Magnetic interaction of wire and rectangular loop

[Prinsen]

Hi. In this video:


he ignores the magnetic field from current 2(I2). Could anyone give me a good explanation why?

 

[HallsofIvy]

The magnetic fields from two wires in opposite directions cancel. He says that in the video.

 

[Prinsen]

Thanks, but when in the video does he say that? I can't seem to find it.

 

[davenn]

Thanks, but when in the video does he say that? I can't seem to find it.

between 2:15 - 2:20

just keep in mind that he is interested in the effect of the magnetic field generated by I1 on the loop
the magnetic field generated by the loop current is nulled out

 

[Prinsen]

Thanks! sorry for the late response, but.. why is the magnetic field generated by the loop current nulled out?

 

[davenn]

because the fields ( the ones generated by the current flowing in the loop) in the 4 sides of the loop are equal and opposite

that is ... the field in the left long side is equal and opposite to the one in the right long side, so those 2 cancel out
and the one in the top short side is equal and opposite to the one in the bottom short side, and again those 2 cancel out

This leaves only the effect of the magnetic field generated by the straight wire has on the loop
again, the one in the top short side is equal and opposite to the one in the bottom short side so there is no
force on the top and bottom sections

BUT

there is a difference in force on the 2 side sections from the straight wire generated field.
The left side ( the one closest to the straight wire) is subject to a stronger force than the right side
as he shows with the larger arrow pointing to the left than to the right.
This causes the loop to move sideways to the left ( towards the straight wire),

Dave