Date: Thu Mar 19 20:58:42 2009 Back to Contents ------------------------------------------------------------------------

Author: Bernard Cleyet

Subject: Re: Power Lines and cows?

Post:


Right! (field day)

Wiki. has a good matrix on the relative motion(s): [go to disk type
gen.]


http://en.wikipedia.org/wiki/Homopolar_generator


On 2009, Mar 19, , at 08:29, cbettis@unlserve.unl.edu wrote:

> Paul,
>
> The thing you need to be careful about is the values for the E and
> B fields in different frames will not necessarily be the same but
> everyone should agree on things like galvanometer readings even
> though their explanations of those readings might differ. If you
> think of a closed vertical loop running east and west, is there a
> changing magnetic flux in it? What gets tough is when you try
> explain what is meant by being at rest WRT the magnetic field. In
> other words can you tell if a bar magnet is rotating about its
> magnetic axis of symmetry? I think enthusiasts have a "field" day
> with homopolar generators trying to come up with ways of creating
> energy out of nothing.
>
> Cliff
>
> Quoting Paul Nord :
>
>> Cliff,
>>
>> That's the demonstration I was thinking of. Lots of references out
>> there on homopolar generators.
>>
>> Is there a reason we can't do this with the earth? There must be
>> some
>> effect. Perhaps it's small. Or are you saying that you need to
>> complete the circuit in an external reference frame?
>>
>> Paul
>>
>>
>> On Mar 19, 2009, at 8:38 AM, cbettis@unlserve.unl.edu wrote:
>>
>>> Paul,
>>>
>>> If you have a homopolar generator around, give this a go: my
>>> version has two disks on a common axis; on disk is a magnet with
>>> the magnetic axis parallel to the axis of rotation; the other
>>> disk is made of aluminum; there are a couple of brushes mounted
>>> radially on the aluminum disk and fixed to the lab frame (i.e.
>>> they don't spin with the disk). If you spin the aluminum disk
>>> and put a galvanometer across the brushes you get a current. If
>>> you hold the aluminum disk fixed and spin the magnetic disk you
>>> don't get a current. If you spin both disks you get a current.
>>> If you don't use brushes and attach your galvanometer directly
>>> to the aluminum disk putting it in its frame, you see nothing no
>>> matter what combination of disks you spin.
>>>
>>> This is a tricky problem and it takes some effort to understand
>>> (it was one of the experiments that inspired Einstein's paper
>>> "On the Electrodynamics of Moving Bodies")
>>>
>>> Cliff
>>>
>>> Quoting Paul Nord :
>>>
>>>> Sorry... that might have been a little too flip.
>>>> It doesn't matter that the source of the field is also rotating.
>>>> Whether you are being difficult or dense surely is of
>>>> consequence. No
>>>> slight was intended there.
>>>>
>>>> I should say more about the claim. Consider a rotating metal
>>>> shaft.
>>>> If I place a strong magnet very near to the end of the shaft, there
>>>> will be some magnetic field passing through the shaft at an
>>>> angle to
>>>> its direction of motion. The electrons in the shaft will be
>>>> caused to
>>>> move by that field and will setup a potential difference. Next
>>>> consider that I have the same magnet attached to the end of the
>>>> shaft.
>>>> The magnetic field in the shaft will be virtually unchanged.
>>>> Why then,
>>>> would it not produce the same potential when the shaft is
>>>> rotated? How
>>>> would the electrons "know" whether the magnet is attached or
>>>> removed at
>>>> some distance? All we can say about the forces on the electrons
>>>> is A)
>>>> they are being accelerated around in a circle and B) they are in a
>>>> magnetic field.
>>>>
>>>> Paul
>>>>
>>>> On Mar 18, 2009, at 10:24 PM, Paul Nord wrote:
>>>>
>>>>> It doesn't matter.
>>>>>
>>>>> On Mar 18, 2009, at 5:48 PM, Bernard Cleyet wrote:
>>>>>
>>>>>> Are you being difficult or am I too dense?
>>>>>>
>>>>>> So is the field. There is no relative motion between the
>>>>>> field and the conductors.
>>>>>>
>>>>>> bc
>>>>>>
>>>>>>
>>>>>> On 2009, Mar 18, , at 15:00, Paul Nord wrote:
>>>>>>
>>>>>>> What do you mean they don't move?
>>>>>>> Of course they move. We're all moving. The earth is spinning.
>>>>>>>
>>>>>>> Paul
>>>>>>>
>>>>>>> On Mar 18, 2009, at 2:43 PM, Bernard Cleyet wrote:
>>>>>>>
>>>>>>>> I thought we were discussing power lines that don't move
>>>>>>>> WRT the earth and its mag. field.
>>>>>>>>
>>>>>>>> bc must reread recent posts?
>>>>>>>>
>>>>>>>>
>>>>>>>> On 2009, Mar 18, , at 12:25, Paul Nord wrote:
>>>>>>>>
>>>>>>>>> BC,
>>>>>>>>>
>>>>>>>>> No... or probably not much. But the earth rotates. And
>>>>>>>>> a conductor moving in a magnetic field will produce an
>>>>>>>>> induced voltage.
>>>>>>>>>
>>>>>>>>> Paul
>>>>>>>>>
>>>>>>>>> On Mar 18, 2009, at 2:08 PM, Bernard Cleyet wrote:
>>>>>>>>>
>>>>>>>>>> The earth's mag. field rotates?
>>>>>>>>>>
>>>>>>>>>> bc
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 2009, Mar 18, , at 07:40, Paul Nord wrote:
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Do north-south power lines induce a DC current as they
>>>>>>>>>>> spin through earth's magnetic field?
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>
>>>
>>>
>>>
>
>
>
>
>


From tap-l-owner@lists.ncsu.edu Thu Mar 19 20:58:42 2009

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