I am having some difficulties solving a combination circuits problem

  • #1
EddyBenzen122
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Homework Statement:
The current in the 8 - ohm resistor in the drawing is 5 A. Find the current in the 20 ohm and 9 - ohm resistor.
Relevant Equations:
V = IR
I'm having difficulties finding the current for a 9-ohm resistor only, not the 20-ohm

My attempt was to find the voltage of a 20ohm resistor which is V = (0.75A)(20-ohm) = 15V. I got the 0.75A by adding the currents of the two resistors (8-ohm and 16-ohm) together that were in parallel and it becomes the current that will flow through the 20-ohm resistor. After getting 15V, I don't know what to do next in order to find the current in the 9 - ohm resistor.
Also, look at the image for more information.
 

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  • #2
haruspex
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Homework Statement:: The current in the 8 - ohm resistor in the drawing is 5 A. Find the current in the 20 ohm and 9 - ohm resistor.
Relevant Equations:: V = IR

I'm having difficulties finding the current for a 9-ohm resistor only, not the 20-ohm

My attempt was to find the voltage of a 20ohm resistor which is V = (0.75A)(20-ohm) = 15V. I got the 0.75A by adding the currents of the two resistors (8-ohm and 16-ohm) together that were in parallel and it becomes the current that will flow through the 20-ohm resistor. After getting 15V, I don't know what to do next in order to find the current in the 9 - ohm resistor.
Also, look at the image for more information.
What other voltages can you determine?
 
  • #3
EddyBenzen122
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What other voltages can you determine?
4Volts from the parallel circuits with an 8-ohm and 16 - ohm
 
  • #4
haruspex
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4Volts from the parallel circuits with an 8-ohm and 16 - ohm
So what is the voltage across the ##9 \Omega ##|| ##18 \Omega## pair?
 
  • #5
EddyBenzen122
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So what is the voltage across the ##9 \Omega ##|| ##18 \Omega## pair?
that's what I'm looking for in order to find its current next.
 
  • #6
haruspex
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that's what I'm looking for in order to find its current next.
What is the relationship between the voltages:
- across the 9Ω|| 18Ω pair
- across the 8Ω|| 16Ω pair
- across the 20Ω resistor
?
 
  • #7
EddyBenzen122
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What is the relationship between the voltages:
- across the 9Ω|| 18Ω pair
the voltages are the same for both resistors
- across the 8Ω|| 16Ω pair
the voltages are the same for both resistors
- across the 20Ω resistor
?
the voltage is the sum of the currents for the 8-ohm resistor and the 10-ohm resistor multiplied by the 20-ohm resistor.
 
  • #8
haruspex
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the voltages are the same for both resistors
I didn't ask how the voltage across one of a pair compared with the voltage across the other of the pair. I asked about the relationship between the voltage across the 9Ω|| 18Ω pair (taken as a pair), the voltage across the 8Ω|| 16Ω pair (taken as a pair) and the voltage across the 20Ω resistor. Three voltages, one relationship.
the voltage is the sum of the currents for the 8-ohm resistor and the 10-ohm resistor multiplied by the 20-ohm resistor.
I have no idea how you arrive at that.
 
  • #9
EddyBenzen122
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I have no idea why you're asking me about the relationships between them, are you just giving me a hint?
 
  • #10
haruspex
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I have no idea why you're asking me about the relationships between them, are you just giving me a hint?
Of course I am giving a hint!

Call the voltage at the right of the 20 Ohm resistor 0.
So what is the voltage at the right of the 9||18 pair?

What is the voltage at the left of the 20 Ohm?
So what is the voltage at the left of the 8||16 pair?
So what is the voltage at the left of the 9||18 pair?
 
  • #11
EddyBenzen122
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Of course I am giving a hint!

Call the voltage at the right of the 20 Ohm resistor 0.
So what is the voltage at the right of the 9||18 pair?
What is the voltage at the left of the 20 Ohm?
So what is the voltage at the left of the 8||16 pair?
So what is the voltage at the left of the 9||18 pair?
K, i got the voltages: 4volts for 8||16 pair, 15V for 20ohm, and 19V for 9||18 pair.
Are they correct?
 
  • #12
haruspex
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K, i got the voltages: 4volts for 8||16 pair, 15V for 20ohm, and 19V for 9||18 pair.
Are they correct?
Yes.
 
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  • #14
EddyBenzen122
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Yes.
Thanks for helping me, I really appreciate it!
 

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