AN: GuruGram #39

... is now available for free download as

It is on fun with fields.

Additional gurugrams at

True... what confused me is that if a bar magnet is being rotated pole- over-pole, the field itself is still uniform, but it's easy to observe its rotation from a fixed point. Ergo there *is* such a beast as a rotating magnetic field.

Still, this is a neat .PDF. Thanks for posting it, Don.

-- jm

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He means that the rotating field is symmetric and is || to the axis of rotation. Therefore the flux remains constant throughout the revolutions.

IOW if you flip the bar magnet around, the field is *not* uniform - it changes.

It depends on *where* the observer is - on the magnet or not.

Nope. Not even wrong.

Read the provided link. The entire detailed explanation is found in the Feynman lectures among many other places.

Take a cylindrical magnet and hang it on a string. Place another cylindrical magnet below it on the same axis.

Rotate the bottom magnet. The Top one does NOT rotate.

Because you can not tell if a perfectly uniform magnetic field is stationary or rotating. General relativity DEMANDS none other.

Again, if you don't like my coverage, see Feynman. The outcome will remain the same.

-- Don Lancaster

Cute. I'm just reviewing some of this stuff after (mumble) years, so timely too. I guess it's doing the calculations inside the PDF, and that's why its so slow.

I did a 3-D calculation to find the voltage gradients in a electroplating situation back in school. It was fun.

Best regards, Spehro Pefhany

But there is ---> no

Nice pics!. I've no idea of Postscript but another pretty field plotting method that can be progged in just a few lines of Basic, C etc is the 'inverse square law' that applies to gravity and charge. Basically goes as, the force on any point in space is due to the sum of the effect of every other point in space. For progging, step through each screen pixel in turn and calc the pythoras distance (H) to every pixel that has been prefilled with a charge (+/-) (or galaxy +!) and then sum up as 1/H^2. Plot the pixel colouring according to its strength. Also works nicely in a 3D volume but is just a tad slower :-) regards john

Gasp!, you're an 'ard man Spehro :-). My favourite heavy calc's in school were figuring how many ciggies I could buy with my dinner money. regards john

I read in sci.electronics.design that bee wrote (in ) about 'AN: GuruGram #39', on Fri, 10 Dec

It is important to clarify the orientation of the magnets with respect to the string.

ASCII art: use Courier font:

| | | | | | _|_ _____|_____ | | |___________| | | |_|

_ ___________ | | |___________| | | |_|

No rotation Rotation

------------------- But all that changes is whether it looks like the fields are electric or magnetic, and how much and which orientation of each.

-Steve

The whole point is that field cutting "lines" do not exist and never did. The concept is fundamentally bogus.

Assume a uniform magnetic field in a given area. Then rotate that field about a central axis that is parallel tothe field lines. Impossible to detect.

That is the only case i can think of right now; is one sufficent?