• Question: If you can't create or destroy energy, how do you convert it?

    Asked by 773xmasen24 to Tom C, Thomas, Saiful, Piers, Laurence, Karl, Javier, Chris on 9 Jan 2017.
    • Photo: Piers Barnes

      Piers Barnes answered on 9 Jan 2017:


      Energy can be seen as a concept that allows us to quantify certain properties of matter that can be converted into other properties. Examples of types of energy that you might be familiar with are potential energy (gravitational, or in a spring, or electrical), kinetic energy in moving thing, chemical energy (which depends on the relative strength of the chemical bonds in a material), radiant energy in light, and thermal energy (the energy stored in random vibrations in a material that gives it its temperature above absolute zero). It’s probably easiest to think about the general conversion between types of energy using an example.

      You could convert kinetic energy to heat by rubbing something. For example a piece of wood sliding against another piece of wood would transfer the kinetic energy of the collective movement of the atoms in the wood into heat energy (this would increase the temperature of the wood – note that temperature is not the same as energy, but is closely related to heat). The collective motion of the atoms in the two pieces wood has been transferred to random vibrational motions of the atoms in the wood. In in this example not all the kinetic energy of the wood is transferred into heat, a fraction of this energy will also be used to break chemical bonds in the wood resulting in ‘saw-dust’. Generally speaking it requires some external energy to break chemical bonds, this could come from the energy in the vibrations (on a small scale this is kinetic and potential) or from a photon (light). When a bond is formed we can think of energy being, released often as heat or light, in some contexts this a known as a ‘binding energy’.

      If the rubbing were to continue, enough heat energy might be transferred to the wood to raise the temperature sufficiently for combustion to start. If this were to happen we then start a chain reaction where the overall heat released by breaking the chemical bonds in the wood and reforming new bonds with oxygen (which releases more energy than was required to break the initial bonds) is sufficient to break more bonds and start burning the wood. The chemical energy in the wood is then converted to heat energy, and light in the form of photons. Some of these photons might then be absorbed by a nearby leaf which would promote an electron into an excited state where it could then form a chemical bond ultimately resulting in the growth of new wood. The efficiency of converting chemical energy (wood) to light back to chemical energy (wood) is very low because so much of the initial chemical energy is converted into other things like heat and light that is not absorbed.

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