And indeed the many different units devised for this physical quantity are baffling. Joules, ergs, hartrees, electron volts, wavenumbers, calories and the list goes on with each sub-discipline and approach seemingly requiring yet another one. What's the point of them all?
To see why, we need to consider why we have units in the first place and how we choose the standard.
Science involves a lot of measuring things. Usually when we measure something, what we want to find out is how much it undergoes some process in order to compare it with other objects in the same process. That way we can get an idea of the rules behind it all. Obviously, in comparing objects we need a standard.
Standards can be picked for a variety of reasons. Convenience is a good one - the familiar everyday units like inch, kilogram, minute and so on all refer to quantities we are likely to deal with every day. Measurability is an important aspect - think of how the Celcius temperature scale is defined . Tied to these is universality, choosing a basic unit that has some kind of physical analogue. The charge on the electron is the basic measure of charge and is the smallest charge you can get without a big particle accelerator (although the internationally agreed base unit for charge is actually the coulomb, much much larger for reasons of convenience).
Ultimately, we need different units for different purposes, so we all choose different ones. Frequently scientists choose units to make their equations simpler and get rid of loads of constants of proportionality. So if you pick your units such that as many constants as possible are 1, this is much easier. For example, we define volts (the unit of potential difference) in terms of amps (the unit of current) and ohms (the unit of resistance). 'One volt' is 'the potential difference required to push a current of one amp across a resistance of one ohm'.
Eventually, scientists got together and defined a set of base units called the SI (Le Systeme International). These are simple units from which everything else can be defined:
[click to make it big enough to read]A nice, organised system and like most nice organised systems people ignore it completely whenever is convenient.
It's easier in practice to use whatever comparative standard comes to hand. For particle physics, we use the electron volt, the energy an electron gains when placed in an electric field of one volt. For physical chemistry, the hartree which is the 'absolute value of the electric potential energy of the hydrogen atom in its ground state'. For spectroscopy, the wavenumber, the energy of a photon with a wavelength of one centimetre. Using such things avoids endless prefixes (all that pico-, femto-, micro-), excess constants (the hartree was invented specifically to incorporate a pile of irritating numbers into its value. Now you can make a nice clean calculation and convert back to joules at the end or, more usually, leave the answer in hartress in the confident knowledge that everyone knows what it means) and irrelevant definitions.
But they make learning physics a nightmare.
PS. If you're wondering why SI amps are defined in terms of newtons, an SI-derived unit, I have no idea. For some reason they wanted current to be the base quantity rather than charge. Given that current is a flow of charged particles while charge is a fundamental property of certain particles, I have concluded that the people defining these are being deliberately obtuse.
2 comments:
I vaguely recall that amps were defined in terms of newtons because of the magnetic force between parallel currents, which depends on the strength of the currents and the distance between them. Two currents of such-and-such a strength separated by thus-and-so a distance attract each other with a force of 1 newton, thus defining a unit of current.
Aye, such is my understanding. But it seems messy to me to define a unit like this when most people deal with amps being a function of charge rather than the other way round. Plus charge has a very obvious base unit as it it - the charge on the electron (or proton, if we want to keep our negatives the same way round).
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