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How does one find the speed of rotation of the magnetic field around a current-carrying wire? or How fast is it circulating around the wire?

How does one identify the speed of rotation of the magnetic field within a solenoid?

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2 Answers 2

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You seem to have a misunderstanding. The magnetic field surrounding a wire (or solenoid) carrying a d/c current is static - it is not changing or rotating. I.e. it has a fixed field strength and direction vector at each point in space. Therefore, it does not have a 'speed of rotation'.

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  • $\begingroup$ how does a solenoid valve work then? how does the solenoid valve move the metal pin within it depending on the polarity of the field? or the magnetic field in a tightly wound toroid doesn't rotate? $\endgroup$
    – dRobert
    Commented Aug 20, 2018 at 21:17
  • $\begingroup$ in this video youtube.com/watch?v=JULySBxDelo the mercury is moving. how is this possible if the field does not move? $\endgroup$
    – dRobert
    Commented Aug 20, 2018 at 21:44
  • $\begingroup$ @dRobert a solenoid valve works because the magnetic field from the solenoid coil causes the spinning electrons inside the core to align, which turns the core into a magnet itself. This results in a force on the core that pulls it into the coil. It's not because the magnetic field is 'moving'. I imagine the situation with the mercury is similar. $\endgroup$
    – Time4Tea
    Commented Aug 21, 2018 at 17:31
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From wikipedia: the last sentence is key, a solenoid typically has the metal slug only part way in the field as its initial position.

"The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil (see Faraday's law of induction). The force applied to the armature will always move the armature in a direction that increases the coil's inductance."

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