, so the hospital needs less electricity, although the panels themselves generate none. (c) In the diesel engine the burning fuel pushes on the engine’s moving parts, which turn the generator directly: there is no boiler, no steam and no turbine. (d) Geothermal energy comes from hot rocks inside the Earth and the tides mainly from the Moon’s gravity, so neither comes from the Sun’s radiation. The wind comes from the Sun heating the air, and diesel is made from crude oil, which formed from living things that got their energy from sunlight.

A full answer includes

• (a) I described water pumped down to the hot rocks.
• (a) I wrote that it comes back as steam, which turns a turbine and generator.
• (b) I explained that the panels heat the water with the Sun’s radiation.
• (b) I explained that the electric heaters are then needed less, so less electricity is used.
• (b) I did not write that the panels generate electricity.
• (c) I stated that the diesel engine has no boiler and no steam turbine.
• (c) I stated that the burning diesel moves the engine’s parts, which turn the generator.
• (d) Supplement: I stated geothermal and the tides.
• (d) I did not choose the wind or diesel: their energy came from the Sun.

Also accept

• (a) water pumped to hot rocks returns as steam that turns a turbine and generator (b) panels heat the water, so less electrical heating is needed (c) no boiler or steam turbine (d) geothermal and the tides

Watch for

• “Solar panels on the hospital roof generate electricity for its machines.” — Solar panels heat water. It is solar cells that generate electricity.
• “Diesel’s energy cannot come from the Sun, because oil comes from underground.” — Crude oil formed from ancient living things that got their energy from sunlight, so its energy came from the Sun long ago.