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Old 9th Sep 2017, 11:27 pm   #1
ukcol
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Default An opportunity to be SMUG

Back in 2009 I started a thread (now closed) about using milli-amps and k-ohms in the Ohm's law equation.

See here -

http://www.vintage-radio.net/forum/s...ad.php?t=42641

I was very pleased (SMUG) to read that no less an authority than M.G.Scroggie (Cathode Ray) had offered the same advice in the February 21st 1936 issue of Wireless World in an article entitled "Ohm's Law".


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BTW there is a very comprehensive collection of Wireless Worlds available for download here -

http://www.americanradiohistory.com/...d_Magazine.htm
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Old 10th Sep 2017, 7:52 am   #2
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Um, no, that's a terrible idea.

It's conflating two steps in the maths, and it's prone to introducing errors when you square milli-somethings and forget that the answer is now in micro-(those things squared). There are a million square millimetres in a square metre. If you aren't extra-careful, you can be out by 1000 times.

Not so bad if you write everything out fully in exponential notation (even the modified, engineering form where the exponent is normalised to be a multiple of three so as to align with a prefix) then do the usual thing of adding the exponents and multiplying the mantissas to simplify.

But remembering pairs of prefixes which annihilate one another is a treacherous path.
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Old 10th Sep 2017, 8:10 am   #3
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You do have to be careful, but yes volts = milliamps x kilohms works for me.

When you square them, milliamps-squared x kilohms for instance, you do get an extra milli (so answer is in milliwatts) but I still find it easier to do (in my head) 9mA in 2kilohms giving 162mW than 0.009A squared and 2,000Ω giving 0.162W. So OK to use a 1/4W resistor.

Much more than this and out comes the paper and pencil.
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Old 10th Sep 2017, 8:32 am   #4
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Default Re: An opportunity to be SMUG

I've used that method often, on the theory that the noughts cancel out, the noughts aren't positive or negative noughts but that's the way I've always looked at them, such has been my life long difficulty in understanding maths, that said I usually end up with the correct results most of the time.

Been trying to understand wave equations and what led to them (Maxwell. Faraday etc etc)....nightmare... what does a dot in a formula mean when the formula is all in letters and strange (to me) symbols.

Lawrence.
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Old 10th Sep 2017, 8:39 am   #5
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I like the idea. One should have a rough idea what the result should be before starting the calculation, thus avoiding being out by a factor of a thousand. A resistor in a domestic electronic product will not be dissipating hundreds of watts, for example.

Those who remember slide rules will be familiar with the concept of estimating the answer and then using the rule to a more precise figure.
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Old 10th Sep 2017, 9:05 am   #6
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Default Re: An opportunity to be SMUG

Quote:
Originally Posted by julie_m View Post
Um, no, that's a terrible idea.

It's conflating two steps in the maths, and it's prone to introducing errors when you square milli-somethings and forget that the answer is now in micro-(those things squared).................
I'm a bit disappointed in that response Julie, remember I'm suggesting this method for simple OHM'S LAW calculations done in the head.

You give the example of when it comes to squaring milli-amps [as in calculating I squared R for power?] and suggest one would forget that squaring milli-amps gives micro-amps. I would suggest that even if you managed to forget that fact the error would be so large that it couldn't be missed by any half decent electronics engineer.

Yes, always use base units and scientific notation if you are a design engineer doing many calculations that happen to include Ohm's law, but for quick calculations when working on a radio set I see no problem with the method.
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Old 10th Sep 2017, 9:15 am   #7
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Originally Posted by ms660 View Post
.......what does a dot in a formula mean when the formula is all in letters and strange (to me) symbols.

Lawrence.
I have the same problem Lawrence. The (advanced) sciences use far too many Greek characters in equations too. I suffer something akin to dyslexia when trying to remember things like theta and rho. Bring back simple characters!

Colin
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Old 10th Sep 2017, 10:42 am   #8
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Problem is that you quickly run out of English letters in both cases. Hence the use of the Greek alphabet. In pure mathematics Hebrew letters are also used on occasion because they run out of Greek letters (Like Aleph-0 for the set of all natural numbers).

But I did a lot of theory in my Electronics/Physics at Southampton, so I'm very comfortable with all that stuff.
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Old 10th Sep 2017, 11:14 am   #9
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Quote:
Originally Posted by julie_m View Post
Um, no, that's a terrible idea.

It's conflating two steps in the maths, and it's prone to introducing errors when you square milli-somethings and forget that the answer is now in micro-(those things squared). There are a million square millimetres in a square metre. If you aren't extra-careful, you can be out by 1000 times.

Not so bad if you write everything out fully in exponential notation (even the modified, engineering form where the exponent is normalised to be a multiple of three so as to align with a prefix) then do the usual thing of adding the exponents and multiplying the mantissas to simplify.

But remembering pairs of prefixes which annihilate one another is a treacherous path.
I can see both sides of this discussion, but part of the answer must be to always quote your units at every stage of the calculation - something which was drummed into me early on! It certainly helps one keep a grasp of what one is trying to do, apart from merely manipulating numbers.
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Old 10th Sep 2017, 11:38 am   #10
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I can see both sides of this discussion...........
Yes both views are valid in there context. I was only advocating use of the method when wanting to find the 3rd value (I, R or V) when you know the other two and in the context of radio service and restoration work. I do not advocate its use in, say the design context for the reasons pointed out by Julie.

I don't BTW use it to calculate power because it is easier to use base units and the method loses it advantage, therefore, as a short cut tool.
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Old 10th Sep 2017, 11:58 am   #11
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It is a very interesting argument and I can see both sides.

But on balance I'd have to agree with Julie M.

As an example, one reason is that the M.K.S (Meter, Kilograms, Second) system of units was rationalized to an absolute system of fundamental units for good reason. Prior to that, the previous (for example) C.G.S system of centimetre, gramme, second system was problematic, especially for electrical engineering.

The problem cropped up because in the case of electric or magnetic systems a 4th concept was required, that of permeability (or permittivity). In CGS these two items in the case of free space were both assigned a value of unity. But because they cannot have the same value, this "assumption" resulted in two systems; "electromagnetic CGS" and "electrostatic CGS".

This was the ghastly price to pay for not using MKS. It was only by 1904 that it got figured out by Robertson (UK) and Giorgi (Italy) simultaneously (much like the Newton vs Libneitz calculus debacle) that if MKS was used that permeability got assigned 10 -E7 and everything in the electromagnetic system and electrostatic system would become absolute units in their own right. It took until 1950 before the International Electrotechnical Commission officially recognized this.

Permeability need not be the 4th concept though, it can just as easily be current. So from the fundamental units of mass length and time and current (in amps) one can derive the value of permeability.

Probably, also since its easier to understand current than permeability, for a beginner, often this was the training where a student was taught that a current of one ampere is defined as the current which flowing in two infinitely long conductors with their centers one meter apart, would produce a force between them of 2 x 10E-7. Newtons per meter.

So I think it doesn't pay on the whole to mess with the fundamental units or pot holes abound for the unwary.
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Old 10th Sep 2017, 1:51 pm   #12
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Quote:
Originally Posted by ukcol View Post
Quote:
Originally Posted by julie_m View Post
Um, no, that's a terrible idea.

It's conflating two steps in the maths, and it's prone to introducing errors when you square milli-somethings and forget that the answer is now in micro-(those things squared).................
I'm a bit disappointed in that response Julie, remember I'm suggesting this method for simple OHM'S LAW calculations done in the head.
I'm inclined to agree with ukcol, I do like the idea of using manageable units for mental calculations! Volts, milliamps, kilohms works well. Capacitance - microfarads, and if frequency is in Hz then reactance is in megohms. Which is fine with valve circuitry! On the few occasions I need it, for propagation delays, speed of light = 1 foot per nanosecond is convenient.

More complex calculations and out comes the pencil, paper, and calculator. And in that case, I do work entirely in base units. But that doesn't invalidate the rough-order-of-magnitude calculations done first, before reaching for said paper and pencil, it just refines them.
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Old 10th Sep 2017, 2:07 pm   #13
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Originally Posted by kalee20 View Post
Capacitance - microfarads, and if frequency is in Hz then reactance is in megohms.
I must admit that I do the 1meg 1uF one second time constant rule in my head all the time without even thinking about it.
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Old 10th Sep 2017, 2:12 pm   #14
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Default Re: An opportunity to be SMUG

I thought everyone used this method, but then I tend toward the practical, finding mathematics beyond the basics to be difficult to grasp. Even so, I always use pen and paper and if unsure, back it with a calculator and, of course, I have a good idea of the value I'd expect, even before I start. So yes, Scroggie's (and Colin's) approach gets my vote.
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Old 10th Sep 2017, 2:15 pm   #15
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The general rule with calculations is that if you know what you are doing then you can take short cuts (such as k x mA = V). Problems arise if you don't know this is a short cut, or only know the short cut.

A dot over a quantity generally means differentiate with respect to time. Two dots means second differential etc. A dot between two quantities generally means 'dot product' or 'inner product' or 'scalar product' (which are different names for the same thing: essentially combining two vectors to obtain a number which tells you something about the 'length' of the vectors and the 'angle' they make).

One advantage of learning maths (and physics) is that when visiting a Greek-speaking country you can just about make out signs and, in some cases (if your English is good enough to know some etymology), know what they mean. A friend of mine who was studying cosmology many years ago ran out of Greek letters and started using one of the Indian languages.
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Old 11th Sep 2017, 12:29 am   #16
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Some of this brings back memories. At school we mostly used the normal feet and inches, except chemistry and physics which used cgs. But 5th year physics we were presented with poundals and suchlike to confuse us (me).
Technical college the following year, where we were back to cgs for physics and chemistry, but where 2nd year physics changed to MKS. Only later in life did they try to impose the SI, where some of the units strike me as ridiculous. Of course we had met Barns for area in atomic physics, and in industry I came upon degrees Twaddel. And there were the angstrom units and so many more, all relevant to their usage, but unacceptable to SI. Where are my beloved inches water gauge and degrees of viscosity?
When we met the amp flowing through the infinitely long conductor, I was mentally screwed, having previously accepted that an infinitely long conductor MUST have infinite resistance, thus the current could only be zero!
And all those Greek letter abbreviations could get confusing when the same ones were used for different things in different disciplines (physics, chemistry), and where say the physics lecturer one year used one letter, but the different lecturer the following used a different letter for the same purpose.
Usually, "r" was meant radius, but one physics lecturer insisted on "a" for radius. (q/t=pi.p.a4/8."eta".l) comes to mind, where the a4 meant the fourth power of radius. -- Poiseule's equation if I recall, but it was at least 55 years ago.
Of course I buy my petrol for the bike in litres, but always calculate my mpg figure. One bike only had a kph speedo, so I was mentally calculating mph as I rode along, but that was over 30 years ago, when my brain was younger.
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Old 11th Sep 2017, 1:07 am   #17
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When we met the amp flowing through the infinitely long conductor, I was mentally screwed, having previously accepted that an infinitely long conductor MUST have infinite resistance, thus the current could only be zero!
Les,

Ha Ha, I like that. I guess if they had defined the wire as say being 1000 metres long so the length effect in the middle of it would not significantly corrupt the measured force value over a meter, then another set of equations would have been required to prove the effect was minimal. (also that would be a great response in an exam situation if one had forgotten how to resolve the forces between the conductors, by claiming the question was defective and didn't dignify an answer)

A similar dilemma cropped up in electromagnetism about relative motion of conductors in a magnetic field or currents, so the reference point was taken as a "far off star" but later relativity theory solved this. So if you are travelling along a long wire at the same speed as the charges, the magnetic field around the wire vanishes, the magnetic field is only there when the current has motion with respect to your frame. Likewise if you move with respect to some charges elsewhere you will see a current & magnetic field due to the relative movement.
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Old 11th Sep 2017, 5:13 am   #18
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I seem to remember (but it's a long time since I looked at this) that if you know the force between two 'infinitely long, zero diameter, straight...' wires you can use purely geometric/mathematical methods to work out the force between finite wires, coils, etc. So you make your current balance with a pair of coils, and use said maths to work out what the current must be for the measured force between them.

The electrostatic and electromagnetic units were mentioned in passing earlier. The constant (1/(4*pi*e0) -- where e0 is the permitivity of free space) turns up a lot in electrostatics, and it is often said (by schoolteachers, etc) you can take that as 9*10^9 in SI units. What other (very common) approximation is that exactly equivalent to? (none of the teachers at my school had spotted that, or could answer said question when I asked it).


Incidentally, if you square 'milliamps' (e.g. to work out a power) you do NOT get microamps. You get micro (amps^2). Dimensions are important!

To get back on-topic, I use the V, mA, kOhms trick all the time. I can't remember where I read it (if indeed I did read it, rather than it being something that I spotted as obvious). It's never caused me any problems. I think it's a matter of understanding why it works, rather than just using it blindly.
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Old 11th Sep 2017, 12:52 pm   #19
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If the SI system can have the kilogram as a fundamental unit, then I can have a system of units based on kilohm, volt, millamp~, milliwatt, etc.

Where:
1 of my kilohms = 1,000 SI ohms
1 of my volts = 1 SI volt;
1 of my milliamps~ = -0.001 SI amp
1 of my milliwatts = 0.001 SI watt

and everything is consistent.

(Note that my scaling factor between my milliamp~ and the SI amp is -0.001, (hence the tilde), I chose a negative scaling factor so that my milliamps~ can go in at the hot cathode and come out at the anode, same as what electrons do.)
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Old 11th Sep 2017, 3:09 pm   #20
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I never understood all the fuss about MKS and CGS. The factors in both cases are of ten, so no fundamental difference can apply between the two.
You can express the same quantity in either system with a simple rule:
For MKS to CGS, multiply the Metres by 100, and the Kilograms by 1000.
For CGS to MKS, divide the Centimetres by 100, and the Grams by 1000.

The whole palaver comes about because of a fundamental flaw in the French Revolutionary system of measurement. The unit of Mass, the Gram (or Gramme) is not in the same order of magnitude as the unit of Length, the Metre.
A basic mistake, clearly visible to anyone who actually USES measurements in real life.
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