Conservation of Energy and Efficiency: a full term of science

Ten ready-to-teach lessons for Year 9 Physical sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards or exit tickets, an assessment kit and worked answers.

Start here: five minutes to Monday

1. Skim the term at a glance on the next page.
2. Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards or tickets.
3. Gather the few items under “You need” on the plan. Most come from the science prep room or the supermarket.
4. Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
5. Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.

For any Year 9 science teacher

The pack suits a physics specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: energy is never made or destroyed, only transferred and transformed, so in every device the energy going in equals the energy coming out. What changes is how much of it does the job. The useful part divided by the total input is the efficiency, and the rest, usually spread out as heat, is wasted. No real device is 100% efficient, and choosing devices well is about the energy and the cost of the waste.

Two ways to run each lesson

Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 5 needs an electric kettle with its power on the label, Lesson 6 needs a retort stand and a mass on a string, and Lesson 7 needs spring balances and a board to use as a ramp.

Safety and care in one look

The kettle in Lesson 5 is used by the teacher only, on a stable bench, well away from students. Balls are dropped, not thrown. Pendulums swing in a clear space at bench height, and the mass is fixed firmly. Spring balances are pulled gently, in line with the ramp, and are never stretched past their scale.

The term at a glance

One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.

How the sequence builds

Lessons 1 to 3 follow energy through transformations: the forms it takes, a bouncing ball that never returns to its height, and outputs that are useful or wasted. Lessons 4 and 5 turn this into numbers, with Sankey diagrams and a measured kettle. Lessons 6 and 7 find where the waste goes in a pendulum and on a ramp. Lessons 8 and 9 take efficiency into choices people make, from light globes to power stations, and Lesson 10 brings it together.

Curriculum links (Australian Curriculum V9)

The whole term teaches the Science Understanding descriptor AC9S9U05 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:

Assessment in this pack

• Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 9 voice, with working.
• The assessment sheet near the front has a class observation checklist and a three-level rubric.
• Lesson 10 is the summative pair: the school energy task, plus the “Show what we know” check.

Materials for the whole term

One gathering session covers all ten lessons, and most of it is already in the science prep room. Three things are worth doing ahead: check the kettle for Lesson 5 has its power in watts on the label, set up a pendulum stand for Lesson 6, and find a smooth board to use as a ramp in Lesson 7.

The one-trip list

• From the prep room: retort stands and clamps, 100 g masses, spring balances, wooden blocks with hooks, trolleys, a thermometer and timers.
• From the staffroom: an electric kettle and a measuring jug.
• From home or a cupboard: rubber balls, string, sandpaper, a torch, a small fan, a wind-up toy and old globe boxes.
• For Lesson 7: a smooth board about 1 m long and something to prop it on.

Safety and care in one look

• Only the teacher uses the kettle, on a stable bench away from students.
• Balls are dropped, not thrown; pendulums swing in a clear space with the mass fixed firmly.
• Spring balances are pulled gently and never past the end of their scale.
• Mains appliances are only shown switched off and unplugged, except the kettle in Lesson 5.

Assessment without extra work

The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the task, in which students analyse a school’s energy for a day with efficiencies, a Sankey diagram and running costs, and the final check. This sheet is the place to note what you see along the way.

The three levels

Class observation checklist

A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.

Word wall cards

Cut out the cards and build the wall as the words arrive: energy, joule, energy transfer, energy transformation, kinetic, gravitational, elastic and chemical energy in Lesson 1; law of conservation of energy in Lesson 2; input, useful output and wasted energy in Lesson 3; efficiency and Sankey diagram in Lesson 4; power in Lesson 5; friction in Lesson 6; kilowatt hour and energy rating in Lesson 8.

Dear families

This term in science, Year 9 studies energy and efficiency. Energy is never made or destroyed, only moved and changed from one form to another, so every joule that goes into a device comes out again; the question is how much of it does the job. Students measure, calculate and compare efficiencies, and draw diagrams that track where the energy goes.

Along the way your student will measure a bouncing ball, sort useful and wasted energy, time a kettle and work out its efficiency, watch a pendulum’s swing die away, pull a block up a ramp on different surfaces, and compare the running cost of two light globes.

Try this at home

• Find the energy rating label on a fridge or a washing machine. What do the stars tell you?
• Feel a phone charger after charging. Where did that warmth come from?
• Check a globe box: how many watts does the globe use, and how bright is it?
• Drop a ball and watch it bounce. Why does each bounce get lower?

What to ask your scientist

• If energy cannot be destroyed, why do we talk about wasting it?
• How do you work out how efficient something is?
• Why can no machine be 100% efficient?
• Which costs less to run: an LED globe or an old-style globe, and by how much?

A small note on care: only the teacher uses the kettle, and students handle balls, pendulums and spring balances with care.

Warm regards,

The Year 9 science team

Printed from the free seegongsik Conservation of Energy and Efficiency teaching pack · seegongsik.com/au/y9/physical/AC9S9U05/pack

Forms and transformations

Energy is measured in joules (J), and it comes in forms: the kinetic energy of anything moving, the gravitational energy of anything raised up, the elastic energy of a stretched spring, the chemical energy in food, fuels and batteries, and the electrical, light, sound and thermal energy that devices pass along. An energy transfer moves energy from one place to another; an energy transformation changes it from one form to another. Every device takes energy in, in one form, and gives it out in others: some does the job and some is wasted, usually as heat. Students trace the energy through six devices, from the form going in to the forms coming out.

We are learning to

• name the forms of energy and tell a transfer from a transformation,
• trace the energy through a device from its input to its outputs,
• sort the outputs into useful and wasted.

Success criteria

• I can name the input, the useful output and the wasted output for six devices.
• I can draw an energy chain for one device, with every transformation.

You need

• a torch, a small fan and a wind-up toy to show,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the torch, the board and the table in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Energy gets used up.” It changes form and spreads out; the total stays the same.
• “A device makes energy.” It only transforms the energy it is given.
• “Heat is a form of energy only in heaters.” Nearly every device gives out some heat, wanted or not.

Make it easier, make it bigger

• Easier: give the forms of energy as a list on the board to choose from.
• Bigger: trace the energy that lights a torch back to the Sun, step by step. Where did the battery’s chemical energy first come from?

Answers and look-fors

• Part A: a torch: chemical (in the battery) in, light useful, heat wasted; a hair dryer: electrical in, heat and kinetic (moving air) useful, sound wasted; a ceiling fan: electrical in, kinetic (moving air) useful, heat and sound wasted; a bike going downhill: gravitational in, kinetic useful, heat and sound (friction) wasted; a solar panel: light in, electrical useful, heat wasted; a wind-up toy: elastic (in the spring) in, kinetic useful, heat and sound wasted.
• Part B: chemical energy (battery) → electrical energy (wires) → light (globe), with heat given out at the battery and the globe.
• Part C: 1. A transfer moves energy from place to place in the same form; a transformation changes its form. 2. It spreads into the surroundings, mostly as heat, warming them a little. 3. Moving air is the job of the fan; in the hair dryer, heat and moving air are both wanted. 4. No: the total out equals the total in; the device only changes its form.
• Bigger: for example, a power station burnt coal made from ancient plants, which had stored the energy of sunlight by photosynthesis, and that electricity was used to make the battery’s chemicals.
• Exit tickets: 1. Elastic energy. 2. Electrical energy. 3. Heat.

Forms and transformations

A. Six devices

B. The torch, step by step

Draw a box for each form of energy, joined by arrows. Add a side arrow wherever energy is wasted, and label it.

C. Questions

Exit tickets

Cut out one ticket for each student.

The bouncing ball

A ball held up high has gravitational energy. As it falls, that becomes kinetic energy; as it hits the floor and squashes, it becomes elastic energy; as it springs back, kinetic again, and then gravitational as it rises. If every joule came back, the ball would rise to the height it was dropped from. It never does, and that is not because energy is destroyed: at each bounce some of it becomes sound and some warms the ball and the floor. Because gravitational energy is proportional to height, the bounce height shows what share of the energy came back. Students measure five bounces and work out the share kept each time.

We are learning to

• trace the energy transformations in a bouncing ball,
• use heights to compare the energy before and after a bounce,
• apply the law of conservation of energy to the energy that does not come back.

Success criteria

• I can graph the bounce heights and work out the share of energy kept at each bounce.
• I can say where the rest of the energy went, without saying it was destroyed.

You need

• for each group: a rubber ball and a metre ruler fixed upright against a wall; a phone to film the bounce if one is allowed,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Care: balls are dropped, not thrown; groups spread out so that balls do not roll under feet.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the bounces in Session A, and Parts B to D in Session B.

Watch for these ideas

• “The ball runs out of energy.” The energy is still there, spread through the ball, the floor and the air.
• “A heavier ball would bounce back higher.” The share kept depends on the ball and the floor, not mainly on its mass.
• “Sound has no energy.” Sound is energy carried by the air; the bounce’s thud takes some away.

Make it easier, make it bigger

• Easier: measure only the first bounce three times, and use the example heights for the rest.
• Bigger: test the same ball on carpet and on a hard floor. Predict which keeps more energy, and explain why.

Answers and look-fors

• Part A, example: bounce 1 72 cm, bounce 2 52 cm, bounce 3 37 cm, bounce 4 27 cm, bounce 5 19 cm.
• Part B: the heights fall by a smaller amount each time, curving down towards zero.
• Part C: bounce 1 72%, bounce 2 72%, bounce 3 71%, bounce 4 73%, bounce 5 70%: about 72% each time.
• Part D: 1. Gravitational, then kinetic, then elastic as it squashes, then kinetic and gravitational again. 2. Into sound and into heat in the ball, the floor and the air. 3. The same height as it was dropped from, every time. 4. Yes: the energy is all still there, as heat and sound; only the useful share fell.
• Bigger: the hard floor, as carpet squashes and turns more of the energy into heat.
• Exit tickets: 1. Gravitational. 2. Sound and heat. 3. It is spread out, not destroyed.

The bouncing ball

A. Bounce heights (cm), dropped from 100 cm

B. Graph

C. Energy kept each bounce

Share kept = height after the bounce ÷ height before it × 100%.

D. Questions

Exit tickets

Cut out one ticket for each student.

Useful or wasted?

Whether energy is useful or wasted is not a property of the energy itself; it depends on the job. The heat from a toaster is useful, because browning bread is the job; the heat from a laptop is wasted, because computing is the job and a fan has to remove it. The same output can even be useful in one use and wasted in another: the heat from an old-style globe is wasted in a reading lamp but useful in a lamp that warms a reptile tank. Wasted energy is never destroyed: it spreads into the surroundings, usually as heat, where it is hard to use again. Students sort twelve outputs and explain their choices.

We are learning to

• decide whether an output is useful or wasted by asking what the job is,
• explain that wasted energy is spread out, not destroyed,
• recognise outputs that are useful in one use and wasted in another.

Success criteria

• I can sort the twelve outputs and give the job behind each choice.
• I can explain one card from the “depends on the job” pile.

You need

• the output cards (third sheet), one set per pair, with scissors,
• the worksheet (next sheet), one per student.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the sort in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Heat is always wasted.” In a toaster, a kettle or a heater, heat is the job.
• “Wasted energy disappears.” It spreads into the surroundings, usually as heat, where it is hard to use again.
• “Useful and wasted add up to more than went in.” They always add up to exactly the input.

Make it easier, make it bigger

• Easier: sort into useful and wasted first, then pull out the cards that could go either way.
• Bigger: write two new cards of your own for the “depends on the job” pile, and swap them with another pair.

Answers and look-fors

• Part A: useful B, C, E, H, K; wasted A, F, J, L; depends on the job D, G, I.
• Part B, the reason for each card: A the job is computing; a fan has to carry the heat away; B browning the bread is the job; C making sound is the job; D wasted in a reading lamp, but useful in a lamp that warms a reptile tank; E the picture is the job; F the job is warm, moving air; G wasted on a hot day, but in winter some of it warms the cabin; H warning people is the job; I useful on a dinner table, wasted when the candle only keeps a teapot warm; J the job is heating the bread; K warning people is the job; L the job is moving the clothes through the water.
• Part C: 1. The job the device is for. 2. It spreads into the surroundings, mostly as heat; it is still there, but too spread out to be useful. 3. They add up to the total input, because energy is conserved. 4. The device cannot be 100% efficient: some energy always escapes as heat and sound.

Useful or wasted?

A. Our sort

B. Three cards, three reasons

Choose one card from each pile. Name the job of the device, then explain your choice.

C. Questions

Useful, wasted, or it depends on the job?

Cut out the cards. Lay down the three grey heading cards, then sort the outputs under them.

Teacher note: the piles are not the same size. The answer letters and the reasons are in the plan.

Energy accounts

Because energy is conserved, every device keeps an exact account: the energy in equals the useful energy out plus the wasted energy out. A Sankey diagram draws that account as a flow, with each arrow as wide as the energy it carries: the input enters on the left, the useful output runs straight on, and the wasted outputs bend away. Efficiency reads straight off it: useful output divided by total input, times 100%. Students complete the accounts for an LED lamp, a car engine and an electric kettle, draw two of them to scale, and compare their efficiencies.

We are learning to

• complete an energy account, so that the outputs add up to the input,
• draw a Sankey diagram to scale,
• calculate efficiency as a percentage.

Success criteria

• I can draw a Sankey diagram with every arrow the right width.
• I can calculate the efficiency of all three devices and rank them.

You need

• rulers and coloured pencils,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the diagrams in Session A, and Parts B and C in Session B.

Watch for these ideas

• “The arrows just show direction.” In a Sankey diagram, the width shows how much energy each arrow carries.
• “The useful arrow can be wider than the input.” No output can be larger than the input.
• “An efficiency can be more than 100%.” That would mean making energy, which never happens.

Make it easier, make it bigger

• Easier: draw the LED lamp only, with the input arrow already drawn.
• Bigger: an electric car turns about 85% of the energy from its battery into motion. Draw its Sankey diagram beside the petrol car’s.

Answers and look-fors

• Part A: an LED lamp: input 10 squares, light 4.5, heat 5.5; a car engine: input 10 squares, kinetic 2.5, heat 7, sound 0.5. The output arrows add up to the input’s width.
• Part B: an LED lamp 45%, a car engine 25%, an electric kettle 88%. The most efficient is the electric kettle.
• Part C: 1. The electric kettle. 2. As heat, through the exhaust and the radiator, and as sound. 3. The arrows for the outputs add up to the same width as the input: energy is conserved. 4. The light that comes out is what we pay for; less heat for the same light means a smaller bill.
• Bigger: the electric car’s useful arrow is 85 of 100, against 25 of 100 for the petrol car.
• Exit tickets: 1. Useful output ÷ total input × 100%. 2. 40%. 3. It shows how much energy each arrow carries.

Energy accounts

A. Three accounts (for every 100 J in)

Draw a Sankey diagram for each of the first two devices. One square stands for 10 J, so the input arrow is 10 squares wide.

B. Efficiency

C. Questions

Exit tickets

Cut out one ticket for each student.

How efficient is a kettle?

Power is how fast a device transfers energy, measured in watts: one watt is one joule every second. So the energy a kettle takes in is its power times the time it runs. The energy the water gains can be worked out too: it takes about 4,200 J to warm 1 kg of water by 1 °C. The teacher boils a measured amount of water in a kettle whose power is on its label, and the class works out the efficiency from real measurements. The difference between the two energies warmed the kettle itself, its element and the air around it.

We are learning to

• calculate the energy a device uses from its power and the time it runs,
• calculate the energy gained by water from its mass and temperature rise,
• work out a measured efficiency, and judge how accurate it is.

Success criteria

• I can calculate the kettle’s efficiency from our measurements, step by step.
• I can say where the wasted energy went, and name a source of error.

You need

• an electric kettle with its power on the label, a measuring jug, a thermometer and a timer, run by the teacher,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Safety: only the teacher handles the kettle, on a stable bench away from students; steam and boiling water scald.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the measurement in Session A, and Parts B and C in Session B.

Watch for these ideas

• “Watts are a kind of energy.” Watts measure power, how fast energy is transferred; energy is in joules.
• “A kettle is 100% efficient because all the heat goes into the water.” Some warms the kettle and the air.
• “A hotter kettle uses more power.” The power on the label is fixed; heating more water takes longer, so it uses more energy.

Make it easier, make it bigger

• Easier: use the example measurements, and work the first step together.
• Bigger: boiling only the water you need saves energy. How much energy would boiling half a litre save, if the efficiency stays the same?

Answers and look-fors

• Part A, example: 2,000 W, 1 kg, from 20 °C to 100 °C, 190 s.
• Part B: 1. 2,000 × 190 = 380,000 J. 2. 4,200 × 1 × 80 = 336,000 J. 3. 336,000 ÷ 380,000 × 100% = 88%. 4. 380,000 − 336,000 = 44,000 J wasted.
• Part C: 1. Into heating the kettle’s body and element and the air around it, and a little into steam and sound. 2. For example: the start temperature read wrongly, the timer stopped late, or the label power not exact. 3. Yes: the energy wasted is still there, as heat in the kettle and the room.
• Bigger: half the water needs half the energy, so about 190,000 J is saved each time.
• Exit tickets: 1. Joules. 2. Power × time. 3. 1,000 W × 60 s = 60,000 J.

How efficient is a kettle?

A. Measurements

B. Step by step (use the example or ours)

C. Questions

Exit tickets

Cut out one ticket for each student.

A swinging pendulum

A pendulum trades energy back and forth: at the top of each swing it has gravitational energy and is still for a moment; at the bottom it has kinetic energy and moves fastest. In a perfect world it would swing forever. A real pendulum pushes air aside and rubs at its pivot, and each swing turns a little of its energy into heat and sound, so it rises a little less each time. Because the gravitational energy depends on height, the height of the swing tracks the energy left. Students release a pendulum from 20 cm above its lowest point and measure the height of the swing every 5 swings.

We are learning to

• trace the energy changes in a swinging pendulum,
• measure a slow change with careful, repeated readings,
• explain the loss with friction and air resistance, using the law of conservation of energy.

Success criteria

• I can graph the height of the swing and work out the share of energy kept every 5 swings.
• I can say where the energy goes, and suggest how to make the pendulum swing longer.

You need

• for each group: a retort stand and clamp, 1 m of string, a 100 g mass, a metre ruler and a timer,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Care: tie the mass on firmly and swing it at bench height in a clear space; keep faces out of its path.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the pendulum in Session A, and Parts B to D in Session B.

Watch for these ideas

• “The pendulum stops because gravity runs out.” Gravity stays the same; the energy leaks away as heat and sound.
• “At the bottom of the swing it has no energy.” At the bottom it has the most kinetic energy.
• “A heavier mass would swing for ever.” Every real pendulum loses some energy to the air and the pivot.

Make it easier, make it bigger

• Easier: read only the start and the height after 20 swings, and use the example for the rest.
• Bigger: tape a card sail to the mass and repeat. Predict, then explain, the difference in how fast the swing dies away.

Answers and look-fors

• Part A, example: 0 swings 20 cm, 5 swings 18.2 cm, 10 swings 16.6 cm, 15 swings 15.1 cm, 20 swings 13.7 cm.
• Part B: a gently falling curve, dropping a little less each time.
• Part C: swings 0 to 5, 91%; swings 5 to 10, 91%; swings 10 to 15, 91%; swings 15 to 20, 91%: about 91% each time. After 20 swings it kept 69% of its starting energy.
• Part D: 1. Gravitational energy at the top, kinetic at the bottom, and back. 2. Into heat and sound, by pushing air aside and rubbing at the pivot. 3. A smaller, smoother pivot and less air to push: a streamlined mass, or even a vacuum. 4. No: the energy is still there, as a little warming of the air and the pivot.
• Bigger: the card sail pushes more air aside, so more energy turns into heat in the air each swing, and the swing dies away faster.
• Exit tickets: 1. At the bottom of the swing. 2. Friction and air resistance turn it into heat and sound. 3. It is still there, spread out as heat.

A swinging pendulum

A. Height of the swing (cm)

B. Graph

C. Energy kept every 5 swings

Share kept = later height ÷ earlier height × 100%.

D. Questions

Exit tickets

Cut out one ticket for each student.

Friction on a ramp

Pulling a block up a ramp transfers energy to it: the energy put in is the pulling force times the distance pulled. The useful part is the gravitational energy the block gains, its weight times the height it rises. If there were no friction the two would be equal; friction between the block and the ramp turns some of the energy put in into heat, so more must be put in than is gained. Students pull a 5 N block 1 m up a ramp 0.4 m high on three surfaces and work out the efficiency of each.

We are learning to

• measure the energy put into a system and the useful energy it gains,
• calculate efficiency and compare it across conditions,
• explain how friction wastes energy, and how to reduce it.

Success criteria

• I can calculate the energy put in, the energy gained and the efficiency for each surface.
• I can explain the differences with friction, and say where the wasted energy went.

You need

• for each group: a board about 1 m long propped 0.4 m high, a wooden block with a hook, a 0 to 10 N spring balance, a sheet of sandpaper and a toy trolley,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Care: prop the board firmly; pull the spring balance gently and in line with the ramp, never past the end of its scale.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the pulls in Session A, and Parts B and C in Session B.

Watch for these ideas

• “The ramp makes energy.” The ramp needs a smaller force, but over a longer distance; the energy needed is never less than the energy gained.
• “Friction destroys energy.” It turns kinetic energy into heat in the block and the ramp.
• “Wheels have no friction.” Wheels have much less friction where they roll, but some remains at the axles.

Make it easier, make it bigger

• Easier: test the smooth board and the trolley only, and use the example for the sandpaper.
• Bigger: rub the block on the sandpaper quickly for ten seconds and feel it. What does this show about where the wasted energy goes?

Answers and look-fors

• Part A, example forces: smooth board 2.6 N, sandpaper 3.4 N, on wheels (a toy trolley) 2.1 N.
• Part B: energy gained = 5 × 0.4 = 2 J on every surface; smooth board: 2.6 J in, 77%; sandpaper: 3.4 J in, 59%; on wheels (a toy trolley): 2.1 J in, 95%.
• Part C: 1. On wheels (a toy trolley): least friction, so least energy turned into heat. 2. Sandpaper: most friction. 3. Into heat in the block and the ramp surface, and a little sound. 4. The wheels roll instead of sliding, so much less energy is lost to friction.
• Bigger: the block and the sandpaper warm up: friction turns the energy of motion into heat.
• Exit tickets: 1. Force × distance. 2. 3 J ÷ 4 J × 100% = 75%. 3. Heat.

Friction on a ramp

A. Pulling force (N)

The block weighs 5 N. It is pulled 1 m along the ramp, and rises 0.4 m.

B. Efficiency

C. Questions

Exit tickets

Cut out one ticket for each student.

Choosing a globe

An old-style globe makes light by heating a thin wire until it glows, so most of its energy comes out as heat. An LED globe gives the same light from a fraction of the energy. Electricity is sold by the kilowatt hour (kWh): the energy a 1 kW device uses in an hour. Working out the energy of each globe over a year, and its cost, shows why efficiency matters in a household budget as well as for the environment. Australia began phasing out old-style globes in 2009, one of the first countries to do so, and LED globes are now the usual choice. Students compare a 60 W old-style globe with a 10 W LED globe that gives about the same light.

We are learning to

• calculate energy in kilowatt hours from power and time,
• compare the running costs of two devices that do the same job,
• explain how efficiency, cost and rules help people adopt a new technology.

Success criteria

• I can work out the energy and the cost of each globe for a year.
• I can explain why the LED globe costs less to run, using efficiency.

You need

• globe boxes with their labels, if you have them,
• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the table in Session A, and Part B in Session B.

Watch for these ideas

• “A brighter globe always has more watts.” Watts measure the energy used each second, not the light given out.
• “A kilowatt hour is a unit of power.” It is a unit of energy: power multiplied by time.
• “The cheaper globe to buy is the cheaper one to own.” Running costs over its life can be far larger than the price.

Make it easier, make it bigger

• Easier: work the old-style globe together on the board, then students do the LED.
• Bigger: an LED globe lasts about 15 times as long as an old-style one. Add the cost of replacement globes to your comparison.

Answers and look-fors

• Part A: hours a year = 5 × 365 = 1825; old-style 60 × 1825 ÷ 1,000 = 109.5 kWh, $32.85; LED 18.3 kWh, $5.48.
• Part B: 1. $27.38 a year for each globe. 2. $547.50 a year for 20 globes. 3. The LED turns much more of its energy into light; the old-style globe turns most of it into heat, which we pay for but do not want. 4. For example: lower bills, government rules phasing out old globes, falling LED prices, and people seeing the savings.
• Bigger: the old-style globe needs about 15 replacements in the life of one LED, which adds to its cost.
• Exit tickets: 1. Power (kW) × time (hours). 2. 2 kW × 3 h = 6 kWh. 3. Most of its energy becomes heat, not light.

Choosing a globe

A. A year of light

Each globe is used 5 hours a day for 365 days. Energy (kWh) = power (W) × hours ÷ 1,000. Cost = energy (kWh) × 30 cents.

B. Questions

Exit tickets

Cut out one ticket for each student.

Making electricity

Every way of making electricity is a chain of transformations, and each chain has its own efficiency. A coal-fired power station burns coal to boil water, the steam spins a turbine and the turbine turns a generator; about two thirds of the energy in the coal leaves as heat in the cooling towers and the chimney. A hydro-electric dam turns falling water straight into turning blades and wastes far less. A solar panel turns only about a fifth of the sunlight into electricity, yet its fuel is free and gives off no carbon dioxide. Efficiency matters, but it is not the only thing that matters. Students compare five ways, using rough efficiencies, and weigh up what else counts.

We are learning to

• trace the transformations in a power station,
• compare efficiencies and present them on a bar chart,
• weigh efficiency against cost, fuel and the environment.

Success criteria

• I can work out the electrical energy each way gives from the same input, and draw the bar chart.
• I can explain why the least efficient way can still be a good choice.

You need

• the worksheet (next sheet), one per student, and the exit tickets (third sheet).

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the opening, the board and the table in Session A, and Parts B and C in Session B.

Watch for these ideas

• “The most efficient way is always the best.” Fuel cost, pollution, reliability and where it can be built matter too.
• “Wasted energy in a solar panel costs us money.” Sunlight is free; the waste only means a bigger panel for the same output.
• “Power stations make energy.” They transform energy from a fuel, falling water, wind or light.

Make it easier, make it bigger

• Easier: work the coal station together on the board, then students do the rest.
• Bigger: choose one way to make electricity for a remote town. Argue for it, using efficiency and two other factors.

Answers and look-fors

• Part A: coal-fired power station 350 MJ; gas power station (combined cycle) 550 MJ; solar panel 200 MJ; wind turbine 400 MJ; hydro-electric dam 900 MJ.
• Part B: bars in order from the hydro-electric dam (90%) down to the solar panel (20%).
• Part C: 1. As heat, in the cooling water, the steam and the chimney gases. 2. Its input, sunlight, is free and gives off no carbon dioxide. 3. For example: the cost of the fuel, carbon dioxide and other pollution, whether it runs at night or in still weather, and the land it needs. 4. The rest of the energy is not destroyed: it leaves as heat and sound.
• Exit tickets: 1. The hydro-electric dam. 2. Heat. 3. For example, the fuel is free, or it gives off no carbon dioxide.

Making electricity

A. Five ways (rough efficiencies)

Work out how much electrical energy each way makes from 1,000 MJ of input energy (1 MJ is a million joules).

B. Bar chart

C. Questions

Exit tickets

Cut out one ticket for each student.

Show what we know

The term comes together. In the task, students analyse a school’s energy for one sunny day: the solar panels on the roof, the urn in the staffroom and the LED lights in the classrooms. They calculate an efficiency and two energies in kilowatt hours, compare the lights with old-style globes, and draw the panels’ Sankey diagram to scale. Then they sit the eight-question final check, and the class plays the unit quiz as a game on the board.

We are learning to

• apply the law of conservation of energy to a whole system,
• calculate efficiencies and energies with the right units,
• represent an energy account with a Sankey diagram.

Success criteria

• I can complete every calculation in the task, with units.
• I can answer the final check on my own.

You need

• the task (next sheet) and the final check (third sheet), one each per student,
• the interactive unit open on the board for the quiz; mini whiteboards if you have them.

Lesson flow (about 60 minutes)

Running two half-sessions instead? Do the rules and the task in Session A, and the final check and the quiz in Session B.

Watch for these ideas

• Students who mix up watts and kilowatt hours: ask which one is “how fast” and which is “how much”.
• Students who say the panels destroy most of the sunlight’s energy: ask where it goes.
• Students who draw Sankey arrows that do not add up to the input.

Make it easier, make it bigger

• Easier: give the three rules on the task sheet.
• Bigger: the school pays an example 30 cents per kWh. How much does the switch from old-style globes save each school day?

Answers and look-fors

• Task: 1. 120 ÷ 600 × 100% = 20%. 2. 480 kWh, as heat that warms the panels and the air. 3. 2.4 kW × 2 h = 4.8 kWh. 4. 80 × 10 W × 7 h = 5.6 kWh. 5. 33.6 kWh, which is 28 kWh more. 6. Energy cannot be made or destroyed: the 480 kWh not turned into electricity still exists, as heat.
• Sankey: input 600 kWh as 15 squares (1 square = 40 kWh); electrical 3 squares straight on; heat 12 squares bending away.
• Final check: 1. Made or destroyed. 2. 400 ÷ 500 × 100% = 80%. 3. Heat. 4. Some energy is always transferred as heat and sound, by friction and resistance, so the useful output is less than the input. 5. The amount of energy. 6. 2 kW × 4 h = 8 kWh. 7. 60%. 8. The petrol’s chemical energy becomes kinetic energy of the car, with most leaving as heat in the exhaust, the radiator and the brakes, and some as sound; the total stays the same.
• Bigger: 28 kWh × 30 cents = $8.40 each school day.

A school’s sunny day

On a sunny day, 600 kWh of sunlight falls on the school’s solar panels, and they make 120 kWh of electrical energy. The staffroom urn, rated at 2400 W, runs for 2 hours. The classrooms have 80 LED globes of 10 W each, on for 7 hours.

The panels’ Sankey diagram

Draw it to scale. One square stands for 40 kWh.

Show what we know