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*To*: tesla@xxxxxxxxxx*Subject*: Re: The Joy Of Poynting Vectors.*From*: "Tesla list" <tesla@xxxxxxxxxx>*Date*: Fri, 22 Apr 2005 10:20:16 -0600*Delivered-to*: testla@pupman.com*Delivered-to*: tesla@pupman.com*Old-return-path*: <teslalist@twfpowerelectronics.com>*Resent-date*: Fri, 22 Apr 2005 10:20:53 -0600 (MDT)*Resent-from*: tesla@xxxxxxxxxx*Resent-message-id*: <vyNToD.A.D0G.dRSaCB@poodle>*Resent-sender*: tesla-request@xxxxxxxxxx

Original poster: "Wim Bosma" <wbosma@xxxxxxxxxxx>

Regards, Willem, PA0TW

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx> To: <tesla@xxxxxxxxxx> Sent: Friday, April 22, 2005 3:59 PM Subject: The Joy Of Poynting Vectors.

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

Hi all

>http://www.itee.uq.edu.au/~aupec/aupec00/edwards00.pdf

That's a very interesting paper. If you're a real sucker for punishment you can analyse the power flow in a Tesla coil just the same way. You can also prove that electrical power- whether AC or DC- and no matter what the frequency- does not travel in wires. Poynting vector analysis says there can be no power flow unless both E and H fields are present, and there can be no E field inside a conductor. So the electrical power must be flowing in space around and between the wires of a circuit, just like Paul Nicholson says in his piece on "non-Hertzian" waves. But I digress.

The Poynting vector is the cross product of the electric and magnetic fields. It shows the direction that power is flowing in a system of EM fields. It's the same as the "direction of propagation" of a radio wave that you get taught in your ham radio licence classes.

If you're not keen on math, "cross product" just means the right hand rule thing. If you drew some arrows on a piece of paper with the E field pointing up and the H field pointing to the left, the Poynting vector would be sticking out of the page at you. (disclaimer for US readers- don't sue me if you poke yourself in the eye with it) The magnitude of it is equal to (magnitude of E times magnitude of H times cosine of phase angle between them) IIRC. It's a long time since the last math lecture I slept through.

Anyway back to Tesla coils. If you look at the fields at the surface of the secondary winding, there is a H field normal to the wire (ie along the length of the secondary) caused by the primary current, and an E field along each turn of wire caused by the secondary voltage. If you wave your fingers and thumb around for a bit you can see that the Poynting vector associated with these fields points from the primary coil towards the secondary.

If the relative phase of one of the fields reverses (because the H field of the secondary current overcomes the H field of the primary current or whatever) then the Poynting vector turns round and points the other way. This is what happens at the "notch" in primary current.

This is my interpretation of it anyway. Maybe someone who knows more about these things- like Paul Nicholson- could comment?

Steve Conner

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