# Calculating I4 in Combination Circuit

#### aeln269

Joined Oct 18, 2020
31

Ive tried two methods. The one in the textbook using I4=(R3/R3+R4)I2 and an online method from this video
. Ive followed both to a T and still it marks me wrong. Ive been working on this problem for nearly 2 hours now and cant get it. My initial answer was 0.53 using the method from the book and using the video I got 0.067. What am I doing wrong? Im losing my mind.

#### ericgibbs

Joined Jan 29, 2010
19,076
hi 269,
E

#### aeln269

Joined Oct 18, 2020
31
Hi again Eric,

#### Attachments

• 1.1 MB Views: 5
• 1,008.4 KB Views: 5
• 1.2 MB Views: 5
Last edited by a moderator:

#### aeln269

Joined Oct 18, 2020
31

#### ericgibbs

Joined Jan 29, 2010
19,076
hi 269,
The images should now be visible, please in future post all images to one post.
E

#### ericgibbs

Joined Jan 29, 2010
19,076
hi 269.
For the problem, first calculate and post the value of 3 off 15R resistors in parallel.

OK.
E

#### aeln269

Joined Oct 18, 2020
31
thank you, ill try to in the future but im not too great with computers. how do I put the photos all into one post?

#### ericgibbs

Joined Jan 29, 2010
19,076
Hi,
OK,
Answer the question in post #16.

E
I will explain later ref the posting method,

#### aeln269

Joined Oct 18, 2020
31
1/RT = 1/15 + 1/15 + 1/15

1/RT = 3/15

RT = 15/3

RT = 5 for the 3 parallel 15 ohm resistors

#### ericgibbs

Joined Jan 29, 2010
19,076
hi,
So what is the current in the series 100R and 5R.?
E

#### aeln269

Joined Oct 18, 2020
31
I = V/R so I = 24/105 which is 0.23 I think

#### ericgibbs

Joined Jan 29, 2010
19,076
hi,
So what is the Voltage at the junction of the 100R and 5R.?
E

#### aeln269

Joined Oct 18, 2020
31
Im not sure what you mean

The voltage is 24? I think?

#### ericgibbs

Joined Jan 29, 2010
19,076
hi,
You say you have 230mA [ 0.23A] flowing thru the 100R and 5R in series, so what is the voltage drop across the 100R.?
E

Hint: V=I * R

#### aeln269

Joined Oct 18, 2020
31
Ill be honest I dont remember how to calculate voltage drop though we did it a few weeks ago let me try and find information for it

#### ericgibbs

Joined Jan 29, 2010
19,076
hi.
Hint: V=I * R

E

#### aeln269

Joined Oct 18, 2020
31
So ,

V=IR ?

100 x 0.23 = 23

#### aeln269

Joined Oct 18, 2020
31
Oh I did that earlier I just didn't know it was called voltage drop

#### ericgibbs

Joined Jan 29, 2010
19,076
hi,
OK,
So you know the battery voltage is 24V and you are dropping 23V across the 100R, so what voltage is at the top end of R4.?
E