Electronics Forums > How to find missing resister with a given voltage value?

# How to find missing resister with a given voltage value?

Guest
Posts: n/a

 06-19-2007, 03:19 AM
I have two circuits, where resister voltage are given, and I need to
find resistor value.

first circuit is -- both are serial circuits

E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).

Find R. How do I do this, Also please explain the method.

Second question is

E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
divider rule.

This is what I have done so far for;

formula; v1 = R1*E/R1

2v=R1*20v/R1
2V=20V
=10ohm

I use this method on another serial circuit with all the ohms,
current, voltage, and I didn't get the right answer. There are no
answer at the back of the book, and I don't know if I am right.

John Popelish
Guest
Posts: n/a

 06-19-2007, 04:32 AM
(E-Mail Removed) wrote:
> I have two circuits, where resister voltage are given, and I need to
> find resistor value.
>
>
> first circuit is -- both are serial circuits
>
> E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).
>
> Find R. How do I do this, Also please explain the method.

You have one simple formula relating I to V for R3. I*V=24,
or I=24/V or V=24/I.
You need to come up with a second formula relating I and V
to R value based on the other two resistors and total
voltage. Then combine these two formulas to solve for R3.

The voltage across R3 must be 24 - R1 drop - R2 drop. Each
of these drops is pretty easy to describe in terms of the
common current, which is also pretty easy to describe in
terms of R3.

> Second question is
>
> E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
> divider rule.

The divider rule is

fraction of total voltage that is dropped across Rx
= Rx/Rtotal

So, in this case Vfraction = 2/20 = 0.1
=R1/(R1+R2+R3)

> This is what I have done so far for;
>
> formula; v1 = R1*E/R1
>
> 2v=R1*20v/R1
> 2V=20V
> =10ohm
>
> I use this method on another serial circuit with all the ohms,
> current, voltage, and I didn't get the right answer. There are no
> answer at the back of the book, and I don't know if I am right.
>

Eeyore
Guest
Posts: n/a

 06-19-2007, 09:11 AM

(E-Mail Removed) wrote:

> I have two circuits, where resister voltage are given, and I need to
> find resistor value.

You can do this with Ohm's Law.

Do you know it ?

Graham

Fred Bloggs
Guest
Posts: n/a

 06-19-2007, 10:36 AM

> I have two circuits, where resister voltage are given, and I need to
> find resistor value.
>
>
> first circuit is -- both are serial circuits
>
> E= 24v, R1 = 2ohm, R2=4 ohm, and R3= 24W (watts).
>
> Find R. How do I do this, Also please explain the method.
>
>
> Second question is
>
> E = 20V, R1=2V, R2=40ohm, R3=32ohm. Find resistor R1 using voltage
> divider rule.
>
> This is what I have done so far for;
>
> formula; v1 = R1*E/R1
>
> 2v=R1*20v/R1
> 2V=20V
> =10ohm
>
> I use this method on another serial circuit with all the ohms,
> current, voltage, and I didn't get the right answer. There are no
> answer at the back of the book, and I don't know if I am right.
>

In both circuits E=I*(R1+R2+R3) and you have to solve for an unknown R,
which means you must eliminate I from the equation.
In the first circuit you are given the power in R3 is 24W which I^2*R3
so that I=sqrt(24/R3) and the equation becomes:
E=sqrt(24/R3)*(R1+R2+R3), or 24=sqrt(24/R3)*(6+R3), which you can now
solve for R3.
In the second circuit you are given the voltage drop across R1 is
2V=I*R1 so that I=2/R1 and the equation becomes E=2/R1*(R1+R2+R3), or
20=2/R1*(R1+72), which you can now solve for R1.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post justinp Electronic Projects 6 05-14-2013 08:13 PM krishna_gopal_2 General Electronics Chat 2 05-14-2013 10:04 AM sam General Electronics 2 04-30-2004 06:17 PM Andrew Crook Electronic Design 10 12-21-2003 07:51 PM