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Electrical Units and Pedantry

Discussion in 'Off Topic' started by scaesare, Jan 15, 2017.

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  1. scaesare

    scaesare Well-Known Member

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    The intent of this post is to be informative, rather than come of like a jerk. Not to say I'm not woefully aware I am capable of the latter...

    The issue of not getting units for power (kW) vs energy (kWh) correct has already been covered on the forums many a time. It's an easy mistake to make (or at least typo).

    My topic here is regarding electrical potential (aka "voltage") and it's unit of "volts", and the flow of electrical charge (aka "current" or "amperage") and it's unit the "ampere" ("amp" for short).

    One of the better analogies (remembering that none are perfect) is that of water. Voltage is analogous to pressure (PSI) which is a measurement of potential. Current is analogous to gallons-per-minute, which is a measure measurement of flow

    It seems many posts on the forum use these terms at least awkwardly, and often incorrectly.

    For example, "How many amps are in the pack?". As amps are a measurement of flow, that's like asking "How many gallons per minute are in the faucet?" The much better question is "What is the peak current the pack can produce?" which is akin to "How much flow can that faucet provide?"

    Similar, "Raising the volts will charge faster." is, awkward, "Increasing the voltage" makes you sound smooth like butter.

    Finally, when it comes to current, it is drawn, not pushed. So a motor can "draw" 800 amps. The pack doesn't "push" it.... but it must be capable of providing it.

    OK, where are the steps off this soapbox?...
     
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  2. BerTX

    BerTX Active Member

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    Thanks! Going now to hook my car up to the faucet. :eek:
     
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  3. cpa

    cpa Active Member

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    (1) You did not come off like a jerk.
    (2) Your post is informative.

    That said, the body of knowledge out there in the world that covers all matters is staggering. People who are skilled in these different areas know the jargon. They know the scientific principles or the law or the process. They understand the subtleties and nuances because they work with them every day. Then there are the hobbyists and wannabees who think they know stuff but do not have an acute knowledge. Accordingly, sometimes their answers or statements are inaccurate or just wrong.

    I have dealt with clients and the public in my profession for forty years. The level of ignorance is disappointing but not unexpected. I have had scores of people over the years claim that their (coworker, hair dresser, cousin's next door neighbor--whatever) told them all about income taxation. I could show them the law, regulations, court cases ad nauseum but they would refuse to believe me. Fine. I quit worrying about such things a long time ago.

    Finally, science is quite precise. The product of volts times amps always, always, always is watts. It was that way yesterday, it is that way today, and it will be that way tomorrow. Everything else in life is quite imprecise, unpredictable, inconsistent, random and a whole bunch more. And most of us lead our lives in such imprecise fashion. I think this spills over into conversation, internet fora and elsewhere.

    (3) I hope I came off like you: informative but not like a jerk.


    :)
     
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  4. scaesare

    scaesare Well-Known Member

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    Agreed, these are terms many have rarely needed to be more than passingly familiar with...
     
  5. lklundin

    lklundin Member

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    #5 lklundin, Jan 15, 2017
    Last edited: Jan 15, 2017
    Interesting post.

    Let me add something that seems to apply especially to users of the English language.

    In addition to understanding what the different physical quantities are (e.g. energy versus power), it is also important to distinguish between a physical quantity (e.g. power) and its unit of measure (e.g. Watt).

    When someone writes "voltage times amperage equals wattage", they are merely performing a dimensional analysis, i.e. what is the physical unit (e.g. Watt) of a given product of other units (e.g. Volt times Ampere).

    This is in general _not_ the same as using a physical formula on a problem, although it does work out in many cases, including the above example.

    Consider for example an energy. It can be for example the work E done by a (constant) force F over a given distance D, E = F times D, for example to accelerate a resting vehicle with mass M up to a velocity V. If we look at the kinetic energy E that the vehicle obtained by this work, then this is given by E = 1/2 times M times V squared.

    In this case the dimensional analysis tells us that to get the unit of energy, we must square the velocity and multiply with the mass. But this does not yield the kinetic energy, because that requires us to also multiply by a half (which comes from the physics behind the problem).

    In the same example, the time T taken to cover the distance D is given by D = 1/2 times A times T squared, where A is the acceleration given by F = M times A. Again one cannot simply say (for example) that the footage is the seconds squared times the foot-per-second-squared, except to derive the resulting physical unit.

    So it is useful to distinguish between a physical unit (for example energy) and its unit (for example joule).

    This seems to be especially an issue in the English language, where a range of nouns conflate the physical quantity and its unit:
    footage (as opposed to distance)
    mileage (as opposed to distance)
    acreage (as opposed to area)
    tonnage (as opposed to mass or weight)
    amperage (as opposed to current)
    wattage (as opposed to power)
    voltage (in English I only know the cumbersome 'electrical potential' as a non-conflating term)
    (There are surely others).

    Combined with the terrible, archaic system of non-SI units still widely used in the USA this makes physics unneccessarily difficult for people there. If for example the above mentioned kinetic energy instead describes a rifle bullet (found in any truly American home), the formula does not change a dot outside the USA, it is still E = 1/2 times M times V squared. However, in the USA, conversion factors for the various units (e.g. grain and foot-pound-force) must be introduced into the formula, muddling the physics.

    I would be interested if anyone has examples of other languages with this particularity.
     
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  6. hacer

    hacer Member

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    Voltage is an electromotive FORCE (EMF). The battery potential does in fact "push" on the electrons in the valence band and they move as a result. There is also a reaction force pushing back on them due to the resistance they encounter. If the resistance is very large, then the electrons will "pile up" which also creates a reaction force (this is charging the capacitance of the open circuit). So the battery does indeed "push" the current, but it can only push so hard. In your water analogy, the pump (or static head or whatever the source of pressure is) is "pushing" the water against the valve, even if the valve is shut because the valve is "pushing back".

    I wouldn't bother with this but since you were on a soap box being pedantic...
     
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  7. nwdiver

    nwdiver Active Member

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    This is somewhat of a pet peeve of mine as well... IMO those of use that DO understand the difference between kW, KWh, volts and amps need to be especially careful when we use them. It annoys the crap out of me when someone who should know better uses kW as shorthand for kWh. YOU CAN'T DO THAT! I agree that in context it's usually obvious what you mean (for someone that understands the difference) but it can become confusing for someone that doesn't. Most people oddly don't use these units very often despite paying an electric bill every month. I've had engineers at work ask me how a 10kW solar array can be useful when they use 1500kWh per month (.....seriously.....)

    Understanding volts, amps and power is another issue. When I was helping the protest camp in ND set up some of their donated PV supplies it was depressing how incredibly under-utilized a lot of it was. I saw a guy kill a large trailer mounted 20kWh battery overnight with a 1.5kW space heater... he didn't understand the limits of the PV system. There were also several instances of matching a 40v panel with a 12v battery without the sophisticated MPPT charge controller required to take advantage of the extra power... effectively wasting >50% of the panels output.

    Improving knowledge starts with people that have the knowledge using the correct terms to reduce confusion. If someone asks about your cars battery size either leave the units off or spend the extra half-breathe to include the word 'hour'. Might not be a bad idea to start referring to a 10kW solar array as a 50kWh/day array... :confused:

    ....... meh...... none of this will matter in a week or so anyway........ :(
     
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  8. scaesare

    scaesare Well-Known Member

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    Both EMF and potential are ascribed as being the property defined by the volt.

    And while it's certainly true that the potential described by the voltage dictates the overall current flow in the circuit (as described by I=V/R), nonetheless the point I made earlier is generally held true in describing circuit behavior: a load draws a current flow. A motor will draw 200A, regardless of if the source can supply 200, or 2,000 amps.

    Now if you increase the voltage potential of the source, the load will draw commensurately more current. But once again, the current delivery capability of the source is not what dictates the flow (provided it can at a minimum supply the current the load is attempting to draw).

    This is another case where the water analogy is useful: At a given pressure (voltage), a 6" diameter pipe will only draw 10 gallons-per-minute (amps) from a reservoir, regardless of if that reservoir could supply 100 GPM. Now if you double the pressure, that same 6" pipe will flow more, but again, the pipe will only flow (draw) what it is capable of at a given pressure, regardless of the capacity of the source.
     
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  9. Jack007

    Jack007 Member

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    My head hurts,
    no matter how many times I could read this, I can't remember it.
    So I am going to focus on ONE point. KW v KWH
    If I master it i'll let you know :D
     
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  10. nwdiver

    nwdiver Active Member

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    It's a lower case 'k' and 'h'

    JK... I'm not that pedantic :p
     
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  11. jbcarioca

    jbcarioca Active Member

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    Some romance languages are pretty hopelessly confusing too:
    In Portuguese there are two common words describing electrical potential: 'tensão' for potential directly (IMHO a better analogy than pressure) and 'voltagem' for the designed 'tensão' for a given device. That works for me. Easier than the confusing English.
    However, there is zero capacity in modern colloquial Brazilian Portuguese to draw a one word distinction between 'motor' and 'engine'. Thus a BEV must undergo a pollution test at the exhaust because it has a 'motor'. Poor Leaf owners do have an exemption!
    It is quite difficult to explain to the uninitiated that my Tesla has no engine, partly because of this linguistic quirk.

    I note in this thread must of what we all seem to say is described as pedantic by people who have no idea how fundamental these issues are in modern life. I personally fault the primary education more tahn I do linguistic limitations.

    That said, the use of non-metric measurements in the modern world is one of my most frequent frustrations. Because I live in both a pretty deeply metric country and another stuck in the 17th century of measurement I find myself giving my height and weight in metric in the US where it mostly works because most of my MD team is Indian. Otherwise it's not so easy.

    To be fair to English, the linguistic complexity of the language makes some things easy to understand without deep grammatical study, others not so.
    The '-age' suffix, in most cases simply serves to convert a defined quantity to an undefined mass quantity, often with a precise quantification included in either a question or response, sometimes not:
    (e.g. How much acreage is in your estate? Your estate is 3,000 acres. he has lots of acreage, but I don't know exactly how much.)
    Pretty useful linguistically, actually. Sadly, most native English speakers haven't the knowledge to use the language properly.
    To wit: 'Can I talk to you?' when what one means, hopefully, is "may I talk with you?"

    So we are occupied here with a level of arithmental and physical notation that may well not be taught in primary schools in much of the world, regardless of language. arithmental processes may have themselves been largely supplanted by cheap calculators...

    Reflecting in part my age and educational background, I suppose, these issues are very important to me so I rant and rave about them ad nauseum to anyone who will listen, or pretend to listen.
     
  12. McRat

    McRat Active Member

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    The object of informal writing is to clearly communicate. "U OK Dude?" might not look good in the Lancet, but it works.

    When the writing is not clear, request clarification.

    Clear?
     
  13. deonb

    deonb Supporting Member

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    Speaking of which...

    One thing that bothers me is why does the unit Amp-Hours exist?

    Amperes are coulombs per second. So an amp-hour is a coulomb-per-second-hour. Which is a ridiculous unit of measurement.

    If you go into a gas station and try and buy a quarter gallon-per-minute hours worth of gas, you're going to get your car keyed.
     
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  14. scaesare

    scaesare Well-Known Member

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    Agreed... calcs in metric are much easier to deal with. Not only conversion factors, but consistent (and convenient) base.
     
  15. scaesare

    scaesare Well-Known Member

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    When dealing with units, which by their nature have a precise definition, incorrect "informal" usage can actually obscure meaning.

    For example: as kW is power, and kWh is energy, what is actually intended when somebody refers to a "100kW pack" is different than what they stated.
     
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  16. scaesare

    scaesare Well-Known Member

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    That bothers me as well. Actually, I don't like any of the units that specify a quantity by incorporating a unit of time. Amp-hours, kilowatt-hours... or even light-years. I much prefer a specific unit for that quantity.

    I wish we'd use megajoules for energy. The magnitude of the numbers are within reason for common usage, as 1 kWh = 3.6 MJ. Not only does a "360 megajoule battery pack" sound impressive, it's actually easier to say with 2 syllables less than what we use today.

    And not to mention a single megajoule is within shouting distance of what EV's today need to expend to drive 1 mile... which as at least somewhat convienent

    (Or we could go the "calorie" route, and drop the prefix in everyday usage... as what most people refer to as calories are of the "large" flavor, which actually is a kilocalorie... but as that introduces imprecision, I'd rather not).

    While it would take some time to get used to megajoule for sizing packs, as it's an unfamiliar term, I'd bet that to most of the population beginning to look at EV's, it no more foreign a concept than kWh are. Although they've seen them on their electric bill, the average person has no idea what that means for EV range until they get acclimated to the unit. So why not use GJ's instead?

    It's the same for ICE engine sizes. Who knew what a 3.2 liter engine equated to 25 years ago? I knew that my dad's new van with a 5.2L engine was the same as his old with the 318cid. The 5.9L was a 360. The 5.0 Mustang was a 302. My 7.3L diesel was the 444.

    Now it's familiar and I no longer need to convert & compare in my head to understand the quantity (although I always do for fun).

    I guess there is no Ah equivalent without units of time, is there? At lease there's Parsec instead of light years (but who uses those besides the folks in Star Wars?)

     
  17. deonb

    deonb Supporting Member

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    There is! It's just coulomb. 1 Ah is 3600 coulombs. A lithium ion cell holds about 10'000 coulombs of charge.
     
  18. deonb

    deonb Supporting Member

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    And the folks in Star Wars used it incorrectly...
     
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  19. scaesare

    scaesare Well-Known Member

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    Oh, duh, you said that in your earlier post... I inadvertently thought you were dismissing it as ridiculous when you were referring to the Ah...

    So are we promoting the "kC" as the preferred metric??
     
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  20. AudubonB

    AudubonB Mild-mannered Moderator Lord Vetinari*

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    ...but they were usually confused about Force, too...
     
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