Help on question 25 from BR Physics Section VIII (electromagnetism)

[virtuoso735]

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I'm having trouble understanding question no. 25 from BR's physics section VIII (electrostatics and electromagnetism). The question is the last of the 25 review questions directly after the end of each chapter. I'm not sure if I'm allowed to type the question out word for word, but it goes something like this:

Two parallel wires, each of which are carrying currents in the same direction, should attract each other. Why?

The explanation says something about the right hand rule, but I'm not really sure how to use the right hand rule with this. I know that the thumb points in the direction of the current and the fingers wrap around, showing the direction of the B field, but which way does the force go in each case? Should each wire exert a force on the other going in the same direction in each case? How do I use the right hand rule in this case to figure out the direction of the forces? The book says that the forces created by each current push the two wires together, but I'm not sure how to figure that out.

Thanks for the help.

 

[Ssina]

I'm having trouble understanding question no. 25 from BR's physics section VIII (electrostatics and electromagnetism). The question is the last of the 25 review questions directly after the end of each chapter. I'm not sure if I'm allowed to type the question out word for word, but it goes something like this:

Two parallel wires, each of which are carrying currents in the same direction, should attract each other. Why?

The explanation says something about the right hand rule, but I'm not really sure how to use the right hand rule with this. I know that the thumb points in the direction of the current and the fingers wrap around, showing the direction of the B field, but which way does the force go in each case? Should each wire exert a force on the other going in the same direction in each case? How do I use the right hand rule in this case to figure out the direction of the forces? The book says that the forces created by each current push the two wires together, but I'm not sure how to figure that out.

Thanks for the help.

At first, try to get the magnetic field lines for both wire when they are paralel, next to each other, and moving the same direction (e.g. toward the right).... for this use your thumb in the direction of the current and your fingers curl around the wire. you can see that in the middle, the filed is out of the page for one wire, and into the page for another.... so they cancel each other out (this is only in the middle)

Now work with only one wire (e.g. the bottom wire)... and use FBI (right hand rule)... if the current is going to the right.... then thumb goes toward the right, index finger into the page (since the bottom wire on the outside has it's magnetic field, B, pointing into the page) and you will see your force is pointing upwards... toward the top wire. You can do the same for the top wire, and the force will be pointing downwards, toward the bottom wire.

here is a pic I found online:

 

[virtuoso735]

I'm not sure how you used the right-hand rule in the example. I get that your thumb goes in the direction of the current and the fingers wrap in the direction of the magnetic field, but how did you determine the direction of the force?

 

[Ssina]

I'm not sure how you used the right-hand rule in the example. I get that your thumb goes in the direction of the current and the fingers wrap in the direction of the magnetic field, but how did you determine the direction of the force?

I don't know how you were thought the right hand rule, some people teach it using the 3 fingers while others using 2 fingers and palm of the hand... but I learned it using 3 fingers, FBI:

going back to the first picture, on the bottom wire:
the current, I or thumb, is pointing toward the right
the magnetic field, B or index finger, is pointing into the page/away from you
so if you orient your fingers this way, the force has to be pointing upwards, toward the top wire. You can do the same for top wire, and the force will be pointing downwards... so the wires are attracted to each other.

 

[ilovemedi]

I'm incredibly confused on this question still.

The issue is the question doesn't say anything about whether the magnetic field the current creates is external or internal... just "two || wires carry currents in same direction". If i had that information, I understand the problem via the second RHR.. but how do i know if its external or internal for each of the wires? Yes, thumb = current, then my fingers curl in.. but that's the same for the other wire, too..

edit

so correct me if i'm wrong, but say we focus on one wire.. and curl the hands towards one direction.. so it'll hit the second wire and go INWARD. so B = inward; then for the SECOND wire, we do the same, except the B is hitting the second wire in OUTWARD direction... really weird to visualize... is that correct?

 

[howlovely]

BR is probably talking about the equation F = IL x B (the magnetic force = current*length x B-field strength). Notice that the equation involves a cross product, which means that the Right Hand Rule can be used. If you point your fingers in the direction of the current in the wire and your thumb in the direction of the B-field (which can be found by pointing your thumb in the direction of the current and wrapping your fingers around the wire), then the direction you push with your palm is the direction of the magnetic force.

The forces created are external forces, as a moving current generates a B-field. In addition, the force from the first wire's B-field on the second wire will be equal the the force from the second wire's B-field on the first wire.

 

[ilovemedi]

So is my reasoning correct?

 

[inasensegone]

I'm bumping this to post a link to a youtube video that explains this (in case anyone else has the same issue in the future). TBR didn't cover current and the induced magnetic field in this chapter (8), so I'm not sure how we were expected to know this. My electromagnetism knowledge is super rusty and I was never good at it in the first place so I couldn't figure it out from the chapter alone. Maybe someone else will find this useful.

Couple things:
1. Current in a wire, by convention, is the movement of positive charge.
2. Movement of charge creates a magnetic field as explained in this video. (RHR: point your thumb in the direction of the current and wrap your fingers around the wire. Basically the magnetic field will be circular, coming out of the page to the left of the current when it points up, and into the page to the right of the current when it points up. The video illustrates this. I didn't watch more than a couple minutes because this is explained in the beginning of the video, so I don't know how good the rest of the clip is)
3. The rest can be figured out from information that was covered in this TBR chapter.

Here is the problem and explanation from TBR. Solely utilizing information from the chapter, there is no way to deduce this answer, or to reason it out.

Keywords for future searches: TBR The Berkeley Review Physics Chapter Ch 8 Review Question 25 page 146

(Now I know what this person was talking about)