On 13/8/2007 11:18 PM, John Winterflood wrote:
> Horace wrote:
>>
>> <snip>
>> Suppose the DC power supply, a battery, carries a coulomb of excess
>> positive charge, or a coulomb of excess negative charge?
>
> Charge can always be detected by the field that is around it - provided
> you can get "around" it to measure! Gauss's law (I think it is) says
> that if you construct a closed surface and then measure the electric
> field all over that surface, then the sum (integral) of that measurement
> tells you how much charge is enclosed within that surface (fields are
> always measurable without needing to connect a meter between two points
> - eg by a field-mill).
>
> However if you can't get all around it, and the charge is arranged so
> that the field in the areas where you can measure cancels to some
> extent, then it is not (currently) possible to know that that charge is
> present nearby. This is the situation within a charged sphere (or
> Faraday cage) for instance. While ever you are confined to the cage,
> you cannot tell from measurements that you can make within the cage
> whether you and the cage are charged to a high or low potential.
>
> This is the manner in which potential is unmeasurable - you could be
> surrounded by a layer of charge and at a very high potential, but from
> within the canceling distribution that the charge occupies on the
> external surface of the cage, you cannot measure what the potential (or
> phi) is within the cage.
>
> A similar case is true with gravity - you can't in principle tell
> whether you are floating near to a lot of mass, or in empty space. If
> you can make observations outside your enclosure - such as looking at
> spectral lines from distant disturbances, and compare them to the
> spectral lines from similar disturbances you can create. Then it
> becomes possible to know the difference in gravitational potential
> between where you are and the distant observation - as time ticks at a
> different rate dependent on your gravitational potential and the
> spectral lines will be shifted accordingly.
>
> So it is very likely that there is a similar test that could be done to
> measure your electric potential - but in some way you will have sampled
> the space outside of your cage by means of the signal that has traveled
> from the remote location - through the space with the field in it - to
> where you are.
>
If the success of modern physics lies in its ability to identify and exclude
any conception of nature that cannot be made to fit within a virtual box,
then the cost of this success is that modern physics has become a virtual
prison.
Harry
