# Changing electric field generating magnetic field

• B
I would very much appreciate a clarification on what is meant by a changing electric field in the context of statements such as 'a changing electric field creates a changing magnetic field'. My question is does the electric field actually need to reverse as a lot of examples show where two charged particles change places so that the force changes direction or can it be that a magnetic field can be created by an electric field that is simply increasing and decreasing but not reversing. So does it have to be alternating current that induces magnetism or can just a changing amount of current cause magnetic fields. Perhaps another way of describing my question is to think of the electric field as either a sinusoidal wave - the alternating electric field idea - versus a half-sinusoidal wave - only the top or bottom of a sinusoidal wave - so that the field is increasing and decreasing from 0 to whatever height but never crossing the horizontal axis. In that situation the field is increasing/decreasing but not 'reversing' - would that fluctuation create a corresponding magnetic field? Or does it have to be the sinusoidal case for a magnetic field to be created?

Doc Al
Mentor
My question is does the electric field actually need to reverse
No. (By "reverse" you mean change direction. Not needed.)

or can it be that a magnetic field can be created by an electric field that is simply increasing and decreasing but not reversing.
Yes.

Thanks for the quick and definitive response. This point was not made clear in a lot of material I saw - it always had reversal of direction - so negative and positive charges swapping repeatedly. That being said what difference does that particular scenario make as to the simply increasing/decreasing example?

davenn
Gold Member
Thanks for the quick and definitive response. This point was not made clear in a lot of material I saw - it always had reversal of direction - so negative and positive charges swapping repeatedly. That being said what difference does that particular scenario make as to the simply increasing/decreasing example?

just to be clear ....

changing electric field in the context of statements such as 'a changing electric field creates a changing magnetic field'.

that is the basis of the generation of an electromagnetic wave, the charges ( electrons) need to be accelerating

A steady DC current will generate static electric and magnetic fields