# Finding Missing R3 in Combination Circuit

#### aeln269

Joined Oct 18, 2020
31

Hi everyone, im stuck on this problem for my homework. What ive done so far is R3 = RT - R1 - R2 - R4. Also, the current total is supposedly 10ma

#### MrChips

Joined Oct 2, 2009
31,092
Step 1: What is the current flowing through R1?
Step 2: What is the voltage across R1?
Step 3: What is the voltage across R4?
Step 4: What is the current flowing through R4?
Step 5: What is the current flowing through R3?
Step 6: What is the voltage across R3?
Step 7: What is the resistance of R3?

#### aeln269

Joined Oct 18, 2020
31
For #1, I= V/R right? Also because this is a parallel circuit, does that make all of the voltages the same? So is it also 10ma?

#### MrChips

Joined Oct 2, 2009
31,092
No. You cannot make that assumption until you work through it.

R1 is in parallel with R2.
What is the equivalent resistance of R1 and R2?
What is the voltage across R1?
What is the voltage across R2?

#### aeln269

Joined Oct 18, 2020
31
Ok this is what I did.

because R1 and R2 are in parallel, I combined them to solve for R1R2

1/1000+1/1000

2/1000

1000/2

R1R2 = 500

Then I used ohms law V=IR

V=(10ma)(500)
V=0.01(500)
V=5

which applies for both because they are parallel?

#### aeln269

Joined Oct 18, 2020
31
Then because it has to add up to 10 volts, the other two are 5 volts as well?

#### Ramussons

Joined May 3, 2013
1,418
From the values given in the figure, R3 cannot be determined.

#### MrChips

Joined Oct 2, 2009
31,092
From the values given in the figure, R3 cannot be determined.
What?!!
I can solve this while sleeping.

#### Ramussons

Joined May 3, 2013
1,418
Maybe I should highlight my point.

From the values given in the figure, R3 cannot be determined.

...... Also, the current total is supposedly 10ma ....

#### ericgibbs

Joined Jan 29, 2010
19,122
Hi R,
Consider 10v supply at 10mA, what is the total circuit resistance.?
E

#### Ramussons

Joined May 3, 2013
1,418
Hi R,
Consider 10v supply at 10mA, what is the total circuit resistance.?
E
That 10 mA is not part of the figure. It is on a footnote somewhere else.

#### Dodgydave

Joined Jun 22, 2012
11,367
From the values given in the figure, R3 cannot be determined.
As you Americans say " this is electronics 101" (whatever that means)

10V @ 10 mA is 1K resistance, so R1 and R2 make 500 ohms , therefore R3 and R4 make 500 ohms also, so R3 is ?...