Resistor value ?

[stueelad]

I am trying to find the resistor value of Rx in this diagram, can someone please help.

 

[johnboy6083]

here we go, its been a while since i was at college. total resistance= voltage/current (R=V/I) 20/0.25= 80.
Resistance in series = R= r1+r2, so 80-5= 75. so you have 75 ohms to account for. resistance in paralell = 1/r= 1/r1 =1/r2 . so 1/75 -1/10 = 11.53.

Hope i got it right, and that it helps.

Some of the blokes on here are lecturer's so im sure they will be along soon to mark (and fail) me!

 

[stueelad]

Thats what I did first time round but I have since been told that you have to find the volt drop first.

 

[johnboy6083]

how can you find volt drop, when you dont have length or CSA of conductors?

 

[JUD]

how can you find volt drop, when you dont have length or CSA of conductors?

V = IR
V = 0.25A x 10Ω
V = 2.5V

 

[johnboy6083]

oh, you mean volt drop across the individual resistances.

cheers jud

 

[pushrod]

work out voltage across 10 ohm resistor
unknown resistor will have same voltage
work out voltage across 5 ohm resistor (simple subtraction)
calculate current thru it
work out current thru unknown resistor
work out resiistance of unknown resistor.

 

[pmac10]

work out voltage across 10 ohm resistor
unknown resistor will have same voltage
work out voltage across 5 ohm resistor (simple subtraction)
calculate current thru it
work out current thru unknown resistor
work out resiistance of unknown resistor.

im liking bumping into your posts again matenice one

 

[JUD]

work out voltage across 10 ohm resistor
unknown resistor will have same voltage
work out voltage across 5 ohm resistor (simple subtraction)
calculate current thru it
work out current thru unknown resistor
work out resiistance of unknown resistor.

Go on then.

P.S. Empty your PM box

 

[pushrod]

im liking bumping into your posts again matenice one

Cheers

Go on then.

P.S. Empty your PM box

oops done . 3.5A across the 5 ohm resistor (doh! forgotten the code) so 3.25A thru unknown resistor, then 2.5V/3.25A = 0.71 ohms

 

[JUD]

work out voltage across 10 ohm resistor 0.25 x 10 = 2.5V
unknown resistor will have same voltage 2.5V
work out voltage across 5 ohm resistor (simple subtraction) 17.5V
calculate current thru it 17.5 / 5 = 3.5A
work out current thru unknown resistor 3.5 - 0.25 = 3.25
work out resiistance of unknown resistor. 2.5 / 3.25 = 0.77Ω

My Green

I think

 

[JUD]

Cheers


oops done . 3.5A across the 5 ohm resistor (doh! forgotten the code) so 3.25A thru unknown resistor, then 2.5V/3.25A = 0.71 ohms

I get 0.77Ω.

0.71 is the resistances in parallel [1/r = 1/(1/0.77 + 1/10)]

 

[pushrod]

I get 0.77Ω.

0.71 is the resistances in parallel [1/r = 1/(1/0.77 + 1/10)]

sorry you are right too busy watching the telly, and answering the wife's question as i type and using the crappy calculator on here - never could multi task!!!

 

[pushrod]

Cheers


oops done . 3.5A across the 5 ohm resistor (doh! forgotten the code) so 3.25A thru unknown resistor, then 2.5V/3.25A = 0.71 ohms

2.5/3.25 actually equals 0.77 ohms (don't know what's happening to me)

 

[johnboy6083]

told you id fail

 

[pushrod]

told you id fail

But you got the marks for showing your working out

 

[stueelad]

Thanks guys, now it all makes sense