Author Topic: V*C = I*t  (Read 1476 times)

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Offline bostonmanTopic starter

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V*C = I*t
« on: July 11, 2019, 02:00:10 am »
Someone used this formula and I don't understand how the units equal.

V*C = I*t

The SI units (from what I found are):

Volt = kg·m2·s−3·A−1
Coulomb = A * s
I = A
t = s

What am I missing in the SI units that is not allowing both sides to equal each other?

 

Offline T3sl4co1l

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Re: V*C = I*t
« Reply #1 on: July 11, 2019, 02:33:07 am »
kg.m^2/s^3 == W == V.A ;)

Handy to remember that capacitance is F == s/Ω, and inductance is H == Ω.S. :)


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Online ebastler

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Re: V*C = I*t
« Reply #2 on: July 11, 2019, 07:10:14 am »
The “C” in your formula does not stand for Coulomb (which is a unit), but for capacity (which is a quantity, and its unit is Coulomb/Volt).

The „V“ stands for voltage, but its unit happens to be Volt, so that part is ok. :-)
« Last Edit: July 11, 2019, 07:11:57 am by ebastler »
 

Offline TimFox

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Re: V*C = I*t
« Reply #3 on: July 11, 2019, 01:11:58 pm »
Incidentally, when writing equations carefully, one should put the variable "voltage" V in italics, and the unit "volts" V in roman.  Similarly, italic C is the variable "capacitance" and roman C is the unit "coulombs".
 

Online Nominal Animal

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Re: V*C = I*t
« Reply #4 on: July 13, 2019, 03:43:46 am »
Dimensional analysis, my favourite!

I like to write the units in angle brackets, thus:
    V [ V ] · C [ C / V ] = I [ A ] · t [ s ]

For dimensional analysis, just write the units:
    [ V ] · [ C / V ] = [ A ] · [ s ]
which is equivalent to
    [ C ] = [ A s ]
which is true, 1 C = 1 A s.
« Last Edit: July 13, 2019, 03:52:26 am by Nominal Animal »
 
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Offline bson

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Re: V*C = I*t
« Reply #5 on: July 14, 2019, 08:24:45 pm »
Incidentally, when writing equations carefully, one should put the variable "voltage" V in italics, and the unit "volts" V in roman.  Similarly, italic C is the variable "capacitance" and roman C is the unit "coulombs".
We always used U for potential in school.  And you'd be failed immediately if you pluralized SI units, so 1 volt, 2 volt, 0.5 volt.  2 "volts" would be an incorrect answer on a test.  Temperature: 273 kelvin, 300 kelvin, 10.2 kelvin.  Nor were "colloquial" SI units acceptable, like cm.  (The only acceptable colloquial unit was ˚C as opposed to K, when the context called for it.)  This is in contrast to metric (which is not quite the same as SI plus colloquialisms) and imperial, where pluralization is expected: 1 lb, 2 lbs.  We were also taught never to put a space between values and their SI units or prefixes, but it seems the SI has standardized on a space.  Personally I find that awkward since when used in an expression you definitely don't want spaces; for example, when multiplying 1k*50m the k*m cancel and you're left with 1*50 = 50.  "1 k*50 m" just looks awkward to me.

Oh, and we were also taught to use uppercase U, I etc for DC: operatings points, bias, etc.  And lowercase u, i, etc for small-signal properties.
« Last Edit: July 14, 2019, 08:28:41 pm by bson »
 

Offline IanB

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Re: V*C = I*t
« Reply #6 on: July 14, 2019, 08:41:00 pm »
And you'd be failed immediately if you pluralized SI units, so 1 volt, 2 volt, 0.5 volt.  2 "volts" would be an incorrect answer on a test.

Not quite correct. You shall not pluralize the symbols for units of measure, but the names are English words and follow English grammar rules.

Thus: "1 V, 2 V, 0.5 V", but: "One volt, two volts, zero point five volts."

English grammar style frowns on mixing numbers with words (unless it is a big number or a telephone number), so "2 volts" would get a fail from your English teacher. It should be "2 V" or "two volts".

Another point is that there should always be a space between the number and the unit, so "2 V" not "2V" or "2v" (case matters too).
 

Offline chris_leyson

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Re: V*C = I*t
« Reply #7 on: July 14, 2019, 09:35:18 pm »
Just remembered a mnemonic, CIVIL, for a capacitor I leads V and for an inductor V leads I. Not sure why that popped into my head.
« Last Edit: July 14, 2019, 09:40:35 pm by chris_leyson »
 


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