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The answer is that if you have a magnetic field and some charges, there will be some angular momentum in the field.
would it be significant enough to be observed under practical conditions?
How much is the angular momentum if the magnetic field and electric charge are known?
Just as the Electric potential at any point in space is arbitrary (since we usually only observe effects from a potential difference between two points), the amount of angular momentum in the E and B fields is quite arbitrary. For example, the plastic disc with charges that Feynman mentioned didn't remove all of the angular momentum from the electromagnetic field in space, there was just some difference between when the solenoid was turned on and turned off, the disc acquired angular momentum equal to that difference.
When the current stops, B becomes 0, assuming that no ferromagnetic material is used. According to your formula, it makes angular momentum of the electromagnetic field 0.
Any change in an E field (or B field) in space implies there has been a net flow of energy and it turns out that a net flow of momentum is then inevitable.
Quote from: Eternal Student on Yesterday at 05:25:59The answer is that if you have a magnetic field and some charges, there will be some angular momentum in the field.The questions are then, would it be significant enough to be observed under practical conditions?How much is the angular momentum if the magnetic field and electric charge are known?Re