What resistance must be placed in parallel with a 155 ohm resistor to make the equivalent resistance 112 ohms?

Question

Parallel wiring question....

What resistance must be placed in parallel with a 155 ohm resistor to make the equivalent resistance 112 ohms?

Any help is appreciated!!

Answer 1

403.7 ohms

(1/112 - 1/155) ^-1

Answer 2

Hi,
A formula for two resistances in parallel is this:
R1R2/(R1 +R2)
Now, if we set that equal to 112 (the equivalent resistance we want) and substitute the know resistance for R1, we have this:
155R2/(155 +R2) = 112
155R2 = 112(155+R2)
155R2 = 17360 + 112R2
43R2 = 17360
R2 =403.72

Please be aware that you could never find a resistor of 403.72 ohms. Probably the nearest value you could get would be 400 ohms. But, of course, we do these problems in books. :-)

Hope this helps.
FE

Answer 3

Remember how resistances work in parallel. 1/Re = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn When you have "n" resistors in parallel. Re is the equivelant resistance. Here we have 2 resistors, and we are told R1 = 231, Re = 99.5. Then 1/R2 = 1/Re - 1/R1 = 1/99.5 - 1/231 Get a number for that and then take the inverse to get R2 = 175ohm.

Answer 4

403.7ohms

1/R1 + 1/R2 = 1/Rtotal

Answer 5

Don't cheat.

Do your own homework.