V*C = I*t - Page 1
Forum section: electronics / beginners

author: bostonman
author_rank: Super Contributor
timestamp: July 11, 2019, 02:00:10 am
content: Someone used this formula and I don't understand how the units equal.V*C = I*tThe SI units (from what I found are):Volt = kg·m2·s−3·A−1Coulomb = A * sI = At = sWhat am I missing in the SI units that is not allowing both sides to equal each other?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2019, 02:33:07 am
content: kg.m^2/s^3 == W == V.AHandy to remember that capacitance is F == s/Ω, and inductance is H == Ω.S.Tim

author: ebastler
author_rank: Super Contributor
timestamp: July 11, 2019, 07:10:14 am
content: 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. :-)

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2019, 01:11:58 pm
content: Incidentally, when writing equations carefully, one should put the variable "voltage"Vin italics, and the unit "volts" V in roman. Similarly, italicCis the variable "capacitance" and roman C is the unit "coulombs".

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 13, 2019, 03:43:46 am
content: 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.

author: bson
author_rank: Unknown
timestamp: July 14, 2019, 08:24:45 pm
content: Quote from: TimFox on July 11, 2019, 01:11:58 pmWe 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.

author: IanB
author_rank: Super Contributor
timestamp: July 14, 2019, 08:41:00 pm
content: Quote from: bson on July 14, 2019, 08:24:45 pmNot 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).

author: chris_leyson
author_rank: Super Contributor
timestamp: July 14, 2019, 09:35:18 pm
content: 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.