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Efficiency and Power

Not all energy does something useful. When work is done on an object, usually some energy is used to overcome a friction or air resistance force. This energy becomes internal energy of the molecules of a material. This energy takes the form of a random movement of molecules for a gas or liquid, or randomly oriented vibrational energy for a solid. This energy cannot be recovered to do useful work, and is usually considered to be lost. A lot of the effort that goes into designing machines is in the direction of minimising this 'lost' energy. This can be done by reducing the force of friction using lubricants for example. A lot of research is directed to finding the best lubricants and making better lubricants. Not only would this mean that less energy is wasted: often wasted energy forces the the use of more energy to handle it. For example a car engine produces heat energy which is waste. The engine is often cooled by water pumped around the engine. A computer CPU must be constantly cooled by a dedicated fan or water unit to keep it from overheating. These units themselves consume power which subtract from the overall efficiency. More efficient engines or CPU might mean less heat produced, possibly eliminating the need for these cooling devices.

The efficiency of a machine or process is defined as one of:

Many machines are surprisingly inefficient. Only about 15% of the chemical energy of the fuel for a car is transferred into actual kinetic energy of the car on average. A typical power station is only about 30% efficient. The muscles in the human body are of the same order of efficiency – maybe 18 to 26%.

The useful power can depend on the context. Some machines are designed to be 0% inefficient in the sense that the useful mechanical energy produced is zereo. For example, brakes are DESIGNED to turn kinetic energy into heat energy. The efficiency of brakes according to any definition above is zero, since all the kinetic energy of the vehicle is changed into heat energy which cannot be recovered to do mechanical work but this does not mean the effectiveness of the brakes is low or can be improved. A heating element is designed to turn electrical energy into heat energy, which in many electrical devices would be considered a form of waste energy.