Sagan Jones, We know you can do better than him...

[G0HZU]

Didn't they teach you about mho units (1/ohm). You add them for parallel resistors.

Sure, but everybody to get the parallel resitor was doing R1*R2/(R2+R1), I never saw somebody pressing/using the 1/x key.

Yes, it's an efficient and intuitive way to do it because the 1/x key on an old school calculator acts as a rapid conversion to/from mho. You can also use it with series capacitors or parallel inductors.

In the real world we have access to calculators and this method is therefore more efficient than the classic formula the kid was using. I've been using this 1/x key method for many years and once you have used it a few times it kind of sticks. I hoped a few people would not have seen it before so I thought it was worth posting on here

[zapta]

Apparently mho units are also called Siemens. Never heard of it before.

http://en.wikipedia.org/wiki/Siemens_%28unit%29

[Howardlong]

Apparently mho units are also called Siemens. Never heard of it before.

http://en.wikipedia.org/wiki/Siemens_%28unit%29

Showing your age 

[rob77]

Apparently mho units are also called Siemens. Never heard of it before.

http://en.wikipedia.org/wiki/Siemens_%28unit%29

Showing your age 

it's not about age, it's about location...

i never heard about "mho" before (ohm in reverse ?) , but Siemens is a widely used unit over here in Europe.

[zapta]

Apparently mho units are also called Siemens. Never heard of it before.

http://en.wikipedia.org/wiki/Siemens_%28unit%29

Showing your age 

Yes, too old, Siemens wad not born yet when I was in college.

;-)

[EEVblog]

I meant the trick with the 1/x key on the calculator... I used to setup a formula on my TI-68 like this Zp=Z1*Z2/(Z1+Z2), it was working even with complex impedance too! What a beauty...

It's not a trick, it's that's the most used and standard formula!

1/Rp = 1/R1 + 1/R2

IME the majority of sources that teach parallel resistors use the reciprocal method instead of the product over sum method. Both very often both.
That's why I'm surprised you've never seen it.
Probably more popular than product over sum because it is easier to do on a calculator.
On an algebraic, for a single digit resistance, 7 keystrokes vs 10 keystrokes.

[mtdoc]

Apparently mho units are also called Siemens. Never heard of it before.

http://en.wikipedia.org/wiki/Siemens_%28unit%29

Showing your age 

Yes, too old, Siemens wad not born yet when I was in college.

;-)

In college before 1881?  Wow - you are old! 

[IanB]

Sure, but everybody to get the parallel resitor was doing R1*R2/(R2+R1), I never saw somebody pressing/using the 1/x key.

Then how would you determine the parallel sum of three or four resistors, like R1||R2||R3||R4?

The standard formula for that is:

    1/R = 1/R₁ + 1/R₂ + 1/R₃ + 1/R₄ + ...

It's going to get very messy if you do it any other way. It is completely astounding to me that you have not learned this standard formula in high school, let alone college. (In fact, I think perhaps you are not serious and you are pulling everyone's leg?)

By the way, the origin of this formula is why there was a much easier way to solve the problem on the blackboard than the way used by that child:

    I₁ = 8 V / 8 ohm = 1 A
    I₂ = 8 V / 2 ohm = 4 A
    I_T = 1 A + 4 A = 5 A
    R_T = 8 V / 5 A = 1.6 ohm

An older and more experienced person would have seen this method and intuitively applied it rather than solving the problem in the order given by the question as the child did.

[IanB]

English question, is it draftsman or draughtsman where technical drawing is concerned?

In America it would be draftsman, in Britain it would traditionally be draughtsman, but the American spelling is seen more often in Britain these days due to global American influence.

[lapm]

Few more years and Sagan can take over fundamental fridays 

[coppice]

Yep, but this kid is only 5. 
As the father of 6 and 8 year old boys, I  can tell you the difference between 5 and 7 or 8 is huge.
What's impressive to me is that this kids verbal and writing skills appear to be very good as well.

As only having an almost 4yo, no direct experience with older kids, but it's obvious to any parent there are dramatic differences between kids of the same age. And that even just 1 year can make the difference between chimp level skills, and the kind of stuff we see here. Every brain seems to grow and develop at vastly different rates.

The youngest and oldest kids in a class are generally one year apart in age. In the early years of school it really shows, both physically and mentally.  The mental differences aren't that well documented, but some physical ones are. Hardly any US baseball stars were born in the second half of the year. They get picked for development while young, based on the year they were born and their performance. The kids who are a few months younger stand no chance at all in these selection tests. They never get into the development program, and they never become stars. My experience with the mental abilities of children are similar. Any early selection of children as exceptionally smart in their class will massively favour the oldest in the class.

[Zucca]

It's not a trick, it's that's the most used and standard formula!

I´m not taking about the formula or theory, the point is the 1/x key on the calculator!
In your experience and known world is the most used (and surely your E/E experience is much bigger than mine), but there are EE people you don´t know out there (less than 1% at this point, maybe?) like me who for decades never touched the 1/x key on the calculator for parallel resistance calculation. I promise you I´m not alone. BTW I´m pissed with my well paid professors who didn´t told me:"Dears students just use the 1/x key to save time". Sometime the easiest way can be very clear and simple, but if you don´t know it and you know a way to do get what you need, good luck to understand you are losing time!

Then how would you determine the parallel sum of three or four resistors, like R1||R2||R3||R4?

Rp12=R1*R2/(R1+R2)
Rp123=Rp12*R3/(Rp12+R3)
Rp1234=Rp123*R4/(Rp123+R4)

Very long and stupid way once you know how to use the 1/x key. Is the result a wrong one? No, so that´s why I was keep going like this, of course for massive calculation Excel was my best friend.

Now, wait a minute. Probably once I tipped on mine TI-68

Code: [Select]

(Z1 [1/x] +Z2 [1/x] +Z3 [1/x] +Z4 [1/x] ) [1/x]
where Zn were complex impedance, please note that [1/x] means the 1/x key.

It could be I used even

Code: [Select]

^-1
wihch means 3 keys on the calculator, not sure about that. Probably no, I am more crazy than stupid.

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4 + ...

I think perhaps you are not serious and you are pulling everyone's leg?...

Of course I saw that equation and also used it! The whole point is it never clicked in my mind to use the damn 1/x key, and I thank G0HZU for telling me that.

It also depend on the calculator you had. If you had one which after pressing [1/x] was displaying immediately the result then probably this discussion makes you LOL.
Mine by pressing the [1/x] was just putting the -1 at the exponent and sitting there, to get the result you had to press [ENTER].

Anyway good for you and bad for me. Case closed. I already lost enough time by no using the 1/x key. Please forgive me but in this discussion I have the feeling I am wasting my time by talking about how I wasted my time in the past. I am not irritated at all, I am just facing the crude reality LOL.

PS: I miss now my old trusty TI-68 soo much, we won a lot of battles togheter! It was declared dead two years ago, the display was just showing garbage all the time. May his soul rest in peace.
http://www.datamath.org/Sci/Modern/TI-68.htm.

[IanB]

Of course I saw that equation and also used it! The whole point is it never clicked in my mind to use the damn 1/x key, and I thank G0HZU for telling me that.

It also depend on the calculator you had. If you had one which after pressing [1/x] was displaying immediately the result then probably this discussion makes you LOL.
Mine by pressing the [1/x] was just putting the -1 at the exponent and sitting there, to get the result you had to press [ENTER].

I guess this is a downside of things like S.V.P.A.M. (Super Visually Perfect Algebraic Method) and its various equivalents on calculators in recent years. I dislike these things because they stop you juggling with numbers and seeing the results as you go along.