Somebody please help me out here...

[matt1386]

Hi all, im struggling to get my head around how to find the value of unknown resistors in parallel, here is the question I have to answer :

Two resistors of 48 OHMS and 80 OHMS are connected together in parallel. What would the value of a third resistor have to be, when connected in parallel with the first two, in order to give a combined resistance total of 15 OHMS ?

Please could somebody explain how I work this out in simple terms lol

Im not asking for answers, just for guidance to make me understand

Thanks very much

Matt

 

[Silly Sausage]

Knowing the formula for resistors in parallel, insert the values given, then rearrange the equation to make R3 the result.

 

[Sintra]

Ok first find the combined value of the 2 known resistors in parallel using the formula

1/Rt = 1/R1 + 1/R2

Once you have this value which we will call R? for now use this and your total resistance that you have for all 3 in this formula

R3 = R? X Rt / R? - Rt

Have a go and post your workings then I will give you your marks out of ten

 

[matt1386]

Thank you guys for your quick replies, its much appreciated

Ok Sintra, ive had a go, here is what I got ( probably look an idiot now if this is miles out lol )

1/48 + 1/80 = 1/0.03 = 30 ohms (Rt)

30 x 15 = 450 / 30 - 15 = 15

450 / 15 = 30 ohms ( R3 value )

Am I anywhere near ?

Thanks again

Matt

 

[Sintra]

Looks spot on to me so you get 10 out of 10

 

[matt1386]

Oh brilliant, thanks for that Sintra

Im still not 100% on understanding how its all worked out but now I know the formula I can practice until it sticks

Thanks again, appreciate the help and advice

 

[Sintra]

No probs knowing the correct formulae is always good starting point