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Profusion of Units of Energy

  1. Jul 29, 2014 #1

    Why do we need 12 Units of Energy?

    When the SI Unit of Energy Joule should be the universal standard.

    ...............It makes absolutely No Sense

    Scale is no excuse - there are 20 SI prefixes from 10-24 to 1024

    1 electronvolt = 0.1602176565 aJ

    the ground state energy of H = 2.179872172216155945 aJ

    so, what is the problem with expressing energy strictly in Joule
    on every level - from atomic physics to cheese biscuits (?)

    Joule.............................. J
    electronvolt................... eV = 1.602176565 × 10−19 J
    Hartree................. ....... Eh = 4.35974434 × 10−18 J
    Planck energy................ Ep = 1.956 × 109 J
    Rydberg........................ Ry = 13.60569253 eV = 2.179872172216155945 x 10-18 J
    kilowatt-hour.............. kWh = 3.6 x 106 J
    British Thermal Unit.......Btu = 1055 - 1060 J
    depending on the day of the week
    therm......................... thm = 1.05506000 x 108 J
    calorie.......................... cal = 4.184 - 4.204 J
    depending on the day of the week
    reciprocal centimetre..cm-1 = 11.96266 J
    TNT equivalent (tonne)..... t = 4.184 × 109 J
    horsepower-hour..........hph = 2.6477955 x 106 J

    Last edited: Jul 29, 2014
  2. jcsd
  3. Jul 29, 2014 #2
    I think it would be easier if everyone used the same system of units. In my opinion the SI system is best, just one reason being it is the one I'm most familiar with. That being said we use the set of units which is most useful for the task being worked on. For some branches of physics eg atomic physics it is usually easier to use the eV . When looking at energy bills the kWh is a suitable unit.
    If we used the same units for every problem there would be certain problems where it takes longer to do the calculations. What is important is to know how to convert between units.
  4. Jul 29, 2014 #3
    I cannot think of any problem or situation which could take longer to calculate in Joule than any other unit.

    If Joule were the only unit in existance - no one would even give it a second thought.

    all equations involving energy should be correlated in Joule, especially at the atomic level
    there is just no excuse.

    How is 2.179872 more difficult than 13.60569253 ?

    Electric energy bills in kWh are not in any way more practical than Joule
    1 kWs = 1 Joule where is the difficulty?

    The Watt is not even a unit of energy, it's a unit of work.

    I personally refuse to use any other unit of energy
    Last edited: Jul 29, 2014
  5. Jul 29, 2014 #4
    There are numerous examples where the joule is not the most convenient unit. Here's just one simple example

    How much energy is used when 1kw fire is used for 1hour.
    Answer=1 times 1=1kWh

    In joules the energy used is 1000 times 3600=3 600 000 Joules

    I think most bill payers would prefer to see their energy consumption expressed in kWh
  6. Jul 29, 2014 #5

    1 kWh = 3.6 kJ

    why hours? why not seconds, or kiloseconds?

    the SI unit of time is the second, not the hour.

    Speed should be measured in m/s not km/h

    having to convert to rediculous units like hours or minutes is a waste of calculating time,
    if everything were calculated in SI units, calculating would be amazingly simple and FAST
    since all SI units are correlated.

    Lavoisier conceived the metric system because France was a chaos of unrelated units before the revolution.
    The same chaos of units still exists today, Lavoisier must be rolling in his grave.
    Last edited: Jul 29, 2014
  7. Jul 29, 2014 #6


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    Because historically and in typical usage people need to be able to make quick, easy, simple calculations that can be done in their head or on their fingers, or compared intuitively, while busy with other tasks such as operating machinery, without the aid of wikipedia and a calculator.
  8. Jul 29, 2014 #7

    That's exactly my point... nothing is easier, simpler and more intuitive than metric units,
    in Europe these units are all we have ever known, and in most parts of the world.

    It's just to forget the ancient historical units and use metric units and prefixes
    which have been in constant use for the past 219 years.

    Unfortunately the second is a relic of the sexagesimal horology system of hours, minutes and seconds,

    the first metric unit of time was the day, divided into centidays, and millidays,
    but the new decimal clocks were confusing to ordinary people in the 18th century
    as well as Napoleon - so decimal time based on the day was abandoned.

    Still, it's not so difficult to remember that a day is 86.4 ks


    Last edited: Jul 29, 2014
  9. Jul 29, 2014 #8


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    for the avg non-science person ... yes it is !

    You are not really being realistic

    QP and Dadface both gave good examples of why for the layman ( general public in particular) or even a particular field of science, its easier to work with when done the way it is

    the ol' KISS rule really applies in these cases
    ( Keep It Simple Stupid)

  10. Jul 29, 2014 #9


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    You want to go 400km from A to B in 5 hours. How fast (on average) do you have to drive in m/s?
    That is way easier to calculate in km/h.

    Your electron gun has an accelerating voltage of 30 kV. How much energy do the electrons have afterwards?
    Trivial in eV, but you need a calculator to express it in J.

    Units are chosen to be convenient where they are used. Comparing a nuclear explosion to conventional explosives (TNT equivalent) is easier to understand than some number of EJ.

    Also, dealing with various different exotic SI prefixes for literally every quantity is just annoying.

    No, 1 kWh = 3.6 MJ
    And you cannot expect the general public to understand the difference and conversion between kJ (something a flashlight needs), MJ (something an oven needs) and GJ (what would appear on the electricity bill) or even the scientific notation with powers of 10.

    If we could start a new unit system from scratch today, we could make more constants more convenient - define a coulomb as 1018 times the elementary charge for example, or make the speed of light exactly 300,000km/s. Or let an hour be 1000 seconds and a day 100 hours. But we can't.

    Out of those 12 units you mention, most are not used any more, or not in a context where a conversion would be interesting.
    Oh, and your value for the reciprocal centimetre is completely off.
  11. Jul 29, 2014 #10


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    If you want a windmill to tilt at, try the non-decimal units for time.

    Argue for 100 seconds per minute, 100 minutes per hour, 10 hours per day. Then the year. Oh God the year, how shall we define that? It it a choice that is not without consequences; remember Y2K.

    I'm sure that the result you'll find is that there is no political will to think logically when it comes to units.

    By the way, this is the physics forum. You should really be arguing for c=1, hbar=1, G=1. Those are the truly logical choice of units in physics. The units for time, energy and everything else can be derived from those; the Plank length, Plank time, Plank energy, and so on.
  12. Jul 30, 2014 #11
    there should not be any "non-science persons"

    science should be a mandatory component of standard education

    explain to me why kicking a ball around for an hour a day at school is more important than physics?

    on the contrary, I'm being brutally realistic

    first of all.. forget km - just use meters
    divide 400000 m by 3600 s and multiply by 5 = 22.2 m/s Trivial

    1 Volt x 1 Coulomb = 1 Joule how difficult is that?

    you don't need to be Marcus du Sautoy to use a calculator.

    how are SI prefixes "exotic"? every 7 year old kid in Europe knows them.

    the good news is.. you don't need to use them if you don't like them, just use base units and scientific notation.

    haha.. that was a typo

    I expect the general public to pull their fingers out of their noses and learn science.

    who says we can't?
    We can do anything we choose.. we are autonomous free thinking individuals.. not mindless slaves.

    I can use any system of units I wish, I can construct my own units and prefixes, I don't need permission from the BIPM

    I'm fairly sure all of these are being used by 'someone', for 'some reason'.

    None of these arguments justify having more than 1 unit of energy

    it's completely illogical and only wastes time when multiple units have to be converted
    it's like having to exchange currency every time we travel to the UK from Europe
    it's amazingly annoying and time and money is wasted in the process.

    according to this source :


    1 cm-1 = 1.196266 x 10-2 kJ mol cm.

    1.196266 x 10-2 kJ = 11.96266 J

    I absolutely agree with you there

    a submultiple of the day should be the base unit of time

    the French abandoned the idea 2 centuries ago.. but it can always be adopted if enough people use it.

    Also the Coulomb should be a base unit - not the Ampere, the Ampere is a derived unit!

    Last edited: Jul 30, 2014
  13. Jul 30, 2014 #12


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    I think kinogram is managing to wind you all up. And it's not even April the First.

    (But he is right about the need to obliterate all but SI units.)
    Last edited: Jul 30, 2014
  14. Jul 30, 2014 #13


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    Seriously? 400/5 I can do in my head. What you just did I need a calculator for. Are you really capable of doing that in your head? More to the point: are you really incapable of seeing that what you suggest is more complicated? I'm having a hard time believing you are serious. Hmmm...

    And not your first! So you are arguing against your point by making these mistakes!

    1. 1 kWs = 1 kJ.
    2. The Watt is a unit of power.

    You're actually arguing against three things here, not one:
    1. You are against the English/Imperial system.
    2. You are against the idea of derived units.
    3. You are against the use of prefixes.

    The first is fine. The other two are almost comically nonsensical given that you keep messing them up when trying to prove your point.
  15. Jul 30, 2014 #14
    I can easily calculate this in my head - is that strange?

    I'm an organic being, I do make typos

    I often post without proofreading - which is asking for problems

    Fortunately I'm the only one here who has ever made a typo

    The Watt is a unit of Work-done which is the same as power

    now you're getting into semantics

    I never said I was against any of these things
    I have not even mentioned the imperial system

    mfb said he had a problem with prefixes, so I suggested using scientific notation and base units,
    you should read my posts more carefully, you've misinterpretaed almost everything.

  16. Jul 30, 2014 #15


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    That is not correct. Work-done is Energy and not Power. That is much worse than a mere 'typo'.

    Apart from having a general rant about something that is not clear, what was the purpose of this thread?
  17. Jul 30, 2014 #16


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  18. Jul 30, 2014 #17


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    I am going to just cherrypick one of your more blatant mistakes:

    This does not answer the original question (energy of an electron after being accelerated through a 30 kV potential (or 1 V for that matter)), it simply defines 1 J.

    As a particle physicist I typically work in units of eV with different prefixes and in a system where hbar = c = 1 simply because this is the most convenient system of units available for my applications. For other applications, other units are going to be more appropriate. Working with an arbitrary system of units is not difficult as long as you do not have several units for the same thing (i.e., feet, yards, miles) which are not related through powers of 10. Conversions between different unit systems are just as trivial as using prefixes and scientific notation.

    In addition, defining the basic unit of time as a fraction of the length of a day would be, forgive the bluntness, idiotic for several reasons - the most obvious of these being that the definition would not be constant.
  19. Jul 30, 2014 #18


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    The OP was a sockpuppet of a previously banned member and as such has now been banned. Not sure if this was a pure troll or another bad misunderstanding, but either way it is over now. Thread locked.
  20. Jul 30, 2014 #19


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    Sorry russ, just to clear up a misunderstanding:
    I don't have a problem with prefixes, I like them and I use them every day, but I prefer milli, kilo, Mega and Giga over Yocto and Zepto, and there are not even prefixes to express particle-physics cross-sections as multiple of square meters (sometimes <10-40).
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