Efficiency: Where Does the Energy Go?

Hook: rub your hands

Rub your hands together quickly for ten seconds. They get warm. The work your arm muscles did did not disappear. It turned into thermal energy because of friction.

The same thing happens inside every machine: in a bicycle chain, in a car's wheel bearings, in a pulley's axle. Some of the energy you put in becomes warmth and sound instead of useful work. Engineers call this energy dissipated.

Energy is never lost, but it can be wasted

The law of conservation of energy says energy cannot be created or destroyed. So where does the "lost" energy go? It dissipates: it spreads out into forms that do not help with the job.

• Friction between moving parts turns energy into thermal energy.
• Sound: squeaks, rattles and engine noise carry energy away.
• Vibration: shaking parts use up energy.
• Air resistance (drag) pushes against moving objects.
• Electrical resistance warms wires and motors.

Because of this, the useful work output of a machine is always less than its work input.

Calculating efficiency

Efficiency = useful work output ÷ work input × 100%

• Work input = input force × input distance
• Useful work output = output force × output distance

Efficiency is always less than 100% for a real machine. A higher percentage means less energy is wasted.

Note: in Grade 7 you used "efficiency of a structure" to mean how much load a structure holds compared with its own mass. In this unit, efficiency means how much input energy becomes useful output.

Innovations that make mechanical systems more efficient

Engineers fight energy dissipation in many ways:

• Lubricants (oil and grease) let parts slide smoothly over each other. Engines, bike chains and door hinges all need them.
• Ball bearings and roller bearings let wheels and axles roll instead of rubbing. Rolling friction is much smaller than sliding friction. Skateboards, bicycles, fans and cars all use bearings.
• Streamlined shapes reduce air resistance. Think of a racing cyclist's helmet or the smooth nose of a high-speed train.
• Low rolling-resistance tires and keeping tires properly inflated reduce energy wasted as tires squish.
• Regenerative braking in electric and hybrid vehicles uses the motor as a generator when slowing down, turning some kinetic energy back into electrical energy stored in the battery instead of all of it becoming thermal energy in the brakes.
• Lighter, stronger materials (like aluminum and carbon fibre) mean less energy is needed to move the machine itself.
• Heat recovery: some systems capture waste thermal energy and reuse it, like the milk plant in the food processing lesson.
• Maintenance: clean, tight, well-adjusted machines waste less energy.