Working out Resistance in a parallel circuit

[RUSSELLM]

One for the Auto Electricians and other technically minded

Whilst at college today, the students were discussing working out resistance in parallel circuits.

A House Electrician came up with the formula... 1 over (1 over R1) + (1 over R2) + (1 over R3)

So if R1 is 10 ohms, R2 is 68 ohms and R3 is 10 ohms, you've got 1 over (68/680 + 10/680 + 68/680) = 1 over 146/680 = 4.66 ohms

I think I've worked that out right .... And that's the first formula that Google came up with too.




The lecturer came in and then showed us a slide involving a 12VDC circuit, with the same 10,68 and 10 ohm resistors.

The calculation on that slide showed...
12v divided by 10 ohms = 1.2 amps
12v divided by 68 ohms = 0.18 amps
12v divided by 10 ohms = 1.2 amps

The slide then showed 1.2 + 0.18 + 1.2 = 2.58 ohms



Why are we getting Two different resistance figures ? The only thing I see different is a voltage being present. So is it a different formula if there's no known voltage ?

Thanks for any help

Edited by RUSSELLM on Thursday 12th March 20:47

[M4cruiser]

You've got your ohms and amps mixed up.

2.58 amps

12 v / 2.58 gives 4.66 ohms

So they're both right.

[RUSSELLM]

That's brilliant thanks.... I think the last part of the equation was missing from the slide...

[RUSSELLM]

Mods.... My question's been answered, thankfully, whilst it was in General....

But out of interest, what's 12 volt auto electrics got to do with Homes, Gardens, and/or DIY ?

[Big Al.]

Sorry Russell, my error must have had brain fade.