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EM waves

We are always told that the electric and magnetic fields are at 90 degree and in phase. So why does light continue to travel past the point where both would be zero. Shurly it makes more sense for the two to be out of phase so that the decay of one induces the other and creates perpetual motion. Or am I missing something?
Original post by CajunCrab
So why does light continue to travel past the point where both would be zero.


Please explain.
The rate of change of electric field is what produces the magnetic field and this is at its greatest when the electric field is zero so you might expect (as you do) that the magnetic field would be greatest at that point. However it is the *curl* of the magnetic field that depends on the rate of change of the electric field, not the actual field itself. Imagine the changing field to be a bit like an electric current (as Maxwell did). The magnetic field lines circulate round the direction of current flow, rather like the way the field lines circulate round a current carrying wire.
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CajunCrab
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Original post by Martin Hogbin
The rate of change of electric field is what produces the magnetic field and this is at its greatest when the electric field is zero so you might expect (as you do) that the magnetic field would be greatest at that point. However it is the *curl* of the magnetic field that depends on the rate of change of the electric field, not the actual field itself. Imagine the changing field to be a bit like an electric current (as Maxwell did). The magnetic field lines circulate round the direction of current flow, rather like the way the field lines circulate round a current carrying wire.
Thank you, need to take a look at Maxwells equations again, Cheers
I hope that I was of help and I certainly did not mean to be critical of you.I do not think it is at all obvious that the E and M fields are in phase in a travelling EM wave and, in fact. there are many people who expect them to be 90 deg out of phase. You will see diagrams on the net showing this.My reference to Maxwell referred to his invention of the displacement current, in which a changing E field acts like an electric current. It was just the leap of imagination required to make full sense of the subject.

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