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What is energy?

I've been asked many times what energy is (in a physics context).  It's actually very hard to define energy because it is an intangible concept.  However this got me thinking and I've written the short article below which hopefully sheds some light on the question! 


Whenever an object is pushed or pulled by a force over a distance then work is done on the object and energy is transferred.  This is embodied in the equation Work done = force x distance.  We also have the law of conservation of energy which tells us that the total energy of a system remains constant although energy can be converted from one form to another.  It is this fact that energy is conserved which makes it an extremely useful concept in physics.


One of the difficulties with energy is that it exists in lots of different forms (such as kinetic, chemical and electrical etc) and can be very difficult to keep track of let alone measure. If you carefully count out 1000 grains of rice (uncooked) and then throw them into the air in your back garden it would be very difficult to then find all 1000 grains. Perhaps after many hours of searching you only find 960 grains. Now, you wouldn't conclude that 40 grains have simply vanished but instead you would consider alternative explanations; some may have fallen down cracks in your patio, others may have been eaten and carried off by birds. But you would still have confidence that the 1000 grains must be somewhere, they can't just disappear. 


In this circumstance then, the number of grains of rice is a conserved quantity; there's the same number after some event as before the event; the number doesn't change. Measuring the number of grains is easy; you can see and touch each grain of rice, and you can count them up, so measuring is easy.


With energy you cannot measure it directly; you must take careful measurements of other quantities (like temperature, speed & distance) then combine those with other data and finally use some equations to work out the energy there.


As with the rice, if it appears that energy is not being conserved in a particular scenario then you wouldn't immediately announce to the world that the conservation of energy principle has failed. It is true that a law of physics can turn out to be wrong, but the conservation of energy law is so well established that it is extremely unlikely to be wrong. So instead you would look for alternative explanations; perhaps some energy is in a form you haven't considered, perhaps your measurements are flawed, or perhaps you have discovered some new phenomenon of nature. This latter point is very important; once a law has been established it can help to find other new laws or phenomena (an example is the discovery of the neutrino from studying beta decay).


When I was at school I remember that I wasn't totally convinced about the conservation of energy.  The reason for this was that I could think of various circumstances where energy did not seem to be conserved.  For example, if you drop a book onto the floor it definitely has kinetic energy just before it hits the floor, but a moment later it is just sitting there on the carpet and there is no energy - where has the energy gone?  


The answer to this comes from a closer analysis of what actually happens at the point of impact.  As the book hits the floor it produces a pressure wave in the surrounding air (and the ground underneath) which emanates outwards and conveys some energy.  Any movement of the book against the surface of the carpet will generate some thermal energy by friction. The impact also causes the carpet fibres to compress and/or bend resulting in some stored (elastic potential) energy and further heat. There may be other mechanisms of energy conversion which you can think of but we probably have enough to show that energy could be conserved.


If energy is conserved why do we need to worry about our supplies of energy here on Earth? Most of the energy we use eventually ends up as thermal energy - it is 'lost' to the atmosphere. Of course it is not really lost, the energy is actually there but you cannot use it!  In order to derive useful energy from heat you need a temperature difference. You can create a temperature difference but you need to put energy in to do that (that's why you have to plug your fridge into the mains!).


As recently as one hundred years ago inventors were still coming up with clever designs for perpetual motion machines (ie devices which could continuously do work without requiring an energy source). Although physicists had a sufficient understanding of energy to know that they could not work, the challenge was to explain why they couldn't work. Some of the explanations are really rather subtle; the thought experiment known as "Maxwell's demon" is a fascinating example. As all the inventors of perpetual motion machines have discovered, you cannot get something for nothing, nature will find a way of defeating you!

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