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Potential Difference and Resistors

  • #1

Homework Statement



Suppose the three resistors in the circuit have the values R1 = 180, R2 = 220, and R3 = 350, and that the emf of the battery is 18.0 V
http://session.masteringphysics.com/problemAsset/1126258/3/5416321031_128.jpg

Find the potential difference across each resistor.

Find the current that flows through each resistor.



Homework Equations



V=IR

The Attempt at a Solution



These things really confuse me, but here is my shot:
V=IR
Itotal=18/(1/180+1/220+1/350)=.233

I also know that due to the loop rule, the sum of all of the potential differences is 0. Because of the junction rule, the current flowing into a junction must equal what's flowing out. I'm just not sure how I can figure out either the V or I across a resistor if I have two unknowns...

any help would be awesome ! :)
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
4,433
5
Itotal=18/(1/180+1/220+1/350)=.233
This is wrong.
R2 and R3 are in series and R1 is in parallel with that combination.
 
  • #3
Oh, wow, that definitely was wrong. Stupid mistake because I'm frustrated with this stuff I guess. Itotal=136.8

The problem is more of I'm not sure what to do next though
 
  • #4
rl.bhat
Homework Helper
4,433
5
18 V is common across R1 and (R2 +R3) combination.
So I1 = 18/R1
And I2 = 18/(R2+R3)
 
  • #5
So Voltage is the same throughout all circuits? Or just in parallel circuits? I'm more looking for the concept of the whole thing more than the answer right now. Thanks for your help so far!
 
  • #6
Also, when it wants V1,V2,V3 and I enter 18,18,18, that is not correct.
When it wants I1, I2, I3 (In mA) and I enter 100, 81.8, 51.4, that also is not correct.
 
  • #7
rl.bhat
Homework Helper
4,433
5
So Voltage is the same throughout all circuits? Or just in parallel circuits? I'm more looking for the concept of the whole thing more than the answer right now. Thanks for your help so far!
In the parallel combination the voltage is common across each branch.
In each branch current may be different.
In a branch if there more than one resistance, then
V = V1+ V1 + ......
In the given problem
I1 = V/R1
I2 = V/(R2 + R3).
V2 = I2*R2
V3 = I2*R3
 
  • #8
Oh, right on man. Thanks for your help! I'm pretty sure I understand now.
 

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