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KOSS
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This question comes under the familiar topic "force on a current carrying wire in a magnetic field".
You all know the Lorentz force and [itex]F=LI\times B[/itex]. So I'm teaching a high school class on this and a senior teacher tells me the students will need to explain the force on the wire for their exam in terms of the modified B field lines (due to sum of B fields of the current in the wire plus the magnet). I could not teach this because I did not know how to explain it.
Here's the sketch in words: B field N to S horizontal (left to right say). Current in wire coming at us out of page. RH Grip Rule field around the wire anticlockwise. This adds to the magnet field. Result: Field lines above wire partially cancel. Field lines below wire reinforce. High field below the wire, weak field above it.
From what I could make out the textbook seemed to be trying to explain the upwards motion of the wire being due to movement of the wire from "high magnetic field region to low magnetic field region". Almost as if it were a magnetic field pressure effect.
But I've always thought this force on a wire carrying a current in a B field was just simply the Lorentz force on the charges. How can magnetic pressure or "movement from high to low field intensity" be the cause? Is it additional to the Lorentz force? Is it different and does it dominate the Lorentz force?
I can see that the field around the wire is not uniform because of the superposition of the magnet's field wit the current's field, but is the lifting of the wire still not just the plain Lorentz force?
Any enlightenment would be welcome.
PS. I'll post the textbook text and figure in a mo' once I scan it.)
You all know the Lorentz force and [itex]F=LI\times B[/itex]. So I'm teaching a high school class on this and a senior teacher tells me the students will need to explain the force on the wire for their exam in terms of the modified B field lines (due to sum of B fields of the current in the wire plus the magnet). I could not teach this because I did not know how to explain it.
Here's the sketch in words: B field N to S horizontal (left to right say). Current in wire coming at us out of page. RH Grip Rule field around the wire anticlockwise. This adds to the magnet field. Result: Field lines above wire partially cancel. Field lines below wire reinforce. High field below the wire, weak field above it.
From what I could make out the textbook seemed to be trying to explain the upwards motion of the wire being due to movement of the wire from "high magnetic field region to low magnetic field region". Almost as if it were a magnetic field pressure effect.
But I've always thought this force on a wire carrying a current in a B field was just simply the Lorentz force on the charges. How can magnetic pressure or "movement from high to low field intensity" be the cause? Is it additional to the Lorentz force? Is it different and does it dominate the Lorentz force?
I can see that the field around the wire is not uniform because of the superposition of the magnet's field wit the current's field, but is the lifting of the wire still not just the plain Lorentz force?
Any enlightenment would be welcome.
PS. I'll post the textbook text and figure in a mo' once I scan it.)
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