Negative current?

  • Thread starter Sastronaut
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  • #1
Sastronaut
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Homework Statement



Calculate the current through R3.

Homework Equations



V=IR

The Attempt at a Solution



my attempt is attached. My question revolves around my answer...is a negative current a legitimate answer? What does it mean?
 

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Answers and Replies

  • #2
lewando
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It is a legitimate answer provided you show a direction arrow (which you have done). It means that the current is not travelling in the direction indicated by your direction arrow. It is going in the opposite direction.
 
  • #3
lewando
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Looking at your loop equations, they seem a bit suspect. If you are summing the changes in voltage across each element along a loop, equating the sum to zero, you should get something different.
 
  • #4
ehild
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Homework Statement



Calculate the current through R3.

Homework Equations



V=IR

The Attempt at a Solution



my attempt is attached. My question revolves around my answer...is a negative current a legitimate answer? What does it mean?

Your equations 1 and 2 are wrong. Note, that the potential decreases in the direction of the current.

ehild
 
  • #5
Sastronaut
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What should my equation for loop 1 be?
 
  • #6
ehild
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You start at the negative pole of the battery, and follow the change of potential in the direction, indicated by the green arrow. The potential increases by 5 V traversing the battery. Then it decreases by I3R3 along R3. Then again, it decreases by R1I1, when you return to your initial position. The net change of potential is zero: 5-I3R3-I1R1=0.
Do the same for the other loop. Note that going in direction opposite to the current, the potential would increase.

ehild

attachment.php?attachmentid=61707&d=1378954564.jpg
 
  • #7
Sastronaut
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okay I think I am following you so for loop 2 the equation is then -10V+I3R3+I2R2=0?
 
  • #8
ehild
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okay I think I am following you so for loop 2 the equation is then -10V+I3R3+I2R2=0?

Yes.

ehild
 

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