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Asked by anon-253669 to Adam on 18 May 2020.
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Adam Baskerville answered on 18 May 2020:
Excellent question. Energy is unfortunately wasted all the time in the world around us. The computer you are typing at now is warm which is caused by the electricity warming up the various components due to electrical resistance. This is wasted energy! An incandescent light bulb wastes a huge amount of energy, and is only approximately 10% efficient. When a driver presses the break pedal on their car a lot of the kinetic energy is converted into heat energy as the break pads heat up which is wasted energy. Energy is often wasted due to unwanted heat generation or light being given off. This carries energy away from the system so it can no longer be used.
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Comments
Tom commented on :
The key thing is that energy can be converted from one form to another. Some forms of energy are more useful than others. If you have a weight that is up high, you can use its gravitational potential energy by tying a rope to it and letting it fall. The rope can pull something else uphill, or spin an electric generator.
If you drop the weight without a rope, its GPE is converted first to kinetic energy (speed) and then to noise and motion of the air. It’s quite hard to capture noise energy and use it to pull an object uphill, so some usefulness has been lost.
In theory, one day all the “usefulness” will be lost, and the entire universe will be at a constant temperature. At this point there will be just as much total energy as now, but it will be impossible to do anything with it. This is called the heat death of the universe!
Susan commented on :
There’s a difference between the total energy of a system, and the energy that can usefully be extracted from it and used to do something, e.g. run an engine. For example, if I have 20 litres of water at 90 C, and another 20 litres of water at 10 C, I can use the temperature difference to run a heat engine and do work – for example, generate electricity (converting some of the heat energy of the hot water into electrical energy). But if I just pour the 20 litres of hot water into the 20 litres of cold water to get 40 litres of water at 50 C, the 40 litres of water has the same amount of energy as it did before, but now I cannot use that energy to do anything (unless I connect it to another bath of cold water). So, by pouring the hot water into the cold water, I have wasted useful energy, even though I still have the same total thermal energy that I had to start with.
Similarly, we consider energy to be “wasted” if what we do converts it into a form that isn’t the one we want. The aim of a standard car engine is to convert chemical energy in the petrol or diesel into kinetic energy of the car. The engine does do that, but it also gets hot (which is why your car needs a radiator) and makes noise. The chemical energy from the fuel that is converted into heat and sound is wasted energy from the point of view of the engine designer, because it is not being converted into kinetic energy of the car. When engine designers improve the fuel efficiency of engines, what they are doing is increasing the fraction of the chemical energy that is converted into kinetic energy of the car.
Unfortunately, there are limits to how efficient engines can be – for example, my engine running between two water baths at 90 C and 10 C can never convert more than 22% of the heat energy into useful work. (I could do better if my hot and cold reservoirs were mote different in temperature.) This is governed by the laws of thermodynamics, which were first explored in the 19th century precisely because (steam) engine designers were trying to make their engines more efficient.