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Induced Current GRE Problem

  • #1
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Homework Statement


http://grephysics.net/ans/9277/54

I am confused about this. The explanation for why the induced current is clockwise is because the induced current would resist the change in magnetic flux, therefore work to increase the magnetic field. But if the current induced in the wires is in the same direction as the original current, I use the right hand rule and it seems as if the magnetic field's will work to cancel eachother out.

I'm picturing two circles going around each wire in the same direction (magnetic field) and between the wires one magnetic field will be going up while the other goes down.
Where is my thinking going wrong here?

Edit:
Is it because if the current is moving in the same direction, there will be an attractive force and therefore it will fight to increase the magnetic field? I feel like this logic isn't right because everyone else seems to be deducing the fact that it is going clockwise and then use that information to decide which way the forces are pulling on each side
 
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Answers and Replies

  • #2
rude man
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Homework Statement


http://grephysics.net/ans/9277/54

But if the current induced in the wires is in the same direction as the original current, I use the right hand rule and it seems as if the magnetic field's will work to cancel eachother out.
That statement makes no sense. The induced current will be in the same direction as that of the long wire in one side of the loop and obviously will be in the opposite direction in the other side.

What direction of loop current will increase the flux thru the coil? (Use the right-hand rule).

As to why E is right and D is wrong, you can also solve this dilemma by remembering that for parallel wires, same-direction currents imply attraction while opposite-direction currents imply repulsion.
 
  • #3
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That statement makes no sense. The induced current will be in the same direction as that of the long wire in one side of the loop and obviously will be in the opposite direction in the other side.

What direction of loop current will increase the flux thru the coil? (Use the right-hand rule).

Oops I meant the side of the loop closest to the original wire (not both of them).

As for what direction of loop current will increase the flux thru the coil? And I should use the right hand rule. I'm so bad at this but I really want to understand. It seems like no matter what way the current runs and I point my thumb my hand will curve through the loop one way or another? I want to to make it so the way my hand curls will make the part of the loop close to the original wire compliment eachother? If so it seems like it going counter clockwise would achieve this ;-(
 
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  • #4
rude man
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That statement makes no sense. The induced current will be in the same direction as that of the long wire in one side of the loop and obviously will be in the opposite direction in the other side.

What direction of loop current will increase the flux thru the coil? (Use the right-hand rule).

QUOTE]
Oops I meant the side of the loop closest to the original wire (not both of them).

As for what direction of loop current will increase the flux thru the coil? I'm pointing my thump up and curving my hand and then doing the same thing with my thumb down, but i'm not seeing how to tell which one increases the flux.. it seems like they both will make a magnetic field going through the coil, it seems to me like clockwise will have a greater effect in canceling out the magnetic field from the original wire. Idk help ;-(
Curl your fingers in one direction. That's the direction of the current. Then the thumb points in the direction of the B field. Now, which direction is the B field due to the long wire? You want to point B due to the loop to point in the direction of the B field set up by the long wire, right?
 
  • #5
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Curl your fingers in one direction. That's the direction of the current. Then the thumb points in the direction of the B field. Now, which direction is the B field due to the long wire? You want to point B due to the loop to point in the direction of the B field set up by the long wire, right?
Okay, see the way I was taught this right hand rule was to point my thumb in the direction of the current, then my fingers curl around and create the magnetic field to show if it is going in or out of the page. I feel like if I use this method and point my thumb up for the part of the wire closer to the original wire, the B field is coming out of the page where the original wire is making a B field go INTO the page. I didn't think of B field as pointing in a direction in the same plane as the wire? if that makes sense
 
  • #6
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Now, which direction is the B field due to the long wire? You want to point B due to the loop to point in the direction of the B field set up by the long wire, right?
I do want to point B due to the loop in the direction of the B field set up by the long wire, but since the loop has parts of the current going each ways, do I only focus on the part of the loop that's closest since that will have the greatest effect?
 
  • #7
rude man
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Okay, see the way I was taught this right hand rule was to point my thumb in the direction of the current, then my fingers curl around and create the magnetic field to show if it is going in or out of the page. I feel like if I use this method and point my thumb up for the part of the wire closer to the original wire, the B field is coming out of the page where the original wire is making a B field go INTO the page. I didn't think of B field as pointing in a direction in the same plane as the wire? if that makes sense
No, if you point your thumb up for the closer-in wire, the fingers curl INTO the page. So the B field points into the page.
 
  • #8
rude man
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I do want to point B due to the loop in the direction of the B field set up by the long wire, but since the loop has parts of the current going each ways, do I only focus on the part of the loop that's closest since that will have the greatest effect?
If you examine the geometry carefully you will see that the B field points into the page for both sides. Remember that the current reverses direction when you go from one side to the other.
 
  • #9
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If you examine the geometry carefully you will see that the B field points into the page for both sides. Remember that the current reverses direction when you go from one side to the other.
And if the current went counter clockwise, when I curl my fingers in the direction of the current my palm still points into the coils?
 
  • #10
rude man
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And if the current went counter clockwise, when I curl my fingers in the direction of the current my palm still points into the coils?
What's with the palm? Your thumb will point OUT OF the page.
 

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