Efficiency Engineer

Nothing is lost, but not all of it is useful

Start from the rule that never breaks: energy is CONSERVED. It cannot be created or destroyed, only shifted from one store to another. So when we say a machine WASTES energy, we do not mean the energy vanishes. Every joule you put in is still there afterwards. The question is only WHERE it ended up. ⭐⭐ Some of it did the job you wanted, and we call that the USEFUL transfer. The rest was DISSIPATED: spread out into the surroundings, almost always by heating, where it is now too thinly spread to drive anything. It is not gone; it is just out of reach. ⭐ EFFICIENCY is one simple idea on top of that: of all the energy you put IN, what fraction came out doing the useful job? A motor that puts most of its energy into turning, and only a little into warming itself, is efficient. One that grows hot and barely turns is not. ⭐⭐ And conservation sets a hard ceiling. Because the useful output can never be more than the total you put in, NOTHING can ever be more than 100 per cent efficient. A claim of more than 100 per cent is not a clever machine; it is a mistake. This module is about efficiency alone. The list of energy stores, and the equations for kinetic and other energies, belong to a different module. ⚠️ Carry one question the whole way through: of the energy I put in, what fraction does the useful job, and where did the rest go?

Useful, wasted, and clawed back

The first column is the energy that did the job, the second is the energy that got away, and the third is how an engineer raises the fraction that stays useful.

Tap the two useful transfers

Tap the TWO transfers that are the USEFUL output of the device named, rather than energy that is wasted.

• A lamp: the light it gives out.
• A kettle: the energy that heats the water.
• An electric motor: the warmth of its casing.
• A phone charger: the heat it gives off while charging.

What wasted energy really is

A machine wastes some of the energy supplied to it. What has happened to that energy?

• It has been dissipated: spread out to the surroundings, usually by heating, where it can no longer do useful work
• It has been destroyed and no longer exists
• It has been stored inside the machine to use later
• It is automatically recycled back into useful energy

The language of efficiency

Six terms the topic turns on. The first three describe where energy goes; the last three are the ways to keep more of it useful.

Match each fix to the loss it cuts

Two ways to raise efficiency

Select the TWO changes that would genuinely make a machine more efficient.

• Lubricating the moving parts to reduce friction
• Insulating it to reduce energy lost by heating
• Redesigning it to give out more energy than is put in
• Simply supplying it with more energy

Answering an efficiency question

Efficiency questions are quick marks once you hold a few habits. ⭐⭐ KNOW THE CALCULATION IN WORDS. Efficiency is the useful energy transfer DIVIDED BY the total energy supplied. Multiply by 100 to give it as a percentage. Put the useful output on top and the total input underneath, and it is hard to go wrong. ⭐ CHECK IT AGAINST THE CEILING. The answer must come out at less than 100 per cent, or exactly 100 for a perfect machine that does not exist. If you get more than 100 per cent, you have divided the wrong way round or mislabelled which figure is the input. ⭐⭐ ACCOUNT FOR THE REST. Because energy is conserved, the useful energy plus the wasted energy equals the total supplied. So the wasted energy is just the total input minus the useful output, and it has been dissipated to the surroundings. IMPROVE BY CUTTING LOSSES, NOT BY MAGIC. To raise efficiency you name a specific unwanted transfer, friction or energy lost by heating, and a specific way to cut it, lubrication or insulation. Vague answers like make it better score nothing. ⚠️ A note on wording. Say energy is transferred by heating, rather than calling it a store of heat, which examiners mark as wrong. And describe the wasted energy as dissipated, not simply lost. Keep the one test to hand: of the energy I put in, what fraction does the useful job, and where did the rest go?

Order the efficiency calculation

Put the steps of working out an efficiency as a percentage into order.

• Find the total energy supplied to the device
• Find the useful energy it transfers
• Divide the useful energy by the total supplied
• Multiply the result by 100
• Check the answer is less than 100 per cent

Work out the efficiency

A motor is supplied with 240 joules of energy and usefully transfers 60 joules to turning a wheel. What is its efficiency as a percentage?

The efficiency run

Five quick decisions about efficiency and energy. Three lives.

Carrying water in a leaky bucket

Leave machines aside for a screen. Imagine carrying water from a well up to a tank in a bucket that has a few small holes. ⭐ You draw a full bucket, but by the time you reach the top some water has dripped away, so less arrives than you set off with. The dripped water is not destroyed. It soaks into the ground, spread through the soil, where you simply cannot scoop it back. ⭐⭐ How much of a job you get done is the share of the water you drew that actually reaches the tank. A bucket with tiny holes delivers most of it; a bucket like a sieve delivers almost none, however fast you walk. And there is a hard limit. You can never pour MORE into the tank than you drew from the well, because the water arriving can only ever be the water you started with minus what dripped out. More than that would be water from nowhere. ⭐ So to get more to the top you do the obvious thing: you patch the holes. You do not invent a bucket that arrives fuller than it left. ⚠️ Hold that picture. A machine is the bucket, the energy you supply is the water you draw, the share that does the job is what reaches the tank, and the drips are the energy spread out to the surroundings. You raise the share by plugging the leaks, never by beating the limit.

Finish each efficiency line

The rule that energy cannot be created or destroyed, only transferred, is the _____ of energy. The fraction of the energy supplied that is transferred usefully is the _____ of a device. Efficiency compares the _____ energy transfer with the total energy supplied. Oiling the moving parts of a machine to reduce friction is called _____. Lagging a hot water tank to reduce the energy lost by heating is called thermal _____.

Three efficiency problems on the floor

Three situations arise in a workshop. For each, choose the response that a sound understanding of efficiency supports.

• A supplier claims their new motor is 108 per cent efficient, delivering more energy than it draws.
• A conveyor motor is running hot and doing less work than it should, and its bearings are dry.
• A heater draws 500 joules and usefully warms the room with 450 joules, and someone asks where the other 50 joules went.

Explain and improve a machine

A machine wastes much of the energy supplied to it. Explain what efficiency means, what happens to the wasted energy, why the machine can never be more than 100 per cent efficient, and how an engineer could raise its efficiency.

• Define efficiency as the fraction of the input that is transferred usefully
• Explain what happens to the wasted energy, using conservation and dissipation
• Explain why no machine can be more than 100 per cent efficient
• Give one specific unwanted transfer and a specific way to reduce it
• Say how cutting that loss changes the useful fraction