Physics Help Forum Direction of Magnetic Field

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 Apr 8th 2018, 08:05 PM #1 Junior Member   Join Date: Apr 2018 Posts: 3 Direction of Magnetic Field Hello, I'm a bit confused on determining the direction of the magnetic field in the attached image. From what I understand, for the top wire, the magnetic field points into the page. For the bottom wire, the magnetic field points out of the page. But what is the direction of the magnetic field at point P? Can someone explain this to me? Thank You Attached Thumbnails
Apr 8th 2018, 08:52 PM   #2

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 Originally Posted by ktjj4 Hello, I'm a bit confused on determining the direction of the magnetic field in the attached image. From what I understand, for the top wire, the magnetic field points into the page. For the bottom wire, the magnetic field points out of the page. But what is the direction of the magnetic field at point P? Can someone explain this to me? Thank You
The "right hand rule" predicts that the magnetic field from both wires points into the page at point B. See here, and go to the section "Ampere's right hand grip rule.

-Dan
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Apr 9th 2018, 09:18 AM   #3
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 Originally Posted by topsquark The "right hand rule" predicts that the magnetic field from both wires points into the page at point B. See here, and go to the section "Ampere's right hand grip rule. -Dan

And just to make sure I'm doing this correctly:

For the top wire, my thumb points into the page so that means that the magnetic field goes into the page in the area enclosed by the wire and out above the top wire

For the bottom wire, my thumb points out the page so that means that the magnetic field goes out of the page below the bottom wire and into the page in the area enclosed by the wire

Did I get this right?

Apr 9th 2018, 11:43 AM   #4

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 Originally Posted by ktjj4 Thank You for replying And just to make sure I'm doing this correctly: For the top wire, my thumb points into the page so that means that the magnetic field goes into the page in the area enclosed by the wire and out above the top wire For the bottom wire, my thumb points out the page so that means that the magnetic field goes out of the page below the bottom wire and into the page in the area enclosed by the wire Did I get this right?
I'm sorry, I had the direction of the current in the bottom wire to go to the left. My previous result is incorrect.

The thumb points in the direction of the current. So the top wire has your hand in the positive x-direction (to the right) and the fingers curl such that the field points downward inside the circle. The bottom wire comes out of the page.

$\displaystyle B = \frac{\mu _0 I}{2 \pi r}$

Thus if $\displaystyle I_1 > I_2$ the magnetic field at the center will come out of the page.

-Dan
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Apr 9th 2018, 12:29 PM   #5
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 Originally Posted by topsquark I'm sorry, I had the direction of the current in the bottom wire to go to the left. My previous result is incorrect. The thumb points in the direction of the current. So the top wire has your hand in the positive x-direction (to the right) and the fingers curl such that the field points downward inside the circle. The bottom wire comes out of the page. $\displaystyle B = \frac{\mu _0 I}{2 \pi r}$ Thus if $\displaystyle I_1 > I_2$ the magnetic field at the center will come out of the page. -Dan
This clears up everything, Thank You!!

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