Energy Efficiency and Transfers

Following the energy

Push a toy car and it rolls, then slows and stops. Switch on a lamp and it glows but also gets warm. Charge a phone and the charger heats up. In every one of these everyday events, energy is being transferred from one place or form to another — and some of it is ending up somewhere you did not intend. Understanding how to track energy, and how much of it does the job you actually want, is what energy transfers and efficiency are all about.

This is one of the most powerful ideas in physics because it applies to everything: machines, living bodies, power stations and stars. It builds directly on the idea of energy stores covered in the many forms of energy.

The unbreakable rule: conservation of energy

The single most important law here is the principle of conservation of energy:

Energy cannot be created or destroyed. It can only be transferred from one store to another, or changed from one form to another.

The total amount of energy in a closed system always stays exactly the same. When a ball rolls to a stop, its kinetic (movement) energy has not vanished — it has been transferred, mostly into heat in the ground, the air and the ball through friction. Track carefully enough and you always find every joule accounted for.

Energy is measured in joules (J). If you put in 1,000 J, you will always find 1,000 J afterwards, however it is spread out.

Energy stores and transfers

Energy can be held in different stores:

• Kinetic — energy of moving things.
• Gravitational potential — energy of raised objects.
• Chemical — energy in fuel, food and batteries.
• Elastic (strain) — energy in stretched or squashed springs.
• Thermal (internal) — energy of hot objects.
• Nuclear, electrostatic and magnetic stores too.

Energy moves between stores by a few transfer pathways: mechanically (a force doing work), electrically (a current), by heating, and by radiation (waves such as light and sound). A falling diver, for example, transfers gravitational store to kinetic store mechanically; a kettle transfers electrical energy to a thermal store by heating.

Useful and wasted energy

Here is the crucial twist. In every real transfer, not all the energy goes where you want it. Some is always dissipated — spread out into the surroundings, almost always as heat, and sometimes as sound.

We label the output energy two ways:

• Useful energy — the part that does the job you wanted (light from a lamp, movement from a motor).
• Wasted energy — the part diverted to unwanted forms (heat from friction, sound from vibration).

The wasted energy is not destroyed — conservation still holds — but it becomes low-grade: so thinly spread through the surroundings that we can no longer gather it up to do useful work. This is why energy "runs out" in a practical sense even though it is never truly lost.

Why nothing is ever 100% efficient

No real machine turns all of its input into useful output, because friction between moving parts, air resistance, electrical resistance in wires, and sound always siphon off some energy as heat. A perfectly efficient machine would need zero friction and zero resistance, which is impossible in the real world. So there is always some waste — the only question is how much.

Measuring efficiency

Efficiency tells you what fraction of the input energy becomes useful output:

Efficiency = useful energy output ÷ total energy input

Multiply by 100 to get a percentage. (You can use power in watts instead of energy in joules — the formula works the same way.) Efficiency is always between 0 and 1 (0% and 100%), and a real device is always below 100%.

Worked example 1 — an electric motor. A motor is supplied with 200 J of electrical energy and does 150 J of useful mechanical work lifting a load. Efficiency = 150 ÷ 200 = 0.75 = 75%. The missing 50 J has become heat and sound — wasted.

Worked example 2 — comparing bulbs. An old filament bulb supplied with 100 J of electrical energy produces only about 5 J of light (5% efficient); the other 95 J becomes heat. A modern LED supplied with the same 100 J produces around 40–50 J of light (40–50% efficient). The LED does the same lighting job for a fraction of the energy — which is exactly why filament bulbs have been phased out.

Worked example 3 — work backwards. A device is 80% efficient and must deliver 400 J of useful output. How much input does it need? input = useful ÷ efficiency = 400 ÷ 0.8 = 500 J. The extra 100 J is unavoidable waste.

Sankey diagrams: drawing the flow

A Sankey diagram is a picture of an energy transfer where the width of each arrow shows the amount of energy. The whole input enters as one wide arrow on the left; it then splits into a useful branch and one or more wasted branches. Because energy is conserved, the widths of all the output arrows always add up to the width of the input arrow.

For our 75% motor, the input arrow (200 J) splits into a useful arrow (150 J wide) and a wasted-heat arrow (50 J wide). The wider the useful branch compared with the wasted branches, the more efficient the device — you can judge efficiency at a glance.

Reducing wasted energy

Because waste costs money and resources, engineers work hard to cut it:

• Lubrication (oil, grease) reduces friction between moving parts, so less energy becomes heat.
• Streamlining reduces air resistance on cars and planes.
• Insulation (loft insulation, double glazing, vacuum flasks) slows unwanted heat transfer, keeping useful thermal energy where it is needed.
• Choosing efficient devices (LEDs over filament bulbs, modern motors) means more of the input does the wanted job.

Every one of these does the same thing: it shifts energy from the "wasted" branch of the Sankey diagram into the "useful" branch.

Try it yourself! 🧪

Measure the wasted heat from a small motor — low voltage only. Use only a 1.5–4.5 V battery; never mains electricity.

You need: a small DC hobby motor, a 1.5 V or 4.5 V battery, connecting wires, a short length of thread, a small light weight (such as a few paperclips), and your hand or a thermometer to sense warmth.

1. Tie the thread to the motor shaft and the weight to the other end so the motor can lift it.
2. Connect the motor to the battery and let it wind the weight up. The motor is transferring electrical energy into useful gravitational energy of the lifted weight — plus heat and sound.
3. After it has run for a minute, carefully feel the motor (or hold a thermometer to it). It is warmer than before. That warmth is wasted energy dissipated as heat.
4. Listen — the buzzing is wasted sound. Both prove that not all the electrical input became useful lifting energy, so the motor is well below 100% efficient.

⚠️ Safety: Keep to small batteries (1.5–4.5 V). A motor run hard can get warm — disconnect if it gets hot. Never connect homemade circuits to wall sockets or chargers.

You have now seen conservation of energy and inefficiency together: the electrical energy did not vanish, it simply spread out into lifting, heat and sound — and only part of it was useful.