# I Magnetic fields and currents - question about time delay

1. Oct 15, 2016

### Kiki

Suppose there is a solenoid near an accelerating electron. The electron must create a changing magnetic field. By Lenz's law, an emf is induced in the coil. One question is, how *fast* is the emf induced in the coil?

Is the emf just there instantaneously or does it take time to formulate? And since the emf in the coil creates a current (by the way, how quickly does the current get created?), the coil has produced an equal and opposite magnetic field so now I ask: how *fast* does the resulting magnetic field from the coil spread -- at the speed of light or is the magnetic field instantly existing throughout space without any time delay at all?

If everything happens all at once, it's as if an accelerating electron creates two equal and opposite magnetic fields at the same time, in the stationary frame of reference--is that what is actually happening? If not, something must take some time--what does?

Last edited: Oct 15, 2016
2. Oct 16, 2016

### Staff: Mentor

Solutions of Maxwells equations show that the fields propagate at light speed in a vacuum and a bit slower in media. In wires, propagation speeds of 0.8c are reasonable.