OT Electrical question

John B. · Dec 4, 2010

I had a job interview Thursday and was tested on numerous vehicle systems and shop tools. I wanted to share this one with you.

With a 12 volt electrical supply and two resistors hooked in parrallel, one being 3.2 ohms the other 1,000 ohms then a third resistor of 4.8 ohms hooked in series with the first two what is the total resistance?

I put down the total resistance is 3.2 ohms. Because electricity will seek the path of least resistance and the resistance will be that of the lowest resistance.

Hope I was correct. I posted a drawing of the diagram.

John B. · Dec 4, 2010

I know my drawing isn't the best but should get the point across.

dbernie · Dec 4, 2010

Can't remember offhand, but to get to ground it would have to go thru the 3.2 + the 4.8 =8. However, I am really rusty and seem to remember somethin that that's not correct. really unsure, maybe I shouldn't have chimed in.
Somebody that knows for sure will reply, I imagine.

John T · Dec 4, 2010

What you have is the 4.8 in series with what the 3.2 and the 1000 in paralell computes to.

The 3.2 in paralell with the 1000 = Product divided by the sum:

3.2 x 1000/ 3.2 + 1000 = 3,200/1003.2 = 3.19

So the total resistance is 4.8 + 3.19 = 7.99

John T

george md · Dec 4, 2010

John,

For the parrell part -- 1/r1 +1/r2 = 1/Rt

add that to the 4.8 to get total R

george

Dalex · Dec 4, 2010

I don't have the correct answer either , but the 1k is effectively going to lower the resistance of the 3.2 down a little, some current will flow through it , and the 4.8 adds to the total resistance , just like batteries in series. The answer is going to be around 7.8 ohms.
Just think of it as plumbing .Say the 3.2 is a 3" pipe , and the 1k is a 1/8 copper tube like on an oil pressure guage. Most of the water will go through the 3" pipe , but you will get a tiny amount through the 1/8 tube.........

RayP(MI) · Dec 4, 2010

John is right, when you have a very large resistance in parallel with a very low resistance, it becomes almost a non-player.
I just added the 3.2 and 4.8 and came up with 8, without doing the math! I cheat!

Heat Houser · Dec 4, 2010

Resistances in parallel.....
Your example
1 divided by the sum of 1 divided by 3.2 plus 1 divided by 1000.
Formula
1/(1/R1 + 1/R2)= total parallel resistance

1/(1/3.2 + 1/1000)
1/(0.3125+0.001)
1/0.3135
3.1897926 Total parallel
Resistance in series, add them.
4.8 plus 3.1897926 is 7.9897926 (or 7.99 ohms)

Got 3 or more in parallel?
1/(1/R1+1/R2+1/R3+1/R4....)

old · Dec 4, 2010

Well you have 4.8 plus the 3.2 which should be equal to a total of 8 ohms total. It has been since say 1996 since I have tried to do math like this but I think on this one I may have J-T beat but then it has been a few

old · Dec 4, 2010

See that is why I gave up working with Zap-U=trons. Funny how few people understand my term but hey when it drives you up the wall you find was to make it less hard to work on. At times I still wish I could remember what or how to work with this system. 001,010,100,110,101,011,111, etc. Once year ago or is that many years ago I could tell you what that was in octal but now days it seems the smoke has been let out of this brain LOL

buickanddeere · Dec 4, 2010

Correct

John T · Dec 5, 2010

Sorry Rich my friend, close but no cigar lol, The net equivalent resistance is determined by a formula PRODUCT DIVIDED BY THE SUM......

NOW its true that when a big resistor is in paralell with a very small one (the 3.2 in paralell with the 1000) the big one has little affect and the net resistance is merely a shade less then the smallest resitance i.e the net equates to 3.19 almost 3.2 lol

But he asked a legitimate direct technical question so I wanted to help educate him with the correct technical answer and the formula by which it was determined

The "correct" answer is 7.99 NOT 8 but hey those are soooooooo close I grant you.

Yall take care now

John T

John T · Dec 5, 2010

You agree with me yayyyyyyyyyyyyyyyyyy lol Gotta be right then huh???

John T

TomH in PA · Dec 5, 2010

I probably would've said 8 ohms, and added a small comment on the side that the tolerance on the resisters could be anywhere from almost 0 to 20% so the actual resistance could be anywhere between less than 7 to over 9 ohms.

old · Dec 5, 2010

Ya after I posted I went back and looked at the math on the other post and understand what it should be but then a 0.01 is not enough to be seen by most meters that the common man owns but yep your right.

agpilot · Dec 5, 2010

Hi John T: You are right. I worked in a R&D lab at Collins/Rockwell Int in the early 1960s and most often we used a 10 to 1 rule. If the other resistor is more then 10 times the value of the smaller one, ignor the larger resistance. In any case it is seldom quality equipment will use 2 resistors together like that. Like driving one large tractor and another small tractor side by side both hooked to the same implement.. Not good.. ag.

Heat Houser · Dec 5, 2010

Had to think awhile to pull the formula out of the soupy fog.
Your formula works for 2 in parallel and is correct.

OT Electrical question

John B.

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John B.

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dbernie

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John T

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george md

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Dalex

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RayP(MI)

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old

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old

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