Date: Thu Mar 19 11:48:20 2009 ** **Back to Contents ** ------------------------------------------------------------------------ **

Author: cbettis@unlserve.unl.edu** **Received: from wm-imp-1.unl.edu (wm-imp-1.unl.edu [129.93.2.180]) by uni00ml.unity.ncsu.edu (8.13.7/8.13.8/N.20071210.01) with ESMTP id n2JFRDt4010795 for ; Thu, 19 Mar 2009 11:27:13 -0400 Received: from localhost (localhost [127.0.0.1]) by wm-imp-1.unl.edu (8.13.1/8.13.1) with ESMTP id n2JFTd7O016061 for ; Thu, 19 Mar 2009 10:29:39 -0500 Received: from lecturdemo.unl.edu (lecturdemo.unl.edu [129.93.36.196]) by wm-imp-1.unl.edu (Horde MIME library) with HTTP; Thu, 19 Mar 2009 10:29:39 -0500 Message-ID:

Subject: Re: Power Lines and cows?

Post:

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?
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>
>>
>>
>>





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