How to tell a kilowatt hour from a kelvin week henry

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How to tell a kilowatt hour from a kelvin week henry

Post by weber » Thu, 29 Aug 2019, 18:42

... or a nanometre from a nautical mile.

This is about correct spelling of the symbols for units of measurement. I expect some of you will think this is mere pedantry, but please bear with me, particularly if you'd like your writing to appear more professional, or if you simply want to be respectful to your readers. I'll keep it simple.

Why do I say "respectful to your readers"? Well, for tHoSe oF uS wHo aLrEaDy kNoW tHe cOrReCt sPeLlInG revolution seconds, reading the wrong spelling is kinda like reading that sentence. It's painful.

The title of this topic comes from one of the most commonly misspelled unit symbols on this forum — the symbol for kilowatt hours. I'm not going to deal with the problem of people using kilowatts (instantaneous power) when they really mean kilowatt hours (accumulated energy). That's a battle for another day. I'll stick to the spelling problem for now.

Why do I call them "symbols", and not "abbreviations"? Because an abbreviation is any old shortening of a word. In the case of measurement units, we can't afford to use just any old shortening. There are too many of them and they would be too easily confused. So a specific symbol has been standardised for every unit and every multiplier, like the symbols for the chemical elements.

Using the forum search, I found the following 27 different spellings. Almost all of them are symbols for some metric unit. But only one of them is the symbol for kilowatt hours. I'll give you some simple rules (and the reasons for them) so you can figure out which one is correct.

kwh    kwH    kWh    kWH    Kwh    KwH    KWh    KWH
kwhr          kWhr   kWHr   Kwhr   KwHr
kwhs          kWhs                        KWhs   KWHs
kwh's                                     KWh's
kwhrs         kWhrs  kWHrs  Kwhrs         KWhrs  KWHrs
kwhours       kWhours

Rule 0. Use the standard symbols, or spell all the words in full. Don't make up your own abbreviations and don't mix words and symbols.

In most cases the symbol consists of a single letter for each sub-unit and a single letter for any multiplier. Because there are so many units and multipliers wanting letters, upper and lower case letters have different meanings.

Rule 1. A unit symbol is uppercase if and only if the unit is named after a person.

Uppercase "K" is the symbol for the kelvin, a unit of temperature named after one Lord Kelvin — sometimes called the "absolute" temperature scale. So we don't want an uppercase "K" here.

The watt is named after James Watt of steam engine fame, so it gets a capital "W". A lowercase "w" could mean "week" (although "wk" is preferred).

The hour isn't named after anyone, so it gets a lowercase "h". An uppercase "H" is the symbol for the henry, a unit of magnetic inductance named after one Joseph Henry.

And because the hour is a very common unit, it gets to have a single character. So "hr" is not a symbol for hour. It is a compound symbol for "hour revolution", because "r" is the symbol for "revolution" as in r/min = revolutions per minute.

If you are wondering why bottles have uppercase "L" for litre, I recommend you do an internet search on "Claude Litre". :)

Rule 2. A multiplier symbol is uppercase if and only if the multiplier is a whole number (not a fraction), with a nasty exception — lowercase "k" for kilo.


In general, it's big letter= big quantity, but because we already have uppercase "K" for the unit named after Lord Kelvin, the multiplier kilo (= 1000) gets a lowercase "k".

Rule 3. Don't add "s" to a symbol to make it plural.

The same symbol is used for both plural and singular. You must not add "s", because "s" is the symbol for seconds. Apostrophe "s", as in 2 m's, is also unnecessary. "2 m" is simply read as "two metres".

Now you have all the rules you need to find the one correct symbol for kilowatt hours above, and you can work out what the other spellings are really saying, and understand the title of this topic. And when you consider the misspellings that end in "hrs", you should also understand why I threw the seemingly-random words "revolution seconds" into my hard-to-read sentence example near the start of this article.

And by the way, we never use "p" for "per" in the metric system. That way we can tell the difference between miles per hour "mph" and metres per hour "m/h". So, kilometres per hour is km/h, not kmph and not kph. Those would be kilometre picohours and kilopoint hours.

Rules for the full names
I note that the above rules are only for the symbols, not the full unit names. When you spell out a unit name in full, you make a plural in the normal English manner, i.e. by adding an "s", provided the word doesn't already end in "s" or "x" — and henry becomes henries. And, except at the start of a sentence, you do not capitalise any multiplier when spelled in full, and you do not capitalise any unit name when spelled in full, even if it is named after a person. A lowercase "watt" is a rate of energy conversion. A capitalised "Watt" is a person — a member of Mr and Mrs Watt's family.

So that you can see just how bad it can be, when people use the wrong case for unit symbols, I offer the following "A to Ω of unit symbols".

If you're interested in some of the finer points of unit presentation, you might enjoy this sequel.

A to Ω of unit symbols
------------------------------
a atto (10⁻¹⁸), are (100 m², as in hectare), year (annum)
A ampere (alternative full name "amp" unofficial but very common)

b bit, barn
B byte, bel

c centi (10⁻², deprecated)
C coulomb
°C degree Celsius

cu cubic (not metric, use superscript ³ or "^3" for metric symbols)

d deci (10⁻¹, deprecated), day
D deca (10¹, when a single-character multiplier is required, deprecated)

da deca (10¹, deprecated)
Da dalton

db decibarn
dB decibel

e electron (in "eV" electronvolt)
E exa (10¹⁸)

f femto (10⁻¹⁵)
F farad

g gram
G giga (10⁹)

h hour, hecto (10², deprecated)
H henry

i binary (as in "Ki" for kili = 2¹⁰, Mi for megi = 2²⁰, Gi for gigi = 2³⁰, etc., used with bits and bytes)
I (not used, too easily confused with digit 1 or lowercase letter L)

j (not used)
J joule

k kilo (10³)
K kelvin

kg kilogram
Kg kelvin gram

kn knot (nautical mile per hour)
kN kilonewton

l (litre, but too easily confused with digit 1 or uppercase letter i)
L litre

m milli, metre, mile (in "mph", otherwise "mi", not metric)
M mega (10⁶), molar (deprecated)

mi mile (except in "mph", not metric)
min minute
mo month

n nano (10⁻⁹)
N newton

nm nanometre
NM nautical mile (also "nmi")

o octet (8 bits, when "byte" might be ambiguous)
O (not used, as too easily confused with digit 0)

p pico (10⁻¹²), per (not metric, use "/" for per in metric), point (not metric)
P peta (10¹⁵)

pa per annum (not metric)
Pa pascal

q (not used)
Q (not used)

r revolution
R ohm (alternative when Ω is not available)

s second
S siemens

sq square (not metric, use superscript ² or "^2" in metric symbols)

t tonne
T tera (10¹²), tesla

u micro (10⁻⁶, alternative when µ is not available), atomic mass unit
U rack unit (not metric)

v vreeble*
V volt

w week (but "wk" is preferred)
W watt
Wb weber

x (not used, as too easily confused with multiply sign)
X (not used)

y yocto (10⁻²⁴), year
Y yotta (10²⁴)

z zepto (10⁻²¹)
Z zetta (10²¹)

/ per
µ micro (10⁻⁶) (greek letter lowercase mu)
Ω ohm (greek letter uppercase omega)

The above is not intended to be a complete list of unit symbols — not even a complete list of metric unit symbols. But it does give all single character metric unit and multiplier symbols, and a few double character symbols and non-metric symbols where, if you change the case, you get a completely different but still valid meaning.

* Only joking. See Knuth D, 1957, Mad #33, halfway down the right hand side of page 2.
Page 1
Page 2

Acknowledgement
This series of articles was inspired by this wonderful post by Coulomb, in the My Nissan Leaf forum.

[Edit: Deleted as incorrect: "People have died because drug dosages were described using "mcg" as an abbreviation for micrograms instead of the correct µg." This is not correct. I misinterpreted the problem. The actual problem is with badly-hand-written µg, perhaps looking like ۳ᵍ or ʍg, being read as mg and resulting in 1000-fold overdose. So mcg is actually preferred to µg in handwriting. The recommendation is for doctors to write "micrograms" in full. This is not a problem in typed material.]
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Chuq » Fri, 30 Aug 2019, 06:08

I'm a complete nerd so I love this post. Especially Claude Litre :D You learn something new every day!

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by jonescg » Fri, 30 Aug 2019, 06:45

I stopped correcting Giles on RenewEconomy whenever I would read about a '100 MW battery'. It got to be tiresome.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by antiscab » Fri, 30 Aug 2019, 06:57

Was that 100 MW battery in relation to the hornsdale 100MW/129MWh unit?
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Fri, 30 Aug 2019, 07:01

jonescg wrote:
Fri, 30 Aug 2019, 06:45
I stopped correcting Giles on RenewEconomy whenever I would read about a '100 MW battery'. It got to be tiresome.
What's wrong with "100 MW battery"? It's spelled correctly, and batteries do have power limits as well as energy capacities. Particularly large battery modules that include management systems and inverter/chargers.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Fri, 30 Aug 2019, 07:12

Chuq wrote:
Fri, 30 Aug 2019, 06:08
I'm a complete nerd so I love this post. Especially Claude Litre :D You learn something new every day!
Thanks Chuq. :) And don't forget, next time someone writes that they have a "144 v battery", you can reply: So it only stores 144 vreebles — the amount of heat it takes to raise one blintz of halva by 144 degrees Smurdley? And then link to my article. ;)
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by jonescg » Fri, 30 Aug 2019, 08:31

In the days before the Hornsdale reserve was conceived, all the talk was about the battery power. But there was virtually no mention of the battery's capacity. It mean the inverters were rated to deliver 100 MW of power, but for how long? It could have been a marketing thing - 100 MW is a decent rate of energy delivery, but without any context for how long said power can be delivered for, it was somewhat meaningless.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by bladecar » Fri, 30 Aug 2019, 12:05

Yes, I noted during a time at teacher's college a comment that the science faculty students had the least amount of english skills. Probably a better word than "amount" don't care. Maybe :) if you go further, doctors are worse :):) but probably not. I have noted a certain lack of english ability here from the technical fraternity and mine is far from perfect. I have also noted, while irrelevant, a total push for language and maths skills for the workforce, which seems to have very little effect on one's ability to get a job.
Currently, I value this site for exchanging experiences in the new industry of electric vehicles. I'm hoping we can have an effect on how the new industry of electric vehicles occurs with the aim of guarding against the idea that it would naturally simply follow the previous industry of hydrocarbon vehicles with the built in costs, many of them unnecessary.

So I have brought up the subject of how we might best describe range, battery capacity etc so that we might most innately understand what we are talking about. KW per Km eg. If this is incorrect, it's just one of the bug bears of life. Iff you no wot i mean. If we repeatedly use certain terms to describe things that are therefore becoming easier to understand for most of us, it will be a form of education and will head in the right directionnn.

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Richo » Fri, 30 Aug 2019, 12:44

I regularly see Mm rather than mm - Umm that's a BIG difference people

OR the other I hate is Mega-milli candella.
Compounded prefixes.
:roll:
So the short answer is NO but the long answer is YES.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Richo » Fri, 30 Aug 2019, 12:52

jonescg wrote:
Fri, 30 Aug 2019, 08:31
It means the inverters were rated to deliver 100 MW of power, but for how long?
Correct.
In the good 'ol days the assumption was that it would be continuous-forever like all other power stations.
So the only consideration IS power not capacity.
Like some engineers they live in a box isolated from the rest of the real world.
Its hard changing their attitude so I understand why you'd give up.
So the short answer is NO but the long answer is YES.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by bladecar » Fri, 30 Aug 2019, 15:10

Richo said "I regularly see Mm rather than mm - Umm that's a BIG difference people"

Darn tootin PEOPLE.
What is that? A MILLION m's.
You could really go wrong there.

The best part about these representations is that you can usually see that there is an error.
When you have a go in order to get a go, you don't even know how it can go wrong.

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Chuq » Fri, 30 Aug 2019, 20:42

I sometimes talk about the odometer on my car saying things like "It's done 60k km" I guess that is 60 Mm ?

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by jonescg » Fri, 30 Aug 2019, 21:19

How good would a speedo reading m/s be? "Do you know how fast you were going sir?" "Well under 16!"
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by 4Springs » Sat, 31 Aug 2019, 06:01

Chuq wrote:
Fri, 30 Aug 2019, 20:42
I sometimes talk about the odometer on my car saying things like "It's done 60k km" I guess that is 60 Mm ?
Like this post back in 2011: viewtopic.php?p=30690#p30690
Now I look at that post, I note that I didn't put spaces between the numbers and the symbols. Started out with them, but then decided it looked funny, even if correct.
I think the main thing I've learned from weber's post is that you don't capitalise the unit names when written in full. I went through a phase of using 'Volts' and 'Amps", but then reverted to lowercase when I saw others doing that. Now I have had it explained I might remember it.

The car has now travelled 308 Mm.

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Sat, 31 Aug 2019, 08:36

Hi 4springs. I applaud your use of megametres (Mm) to give rounded odometer readings, and I encourage everyone to do so. I say "rounded" because I wouldn't want to see decimal fractional parts of megametres like 123.456 Mm. That would be better written as 123 456 km.

Part of the reason the entire Australian building and construction industry was able to painlessly convert to metric, almost overnight, is that they decided to use millimetres for everything. Centimetres were outlawed, and fractional metres never used. This meant there were no conversions required between metric units and everything was purely integer arithmetic. Even Joe the bricky could do it, with a thumbnail dipped in tar on the flap torn off a cardboard box.

Industries, and countries, that decided to use centimetres, like the Australian textile and clothing industry, and the US, are still struggling to convert. I wish Australian school teachers would first teach metres and millimetres, and only later teach centimetres, as being archaic units like inches. Kids rulers should show millimetres (with every tenth one numbered) not centimetres. The claim that millimetres are somehow too small for kids minds to grasp is utter nonsense.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Sat, 31 Aug 2019, 10:10

I deliberately didn't get into the finer points of presentation in the original post above, like the space between the number and the unit symbol. I wanted to keep it as simple as possible while addressing the things that actually alter the meaning, namely using the wrong case, adding "s" for plural, and using non-standard abbreviations.

I wouldn't want people to be so self-conscious about getting things wrong that they hesitate to post for fear of embarrassment. And I suggest that if you see someone getting it horribly wrong repeatedly, that you don't call attention to it publicly, but just quietly PM them a link to the original post above. And if they keep getting it wrong, just accept the adage about old dogs and new tricks and assume they are doing their best.

But for those of you who might write the occasional submission to government, or just want your writing to look as professional as possible, I offer the following.

To space or not to space
The reason it looks wrong when you put a space between the number and the unit symbol, and when you don't, is because the truth is half way in between. You're supposed to use a narrow space. And while we're at it, it should be a no-break space, so the number can never get separated from its units.

Standard keyboards and operating systems don't make it easy to type a narrow no-break space (Unicode 202F). But Coulomb and I swear by this brilliant free utility called WinCompose, by Sam Hocevar and team. It only works with Windows, but similar "Compose key" software is available for other operating systems. It's completely painless to install, and once you've installed it, you can right-click on the WinCompose icon in the task bar and choose Options→Composing→Edit, and paste the following line at the end, then close and save.

<Multi_key> <space> <space>	: " "	U202F	# NARROW NO-BREAK SPACE 

This will allow you to type a narrow no-break space as Right Alt space space . If you don't want to give up the right Alt key you can choose any other key as the "compose" key, but right Alt works well because it's right beside the spacebar.

The same narrow no-break space is used to separate groups of 3 digits in long numbers, as in 123 456 km. Commas are no longer used to break up long numbers, because in some countries commas are used as decimal points. By the way, when breaking up long numbers like this, no digit should ever be "lonely", so 4 digit numbers are written as 1234 km, not 1 234 km, unless you are lining them up in columns.

While you're at it, you might as well add the following, so you can easily type the degree sign, the superscripts for squared and cubed units, and several other units-related characters.

<Multi_key> <0> <0>		: "°"	U0080	# DEGREE SIGN
<Multi_key> <space> <0>		: " °"	U202F U0080 # NARROW NO-BREAK SPACE, DEGREE SIGN
<Multi_key> <1> <1>    		: "⁻¹"	U207B U00B9 # SUPERSCRIPT MINUS, SUPERSCRIPT ONE
<Multi_key> <2> <2>    		: "²"	U00B2	# SUPERSCRIPT TWO
<Multi_key> <3> <3>    		: "³"	U00B3	# SUPERSCRIPT THREE
<Multi_key> <@> <@>		: "√"	U221A	# SQUARE ROOT (@ is shifted 2)
<Multi_key> <p> <p>		: "π"	U03C0	# GREEK SMALL LETTER PI
<Multi_key> <u> <u>		: "µ"	U00B5	# MICRO SIGN
<Multi_key> <R> <R>		: "Ω"	U03A9	# GREEK CAPITAL LETTER OMEGA
<Multi_key> <.> <.>		: "·"	U00B7	# MIDDLE DOT

And yes, even the degree Celsius symbol should be narrow-spaced from its number, as in 25 °C. That's why there's a shortcut, Right Alt space 0 above, that gives you the narrow space along with the degree symbol. It saves having to type Right Alt space space  Right Alt 0 0 . And in case you're wondering, those are zeros, not ohs.

Celsius or celsius
While we're on the subject of degrees Celsius, you might think I'm breaking the rules by capitalising "Celsius" here, but I'm not. This is the standard spelling of the full unit name. It's a bit of a fine point, but in this case we really are referring to the person. The unit is the degree of the person Celsius. In the case of the related unit, the kelvin, the unit is just the kelvin, not the degree Kelvin.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Chuq » Sat, 31 Aug 2019, 10:56

I typically don't put spaces between the number and the unit, but then spell check keeps telling me that I'm wrong, so I go back and add them in :P

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Sat, 31 Aug 2019, 12:00

While researching the original article above, I spotted a symbol with the wrong case, in a table of unit symbols on the website of NIST, the premier organisation for units in the US. I emailed them about it, and received a very nice reply almost immediately, thanking me and telling me they had fixed it. My timing was good, as they kindly sent me a link to the very latest edition of the SI Brochure from the International Bureau of Weights and Measures (BIPM) — so new the ink was still wet. ;) The previous edition was published in 2006.

So if you want the last word, or more details, on the stuff I've been writing about above, start on page 29 (PDF page 45) of the following
https://nvlpubs.nist.gov/nistpubs/Speci ... 0-2019.pdf

However I note that this version has been adapted by NIST for the US, so you should ignore all the notes about the alternative spellings of some full unit names. We Aussies use the international spellings. We're not afraid of a few silent "e"s. :)
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Sat, 31 Aug 2019, 15:15

A correction:

I mistakenly wrote in the original article:
"People have died because drug dosages were described using "mcg" as an abbreviation for micrograms instead of the correct µg."

This is not correct. I misinterpreted the problem. The actual problem is with badly-hand-written µg, perhaps looking like ۳ᵍ or ʍg, being read as mg, and resulting in 1000-fold overdose. So mcg is actually preferred to µg in handwriting. The recommendation is for doctors to write "micrograms" in full. This is not a problem in typed material.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by galderdi » Fri, 20 Sep 2019, 07:28

My car gets forty rods to the hogshead, and that’s the way I likes it

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Fri, 20 Sep 2019, 07:57

galderdi wrote:
Fri, 20 Sep 2019, 07:28
My car gets forty rods to the hogshead, and that’s the way I likes it
What's that in furlongs per firkin?
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Sun, 22 Sep 2019, 15:21

Chuq wrote:
Fri, 30 Aug 2019, 06:08
I'm a complete nerd so I love this post. Especially Claude Litre :D You learn something new every day!
Here's one for us nerds, that I just learned today.

You know those prefixes for really humongous quantities, like tera, peta, exa, zetta, yotta, that are increasingly seen attached to "joules" or "bytes", for say worldwide quantities of energy or information. Well, if you're like me, you have trouble remembering their order of increasing size.

It turns out, if you can remember the prefixes used for polygons and polyhedra, namely tetra (4), penta (5), hexa (6), septa (7), octa (8), then you can figure out the order and meaning of these metric prefixes.

tetra → tera  T = 1000⁴
penta → peta  P = 1000⁵
hexa  → exa   E = 1000⁶
septa → zetta Z = 1000⁷
octa  → yotta Y = 1000⁸

The story is that "giga" came from the same Greek root as "gigantic", and "tera" came from the Greek for "monstrous". But then some bright spark noticed that "tera" sounded a bit like tetra, the Greek prefix for 4, and means 1000⁴. So a rule was born: Take the Greek prefix and delete a letter — preferably the first consonant of a blended pair. A few other liberties had to be taken to avoid reusing existing symbols like H and S, or using letters like O that might be confused with digits. And septa is a Latin prefix — the Greek is hepta. They also seem to have started working backwards through the alphabet.

I'll take a punt and say the next one will be:

ennea → xenna X = 1000⁹         [Edit: Bad idea. See below.]
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by Paul9 » Mon, 23 Sep 2019, 13:55

I can't be sure but I think, since converting to electric, my car has done 210 xenna nanometres?

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Re: How to tell a kilowatt hour from a kelvin week henry

Post by weber » Mon, 23 Sep 2019, 20:07

Hmm. I just learned 3 good reasons why the next larger multiplier should not begin with X.
1. When you pronounce it, it will sound like it should be spelled with a Z, and so it can be confused with Z for zetta.
2. When you write a symbol X, some people will assume it is the symbol for "exa".
3. A symbol X can be mistaken for a multiply sign. And its fractional counterpart lowercase-x even more so.
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Re: How to tell a kilowatt hour from a kelvin week henry

Post by coulomb » Mon, 23 Sep 2019, 20:37

Paul9 wrote:
Mon, 23 Sep 2019, 13:55
I can't be sure but I think, since converting to electric, my car has done 210 xenna nanometres?
Whatever the xenna will get called (I'll call it a stupenda for this post), it's big. I mean, you might thing it's a long way down the road to the chemist, but that's just peanuts to a stupenda nanometre.

1 stupenda = 1000⁹ = 10³·10⁹ = 10²⁷. A nanometre = 10⁻⁹ m = 10⁻¹² km. So 210 stupenda nanometres = 210 × 10²⁷ nm = 21 x 10²⁷·10⁻¹² km = 210 x 10¹⁵ km or 210 000 000 000 000 000 km.

I think I can be sure you haven't driven that far. Unless, as is sadly very likely, I've slipped up with my arithmetic.

One of the things that blew my mind on the ABC radio program Cosmic Vertigo is that once numbers get really really big (like 10 to the 10 to the 50), the units really don't matter any more. Micrometres versus megametres only makes a difference of 12 orders of magnitude, and 12 is utterly insignificant compared to 10⁵⁰ (the exponent in the really big number above). Stupendas aren't quite big enough to ignore the units, though.

With apologies to Douglas Adams, RIP.

[ Edit: namo -> nano; seems I made the mistake thrice for good measure. Sigh. ]
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