# When I do I=V/R for the current in this parallel resistor, I get a different answer..Watch

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#1
Here is the video I am watching:

For the current just after the 40 ohm resistor in parallel, he gets it to be 10 ohms, and his explanation is that it is 10 ohms because the total 20A current splits evenly between two resistors of two equal resistances, which I agree with.

However, when I use maths to get the answer, using I=V/R, I=12V/40ohms=0.3A , which is wrong, because the total circuit current is only 0.2A.

Why am I going wrong when doing maths to find the answer? My teachers explanations for resistors in parallel used the same method I have used, and this website has used: http://physics.bu.edu/py106/notes/Circuits.html (I have attached an image of this method to this post too).

Please can you tell me where I am going wrong with the maths? I don't get it.
Attachment 603246603250
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2 years ago
#2
(Original post by blobbybill)
Here is the video I am watching:

For the current just after the 40 ohm resistor in parallel, he gets it to be 10 ohms, and his explanation is that it is 10 ohms because the total 20A current splits evenly between two resistors of two equal resistances, which I agree with.

However, when I use maths to get the answer, using I=V/R, I=12V/40ohms=0.3A , which is wrong, because the total circuit current is only 0.2A.

Why am I going wrong when doing maths to find the answer? My teachers explanations for resistors in parallel used the same method I have used, and this website has used: http://physics.bu.edu/py106/notes/Circuits.html (I have attached an image of this method to this post too).

Please can you tell me where I am going wrong with the maths? I don't get it.
Attachment 603246603250
The battery voltage is across the whole circuit which as you say produces a total current of 0.2A.

This total current must flow through the series resistor (on its own) hence the p.d. developed across that single resistor is:

Vseries = Itotal Rseries = 0.2 x 40 = 8V

The battery voltage is dropped around the circuit so p.d. dropped across the parallel combination must be:

Vparallel = Vbattery - Vseries = 12 - 8 = 4V

i.e. the p.d. developed across the parallel pair is 4V

Then to calculate the current in each of the parallel resistors:

I = Vparallel / Rparallel

I = 4 / 40 = 0.1A

This is the current that flows in each of the parallel arms.

Which means the total series current splits equally (0.2A = 0.1A + 0.1A) at the parallel junction and then recombined at the return junction 0.1A + 0.1A = 0.2A

(NB the parallel combination resistance when replaced by their equivalent series resistance is 40 parallel 40 = 20 Ohms.)

i.e. if the equivalent parallel resistance is used, then the circuit becomes two series resistors (20 + 40 = 60 Ohms) and once again because it's now all series, the total current is used in this instance.
0
2 years ago
#3
(Original post by blobbybill)
Here is the video I am watching:

For the current just after the 40 ohm resistor in parallel, he gets it to be 10 ohms, and his explanation is that it is 10 ohms because the total 20A current splits evenly between two resistors of two equal resistances, which I agree with.

However, when I use maths to get the answer, using I=V/R, I=12V/40ohms=0.3A , which is wrong, because the total circuit current is only 0.2A.

Why am I going wrong when doing maths to find the answer? My teachers explanations for resistors in parallel used the same method I have used, and this website has used: http://physics.bu.edu/py106/notes/Circuits.html (I have attached an image of this method to this post too).

Please can you tell me where I am going wrong with the maths? I don't get it.
Attachment 603246603250
Long story short: The circuit has two 40 ohms resistors in parallel and they are in series with another 40 ohms resistor.

What you have shown (all resistors are in parallel) is NOT equivalent to the problem (parallel setup + series setup) you are solving.
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