# When a electromagnet is activated, the field must take time to form at both ends and stabilize. What is this time? Or, to put it differently, what is the speed at which the magnetic field forms?

When a electromagnet is activated, the field must take time to form at both ends and stabilize. What is this time? Or, to put it differently, what is the speed at which the magnetic field forms?
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zupa1z
If you are far from the electromagnet and look over time scales long compared to the time light will cross the electromagnet, it is a good approximation to consider that a dipole is instantly created, then the field propagates through space at c
If you are trying to consider short distances and short times, the development of the field will depend on details of the circuit. Each piece of the circuit has its resistance and inductance. The field will form and stabilize in the time that the current distribution stabilizes.
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c0nman56
If you had the disconnected poles coming out of the magnet you would have two magnetic monopoles until they connect. To avoid this the closed field lines must start from the surface of the magnet and spread out. It also takes the electromagnet some time to be switched on anyway. During the continuous change from no magnetic field to the end state the field lines move away from the center.
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