Current Carrying Wires and Magnetic Fields

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Current Carrying Wires and Magnetic Fields

Post by ragtaguzoo » Fri May 22, 2020 11:46 am

I had a question about magnetic fields and current carrying wires. I get that using the right hand rule, you get repulsive forces for wires with currents in the opposite directions. However, I tried to doing the right hand rule for wires with currents running in the same direction, but I can’t get them to point towards each other in as an attraction? They both just point up when I use the rule. For the rule, thumb = v, fingers = B, and palm = force. Maybe you tell where I went wrong. I put an image below of my work. Thanks.
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Re: Current Carrying Wires and Magnetic Fields

Post by NS_Tutor_Mathias » Fri May 22, 2020 8:19 pm

For general reference/to maybe read first: ... irfor.html

For predicting forces:
You've predicted magnetic fields, not forces. The magnetic field generated by wire 1 will generate a Lorentz force on the electrons in wire 2 and vice versa. These forces attract. Slim, but present, chance that this will show up in a passage on the MCAT.

For predicting magnetic fields:
"This pattern indicates... 2 dimensional thinking"

You are using the right hand rule that describes the relation between the direction of travel of an electron in the magnetic field, the direction of the magnetic field lines intersecting that electron's path, and the direction of the force deflecting the electron.

What you want however, is an expression that predicts the magnetic field generated by a current in a wire. In reality, these share a cause (Maxwell's equations/fundamental property of electromagnetism), and so the confusion is understandable - this is less a fault of yours, and more a fault with the minimal amount of depth with which the MCAT approaches these topics, making both the relations and distinctions between these two scenarios much trickier to grasp.

In a nutshell, without providing any extraneous information: You want to use the RHR involving curving your hand, to represent the actual circular/cylindrical magnetic field around the wire.

As for why this works out exactly as an attractive force:
I'm assuming you have this a little bit figured out by now, but if not - go back and check what you know now using the correct RHR about the magnetic fields in the two wires. The field from wire 1 on 2 will point down where it is actually acting on wire 2, and the field from wire 2 on wire 1 will point up (or vice versa, depending on current direction). Consequently, the Lorentz force on the electrons in each wire go in opposite directions - namely, right into each-other.
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