Formula

1/R = 1/R₁ + 1/R₂.

Thanks Mr Tornado

Dear Tornado,

Thank you very much for the reply to Mr.Madness. But I request you

not to reply to such a silly question. You should have reprimanded him for wasting

our time.

Manroop.Chennai

it is the average I think. anyway... Tornado is right. (wiki)

eeewwww....

Go sit in corner.

Average of 1k and 2K is 1.5K

Equivalent resistance of 1K and 2k in parallel is 2/3k.

ya I should... I took electronics in 1989...haven't used it since, for the most part... . but I've learned on cr4 to search for stuff on the net to justify my opinions..so that at least is partly redeeming, because the link is accurate.

No wonder your circuits don't work...
Product over sum
Del

This is a wrong formula for resistors in parallel (in general)

sum over product is for a special case where we have two resistors in parallel

i.e for resistors R1 & R2 in parallel, total resistance R is given by

R= {R1+R2}/{R1*R2}.

For n resistors in parallel always use this formula:

1/Rt=1/R1+1/R2+1/R3+.........1/Rn.

put values into this formulae and see why!

Arrrghhh NO.
IT'S NOT A SPECIAL CASE IT'S JUST F*&%ING WRONG!
It's product over sum
Take two 1K restistors in parallel, sum over product gives 2/1 = 2K which is never right.
Product over sum gives 1/2 k which is correct.
See post #6
Please mark #4 as off topic as it's wronger than something wrong queueing up at the wrong door to be let into the wrong place on the wrong day with the wrong ticket, having forgotten to put it's trousers on in the first place.
Shame on you, so nearly right, line one was fine then you spoit it... go sit on the naughty step until you have had enough.

Del

you were taught by Nuns with a yardstick? (or some other version thereof?)

There is a background SHWACK when you speak like that.. gives me tingles.

thanks for the correction/, i meant to write product over sum.

however product over sum is an easy way/reckoner if values of two resistors are to be found. thats why i said a special case exists.its more complex when you have more than two resistors .i have seen many people trivializing this formula especially in the field thus ending up with wrong results/calcs.

for example,two 4k resistors in parallel, product over sum gives 16/8=2k(correct)

for three 4k resistors in parallel, product over sum gives 64/12=5.33k(wrong!)

know this; in theory for n-resistors in parallel, the total resistance is ALWAYS less than the smallest individual resistance.

i.e three 4k in parallel yields 1/R=1/4+1/4+1/4=0.75 and R=1/0.75=1.33K(correct)

in general for n resistors in parallel 1/R=1/R1+1/R2+1/R3+.......1/Rn

my advise, check your product over sum for n>2 i bet youve been making wrong assumptions over the years, got that?

r4roty

i bet youve been making wrong assumptions over the years, got that?
Nah, not me Guv, I make a much better class of mistake than that.
Del

Assume that in a circuit resistors are connected in parallel R1 and R2... their resultant Resistance is shown as.....

R= (R1 x R2)/ (R1+R2)

For the fun of it...

Lets assume you have two resistors... 1 ohm and 10 ohms in parallel for a total of .909090... ohms, and lets assume that they are both 1 watt resistors for a total of 2 watts.

How much voltage can you put across these resistors before they cook.

For all practical purposes you can ignore the 10 ohm.
when in doubt simplify.
Power = V²/R
Depends what yo mean by cook, the 1W rating is at maybe 50degrees C, so let's say 4W will cook it nicely.
That would be 2 Volts as 2 suared over 1 is 4. Hmmm now I wonder why I chose 4W as the cooking figure.
Del
(Can't believe I bothered to do all that...suckered)
Although it does show imprecise and 'wet finger' most electronics is... you could clamp the resistor to an aluminium plate and it would probably be fine, maybe add some magnets and HHO?