# Calculating Missing Resistor Value In Parallel Circuit

Thread Starter

#### aeln269

Joined Oct 18, 2020
31
Hi everyone, I'm stuck on a homework problem for my Electronics 101 course. My teacher has not responded to any emails and has canceled office hours 5 weeks as well as 4 classes this semester so I haven't had any luck getting help from him. Anyway, this is the question.

Given the circuit below, what is the value of R1?

Because I cant figure out how to copy a photo over, I will describe the circuit to the best of my ability.
Parallel with 3 resistors.
R1 = ?
R2 = 5.1kOhms
R3 = 10kOhms
Vs = 24.0V
10mA

This is the furthest I have gotten so far (with minor variations through trial and error)

I attempted to solve for I2 & I3 through Ohms Law for their individual values (I2=V/R2) & (I3=V/R3) which resulted in 4.7ma & 2.4ma
Then, I subtracted these two values from It (10ma) which gave me 2.9 as I1
I used R=V/I to solve for its resistor value R=24V/2.9mA (0.0029A)
Which gave me 8,275.86 which was incorrect.

Please advise. Thank you for your time.

#### ericgibbs

Joined Jan 29, 2010
18,651
Hi 269
Welcome to AAC.
Is this your circuit.?

E
Use the Attach Files Button to load an image.

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Thread Starter

#### aeln269

Joined Oct 18, 2020
31

#### ericgibbs

Joined Jan 29, 2010
18,651
hi 269,
What is the value of R2 and R3 in parallel.?
E

Thread Starter

#### aeln269

Joined Oct 18, 2020
31
Hi Eric,

Its rather late so I skipped over the tool bar, sorry about that. Ive taken a screenshot and uploaded it for you to see. I believe it is the same as you have posted.

Thread Starter

#### aeln269

Joined Oct 18, 2020
31
Hi Eric,

Thanks for getting back so quickly. I believe they are 4.7ma & 2.4ma

#### ericgibbs

Joined Jan 29, 2010
18,651
hi 269,
What is the Resistance value of R2 and R3 in parallel.?
E

Thread Starter

#### aeln269

Joined Oct 18, 2020
31
I think I see my mistake. Would I be solving for G2 and G3? 1/R2 & 1/R3?

#### ericgibbs

Joined Jan 29, 2010
18,651
Hi,
I was checking to see if you knew Ohms Law.

Using your current method of 4.7mA and 2.4mA,
what do you make the Sum of those two current values.?
and
10mA minus that Sum = ?

E

Last edited:
Thread Starter

#### aeln269

Joined Oct 18, 2020
31
To get the I2 and I3 values, I used I = V/R in accordance to Ohms Law

I subtracted 4.7ma & 2.4ma from 10ma resulting in 2.9ma

I then used R=V/I R=24/0.0029

=8,275.86

#### ericgibbs

Joined Jan 29, 2010
18,651
hi,

Looks OK to me.
E

BTW:
When giving an answer add the units suffix ie: 8275 Ohms or 8.275K, also in your written equations use 24v/0.0029A = 8275 Ohms

Thread Starter

#### aeln269

Joined Oct 18, 2020
31
It was marked as incorrect on the classwork again, maybe its an error on his end?

#### ericgibbs

Joined Jan 29, 2010
18,651
hi,
Did your tutor say what he thought the answer should be.?
E

Recheck your calculations. 4.7ma , you have dropped some decimal places.
E

Last edited:
Thread Starter

#### aeln269

Joined Oct 18, 2020
31
Its an assignment on canvas where they input the answer/ variations of it. It doesn't not state the correct answer unfortunately. I will try to email the teacher and will get back to you once I do. This would not be the first time he's input incorrect answers on our assignments.

#### ericgibbs

Joined Jan 29, 2010
18,651
hi 269,
What you have done incorrectly is round up the 4.7mA

Repost the 24V/5100R value with all the decimal places.

E

#### MrAl

Joined Jun 17, 2014
11,268
To get the I2 and I3 values, I used I = V/R in accordance to Ohms Law

I subtracted 4.7ma & 2.4ma from 10ma resulting in 2.9ma

I then used R=V/I R=24/0.0029

=8,275.86
That result looks a little bit off. Try for a more precise value.
Check your result by calculating all three in parallel then see if you get 10ma.

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