Awkward Questions - Page 1
Forum section: electronics / beginners

author: German_EE
author_rank: Super Contributor
timestamp: March 09, 2015, 06:04:59 pm
content: EEVBlog readers may recall my previous question where I asked for ideas in the education of a seven year old neighbor, well, as he knows how to multiply and divide we did Ohms Law today and the following question came up:Why do we use 'I' for current when it should be 'A' for Amps?To save you looking we have two theories and both of them seem valid.a) the origin of "I" is 'intensité', the French word for current as Ampére was French.b) what we now know to be electric current was, at one time referred to as the 'impetus' of the electric flowPlace your bets now.On a roll I then found out why we use 'L' for current rather than 'H' as the Henry is the unit of measurement, it's to honor Heinrich Lenz who did some research in the area. This way I can look really smart when my young pupil asks the question.

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2015, 06:13:34 pm
content: In general we do not use the letter corresponding to the unit of measurement, so the idea of using A for current would not fit the general rule.For example, we use L for length rather than m for meters.We use T for temperature rather than C for Celsius.We use M for mass rather than k for kilograms or g for grams.You could also introduce your young friend to the idea that there may be different units of measurement. Length could be inches instead of centimetres, and temperature could be Fahrenheit instead of Celsius. Electricity is unusual in having only one way to measure current or voltage.

author: photon
author_rank: Frequent Contributor
timestamp: March 09, 2015, 06:23:46 pm
content: If you read some of the early English guys like Maxwell or Heaviside, they used "C" for the variable name of current. So I guess "I", which is the standard today, must have come from continental Europe.

author: gregallenwarner
author_rank: Regular Contributor
timestamp: March 09, 2015, 06:31:00 pm
content: In agreement with IanB, the explanation I've been told all my life is that it's to differentiate between thevariableand theunit of measurement.I was taught Ohm's Law as E=IR, where E is the variable that stands in for the voltage, I is the variable that stands in for current, and R is the variable that stands in for resistance. The units of measurement for each respectively are V, A, and(Greek Omega). (I don't know how to type it on a US Keyboard.)Take current for instance. If you wanted to write "Current equals 10 Amps" using "A" as the variable name, it might be confusing to see it written: A = 10A. There's not much to indicate that the 2nd "A" is the unit of measurement, not a variable. Instead, we write I = 10A, knowing that we are using the letter "I" as a variable, and thus, "A" is left as the unit of measurement.Feel free to disagree if anyone has any other philosophy. I don't have any sources backing this up, this is just what I was indoctrinated with.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 09, 2015, 06:54:58 pm
content: Quote from: gregallenwarner on March 09, 2015, 06:31:00 pmSame here, almost. The variable for voltage is U, the rest is the same.So:U in VoltsI in AmpsR in Ohms

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2015, 07:09:24 pm
content: Quote from: PA0PBZ on March 09, 2015, 06:54:58 pmI think physicists tend to useEfor potential andUorVfor potential difference. Although in many formulas they may be seen as interchangeable.

author: max666
author_rank: Frequent Contributor
timestamp: March 09, 2015, 07:50:16 pm
content: Quote from: IanB on March 09, 2015, 07:09:24 pmPhysicists use E for the electric Field, unit V/m ... Volt/Meter.The electric Potential is often expressed by(lowercase phi) or(uppercase phi) and yes I've seen V being used too, unit V ... Volt. Potential Difference gets a U, unit V ... Volt.And I say, like it has been said here already, the reason why we don't use the same symbol for a physical quantity and their unit, is because they are not the same.A physical quantity comprises of a numerical value of a quantity and a unit of measurement (exception proves the rule, few are dimensionless).

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2015, 08:11:58 pm
content: There can be variations in how symbols are used. For instanceEcan be used to denote EMF or electro-motive force. When used as such it has units of volts. The symbolEcan also be used to denote standard potentials, such as the standard electrode potential asE°, when it also has units of volts.There are variations in notations and conventions in different fields and in different circles. It is not something that can be categorically right or wrong. (Which is why technical papers and books usually define their chosen nomenclature.)

author: max666
author_rank: Frequent Contributor
timestamp: March 09, 2015, 08:23:04 pm
content: Quote from: IanB on March 09, 2015, 08:11:58 pmConcured!

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2015, 08:59:38 pm
content: It's good you're honest when you're not sure about something, rather than BSing as some of my teachers did when they didn't know something for sure.I can understand why some people feel uncomfortable with using V for voltage but I don't find it that confusing. The worst thing is using lower case variables and upper case variables such as lower case i for instantaneous current an upper case I for steady state current. Capital V and lower case v was even worse because my handwriting is poor. I don't think using lower case e would've been wise as it's easy to confuse with the Euler's number.

author: AG6QR
author_rank: Frequent Contributor
timestamp: March 09, 2015, 09:36:24 pm
content: There are only 26 letters in the Latin alphabet. If we insist on single-letter variable names (makes writing on the chalkboard easier), there are bound to be problems inventing a new letter for everything we want to quantify, especially if we allow ourselves to cross over between disciplines. The Greek alphabet adds some more letters, but not enough.Mathematicians will claim that "i" is the square root of -1, and an "i" is used because it's the first letter of the wore "imaginary". In order to avoid this collision, electrical engineers use "j" for this mathematical concept, reserving "i" for current. Arrgh!When teaching a ham radio license class to new students, covering the basics of Ohm's law and electrical power formulas, I heard an instructor say approximately, "I'm really sorry. There are a boatload of historical reasons why the terms and letters are the way they are, and they don't always make sense. I wish I could go back in time and straighten it all out, starting with the direction of current flow, but I can't. The choices are sometimes arbitrary, sometimes having to do with the names of pioneers in the field. You just have to learn them as they are."

author: tautech
author_rank: Super Contributor
timestamp: March 09, 2015, 11:52:31 pm
content: Quote from: AG6QR on March 09, 2015, 09:36:24 pm+1And thats the way it must be.What, make it easy for all and standardize the EE terminology and units?Sounds like WW III to me.

author: max666
author_rank: Frequent Contributor
timestamp: March 10, 2015, 12:54:14 am
content: Quote from: AG6QR on March 09, 2015, 09:36:24 pmNot enough you say, hmm?How do you like them symbols?And don't worry, if you still need more, our asian friends will help help you out:

author: gregallenwarner
author_rank: Regular Contributor
timestamp: March 10, 2015, 02:11:58 pm
content: Cool! So can we standardize Ohm's Law as the following?

author: Refrigerator
author_rank: Super Contributor
timestamp: March 10, 2015, 03:03:38 pm
content: I have an awkward question too: why do people never use terms like megameters and gigameters, but instead say thousands of kilometers ( kilo + meter ) ?You know like kilo=103, mega=106, giga=109, etc. ?

author: electr_peter
author_rank: Unknown
timestamp: March 10, 2015, 03:26:54 pm
content: Quote from: max666 on March 10, 2015, 12:54:14 amMore symbols does not help at all, it even makes things worse. Why? Because 1) all these symbols are similar to one another (especially in hand writing) and more importantly, 2) you need to know it advance from which symbol subset symbols were extracted. When writing on blackboard, only one symbol subset (or language) should be used, otherwise it quickly becomes incomprehensible. For example, writing Latin and Greek symbols on the blackboard is asking for trouble if it is overused.Main problem is that too much information is being pushed to one symbol, it is just not a good practice. English language itself has some problems with writing/pronunciation dissimilarities, mostly with short words (*ough, *ear, ...). Pushing all the different meanings in one or two syllable words with similar spelling is problematic.If you see one symbol and guess the meaning, something is not right. Try usingsuperscriptandsubscriptin those cases.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: March 10, 2015, 04:13:24 pm
content: Quote from: photon on March 09, 2015, 06:23:46 pmMaxwell was Scottish and they have tended to have better relations with continental Europe than England for a long time. I think that may just be due to their works coming before all this was standardised and Heaviside following Maxwell's convention seeing how influential his work was.TBH my vote is for option number 1, but I can imagine there's no way we can know for certain.

author: magetoo
author_rank: Frequent Contributor
timestamp: March 11, 2015, 03:12:16 am
content: Quote from: Refrigerator on March 10, 2015, 03:03:38 pmSpeculation: I suspect that it is because people tend to keep using what they are familiar with, and "kilometer" is perceived to be a unit of distance on its own. So "a thousand kilometers" is more easy to relate to than a megameter.Around here "mil" (ten kilometers) is commonly used for distances. It's a bit weird when you think about it, but everyone can relate to it and it is what gets used for anything that's more than a couple of kilometers away.

author: willhi
author_rank: Newbie
timestamp: March 11, 2015, 07:00:11 am
content: I think the short answer to the C question is that Einstein made it so. When your Einstein you get to decide things.The longer answer is that the variable c (Latin for "celeritas" meaning "speed) dates way back. However, the most common variable/constant used for the speed of light in the 19th century was the letter V and c was used for the speed of electrodynamic waves. So V or C, either way we would have a problem today.Einstein used V for the speed of light but then switched to C in 1907. I guess they were used to using the variable c and then when Einstein made it a constant he just started using C.That may be close to right.

author: Shock
author_rank: Super Contributor
timestamp: March 11, 2015, 10:29:10 am
content: Current was assigned "I" (International Ampere) as the dimension symbol and "A" (André-Marie Ampère) as the unit symbol. Area uses "A" as the dimension symbol. It's a fair assumption that once Area was assigned it was a done deal.Good enough?