Undergrad Magnetic Field Generated By Moving Charged Objects - Confusion Explained

[hjwu]

Suppose there is a charged line and near that line, there is a magnetic needle lying in the vertical plane of the line. The magnetic needle is radially placed. If the charged line and the magnetic needle are moving at a same constant velocity(parallel to the line, v<<c) towards an observer. I learn from the classic electrohydrodynamics that the observer will find there is a magnetic field and the magnetic needle will rotate. Am I right? Suppose there is another observer who is at rest relative to the charged line and needle, this observer will conclude that there is no magnetic field, so the magnetic needle will not rotate. What actually happened？Dose The needle rotate or not?

 

[Dale]

I learn from the classic electrohydrodynamics that the observer will find there is a magnetic field and the magnetic needle will rotate. Am I right?

No. There is a magnetic field, but there is also an electric field. The forces from each cancel out so that there is no net force.

The needle does not rotate in any frame.

 

[hjwu]

No. There is a magnetic field, but there is also an electric field. The forces from each cancel out so that there is no net force.

The needle does not rotate in any frame.

Thanks