Straight wires carrying DC currents

AI Thread Summary
Two straight wires carrying DC currents will not experience a mutual magnetic force when they are oriented perpendicularly and infinitely long, as the forces on each half of the wire cancel out. When currents flow in the same direction, the wires attract each other, while opposite directions result in repulsion. The net force is determined by the relative direction of the currents, with parallel currents attracting and anti-parallel currents repelling. In the case of perpendicular wires, a torque is generated that attempts to align the wires parallel to each other, but no net linear force acts on them. This phenomenon is akin to the operation of an electric motor, where torque is present without linear force.
fisico30
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Straight wires carrying DC currents...

Dear Forum,

when will two straight wires, both carrying a DC current, not feel any mutual magnetic force?

What reciprocal orientation do they need to have? It looks to me that they will always feel a magnetic force if they are parallel, perpendicular or at any other angle, since they are immersed in each other B field which is not parallel to the wire direction...is that true?

But I have been told that if they are infinite and perpendicular, there will be no net force...
Why? Net force in the sense that there are two forces that oppose each other, or no force on either wire?

thanks,
fisico30
 
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I think the magnetic force depends on the direction of the current in each wire... if the wires are carrying currents in the same direction they attract each other, but if they carry currents in the opposite direction then the force between them is repulsive. So the net force depends on whether the currents are parallel or anti-parallel.
 


If you have 2 perpendicular wires, I believe there would be a torque trying to rotate them about the line which joins them.
 


If two current carrying wires are perpendicular, then the total linear force on one wire will be zero, simply because all the linear forces on one half of the wire will be canceled by forces on the other half. But it will experience a total angular force (a net torque) trying to turn the one wire until it is parallel to the other wire. Draw a little diagram and use F = qv x B at several point along the wire and you can see this pretty quickly. This case is actually very similar to an electric motor with square loops of wire near the outside creating the field and square loops near the axis responding to the field. Each loop feels feels a net torque but no net linear force, so the motor turns.
 
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