Energy and Waves

One question wearing two names

Open this unit and it reads like two subjects that got filed together by accident. Energy stores. Work and power. Efficiency. Power stations. And then, with no warning, waves, the wave equation and the electromagnetic spectrum. ⚠️ MOST BOARDS TEACH THOSE AS TWO SEPARATE TOPICS. THIS ONE DOES NOT, AND THAT IS NOT AN ACCIDENT OF FILING. Every item on that list is an answer to the same question. ⭐ WHERE DOES ENERGY COME FROM, AND HOW DOES IT GET TO WHERE YOU NEED IT? Read the unit again with that question in your hand and it stops being a list. ENERGY STORES tell you where energy is sitting at the moment - in a moving car, in a stretched spring, in a hot cup of tea, in a bucket of fuel. TRANSFERS tell you how it gets out of one store and into another, which is the only thing energy ever actually does. RESOURCES tell you where we take it from in the first place, and whether that source refills. ⭐ WAVES ARE ONE OF THE WAYS IT TRAVELS - AND THE MOST USEFUL ONE, BECAUSE A WAVE NEEDS NOTHING TO CARRY. Energy can arrive from the Sun across empty space with nothing in between. And EFFICIENCY tells you how much of it survived the journey, because none of these transfers is clean and the part that goes astray does not vanish. ⚠️ SO THE UNIT IS NOT ENERGY PLUS WAVES. IT IS ONE STORY: WHERE ENERGY IS, HOW IT MOVES, HOW MUCH ARRIVES. And that gives you something to do with every question in it. ⭐ ASK WHICH PART OF THE JOURNEY THE QUESTION IS ABOUT. Is it about where the energy started, how it travelled, or how much made it? Nearly every question in this unit is one of those three, and knowing which tells you what to reach for.

The journey, in three stages

The whole unit, arranged as one journey. Read the columns left to right, because that is the order energy actually travels in.

Tap what makes a wave useful

Tap the TWO statements that explain why waves matter to a unit about energy.

• A wave can carry energy across a space with nothing in it at all
• A wave moves energy from one place to another without the material itself travelling along
• A wave is one of the energy stores, alongside kinetic and chemical
• Energy carried by a wave arrives without any of it being wasted

Why waves are in an energy unit

Sunlight reaches the Earth across millions of kilometres of empty space. Why does that make waves essential to the story this unit tells?

• Because every other way of transferring energy needs something to do the transferring, and a wave does not, so it is the only route across a vacuum
• Because waves travel faster than any other kind of energy transfer
• Because a wave is an energy store, so it holds energy until it arrives
• Because energy carried by a wave arrives completely undiminished

Five words used exactly

Five terms, each defined by what it is. Where each one belongs in the journey is the next step, and is not given here.

Match each one to its part of the journey

Two that follow from the journey

Select the TWO statements that follow from the way this unit fits together.

• Energy that is described as wasted has still gone somewhere, usually into heating the surroundings
• Whether a resource is renewable is about how fast it refills, not about how clean or cheap it is
• A power station creates the energy that it supplies to homes
• A very well designed machine can be more than 100 per cent efficient

Two numbers from the same journey

A motor is supplied with 1200 J of energy. Of that, 300 J ends up doing the job it was bought for: lifting a load. Efficiency is the useful energy out divided by the total energy in. That is 300 divided by 1200, which is 0.25. As a percentage, 25 per cent. ⚠️ NOW ASK THE QUESTION THAT TURNS THIS FROM ARITHMETIC INTO PHYSICS. WHERE DID THE OTHER 900 J GO? It did not disappear, and it was not destroyed. It was transferred somewhere nobody wanted it: the motor warmed up, the air around it warmed up, and some of it left as sound. ⭐ THE ENERGY IS STILL THERE. IT IS JUST SPREAD OUT IN THE SURROUNDINGS, TOO THINLY TO BE USED FOR ANYTHING. Which is why efficiency is never 1, and why an answer above 1 is always a mistake rather than a discovery. More energy out than in would mean the machine invented some. Now the other number the same journey gives you. Suppose that motor transferred 600 J in 4 seconds. Power is the energy transferred each second, so that is 600 divided by 4, which is 150 watts. ⚠️ NOTICE THAT THOSE TWO CALCULATIONS ANSWER DIFFERENT QUESTIONS ABOUT THE SAME EVENT. Efficiency asks how much of it arrived where you wanted. Power asks how quickly it moved. ⭐ A MACHINE CAN BE POWERFUL AND WASTEFUL AT THE SAME TIME, AND KNOWING WHICH QUANTITY A QUESTION IS ASKING FOR IS HALF THE BATTLE. So before reaching for a formula, decide which part of the journey you have been asked about: where the energy came from, how fast it moved, or how much survived.

Work out the efficiency

A lamp is supplied with 60 J of energy each second, and 45 J of that is transferred usefully as light. Calculate its efficiency as a percentage.

Find the wavelength

A sound wave travels through air at 340 m/s with a frequency of 170 Hz. Using speed = frequency x wavelength, calculate its wavelength in metres.

Order the journey from resource to room

Put the stages in order, from the original resource to the light in a room.

• Energy sits in a store, such as the chemical store of a fuel or the kinetic store of moving air
• A power station transfers it, turning a generator
• It is carried to homes electrically through the transmission network
• A lamp transfers it again, some usefully and some not
• The useful part leaves the lamp as a wave and crosses the room
• The rest warms the lamp and the air, spread too thinly to be used

The energy journey run

Five questions on stores, transfers, efficiency and waves. Three lives.

Complete the energy and waves facts

A place where energy is held at a given moment is an _____. The energy transferred when a force moves something through a distance is _____. The energy transferred each second, measured in watts, is _____. The number of complete waves passing a point each second is the _____.

Three answers to place

Three pieces of student writing. In each case the mark is in the reasoning, not the verdict.

• An answer states that a light bulb "uses up" the electrical energy it is given. What needs changing?
• A student says a wind turbine is renewable because it does not pollute. Is that the right reason?
• A question gives an energy supplied, an energy usefully transferred and a time, and asks for efficiency. A student uses the time in their calculation. What has gone wrong?

Explain where the energy goes

A classmate says this unit is two unrelated topics and cannot see why waves are in it at all. Write them an explanation that follows the energy from a resource to a lit room.

• Explain what an energy store is and give two examples that are genuinely different from each other
• Explain what happens at a power station, being careful about the words you use for what it does to energy
• Explain why a wave is the one kind of transfer that needs nothing to carry it, and why that matters
• Explain what efficiency measures, and say where the energy that is not useful actually ends up
• Finish by explaining why efficiency can never be greater than 1, and what an answer above 1 would have to mean