Efficiency Engineer

Efficiency Engineer

No machine turns all of its energy into something useful. Some is always wasted, and an engineer's job is to measure that waste and then reduce it. Underneath everything here sits the law of conservation of energy: energy can be transferred between stores, but it cannot be created or destroyed. The total never changes. That has a consequence worth holding on to. Wasted energy has not gone anywhere. It is still in the universe, spread through the surroundings, and the only thing that has changed is how useful it is. Answers that say energy is "lost" or "used up" are describing something that never happens.

The missing 90 joules

A filament lamp is supplied with 100 J of energy and transfers 10 J of it usefully as light. Where is the other 90 J?

• Transferred to the surroundings by heating: it still exists, it is just spread out and no longer useful
• Destroyed inside the lamp
• Stored inside the lamp, ready to be used later
• It was never really supplied: the meter over-reads

Useful and wasted

Every device splits its input into two streams. Which stream is which depends entirely on what the device is for, and that is a point worth pausing on: the heat from a kettle is useful, and the heat from a lamp is waste, and it is the same physics either way.

Useful, and wasted

Efficiency

Efficiency measures what fraction of the input actually ends up useful: efficiency = useful output energy ÷ total input energy The answer is a fraction, or a percentage if you multiply by 100. The same equation works with powers instead of energies, and gives the same number. It can never exceed 1, or 100%, and the reason is conservation rather than engineering skill: getting more useful energy out than you put in would mean creating energy. No amount of design improves on that, which is why "more than 100% efficient" is not an ambitious claim but an impossible one.

Work out the efficiency

A lamp is given 100 J of energy and transfers 30 J of it usefully as light. What is its efficiency, as a decimal?

As a percentage

A motor takes in 200 J and delivers 80 J usefully. What is its efficiency as a PERCENTAGE?

The 120% heater

An advertisement claims that a new heater is "120% efficient". What must be wrong with that claim?

• It would mean getting more energy out than was put in, which would be creating energy, and energy cannot be created
• Nothing, provided the heater is extremely well insulated
• Nothing, provided they measured power rather than energy
• They have probably rounded 100% up to 120%

Running it backwards

Exams ask this at least as often the other way round: you are given the efficiency and the input, and asked what came out, or what was wasted. A motor is 0.4 efficient and is supplied with 200 J. Rearranging the same equation, the useful output is 0.4 x 200 = 80 J. Then the wasted energy is whatever is left, because conservation says nothing vanished: 200 − 80 = 120 J. That subtraction is the step people miss. If a question asks for the energy wasted and you give the energy usefully transferred, the arithmetic was right and the answer is still wrong, so read the last line of the question twice.

How much is wasted?

A machine has an efficiency of 0.4 and is supplied with 200 J of energy. How much energy does it WASTE, in joules?

Engineering out the waste

Waste comes mainly from friction between moving parts and from unwanted heating, so the fixes target those two directly:

Diagnose the conveyor

You are the engineer for a factory conveyor. Its motor is supplied with 1000 J and delivers 600 J of useful movement. Work through it.

• Where has the other 400 J gone?
• You find the bearings running hot to the touch. Which fix targets that most directly?
• After lubrication the motor delivers 700 J of useful movement from the same 1000 J. What has happened to the efficiency?
• A colleague suggests wrapping the motor housing in insulation so the waste heat "stays in the system and is not lost". What is wrong with that?

Order the experiment

Put the steps for testing which material is the best insulator, meaning the one that keeps water hot longest, into order.

• Fill identical beakers with the same volume of hot water at the same starting temperature
• Wrap each beaker in a different insulating material, leaving one bare as a control
• Measure the temperature of each beaker after the same length of time
• The water that cooled the least was the best insulated

The efficiency rules

Energy is never created or destroyed: it is _____, so wasted energy has not gone anywhere, it has simply been _____ to the surroundings and become less useful. Efficiency = useful output energy ÷ _____ input energy, and it can never exceed _____%, because exceeding it would mean creating energy.