MadSci Network: Physics

Re: The effect of water surrounding on a cable, such as phone, speaker wire ?

Date: Wed Jun 2 12:43:27 2004
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Physics
ID: 1083639123.Ph

Hi Chelvam,

That's an interesting question.

In general, water is a better insulator than it is a conductor. This has to
be qualified depending on the nature of the water. Pure distilled water
with no minerals is a good insulator. Most tap water, fresh water from
streams and rivers, and of course sea water all contain minerals in varying
degrees. These minerals will increase the conductivity of the water, but
it's far from a good conductor like copper or other metals.

It's easier to answer the question of what happens when we immerse copper
wire into an insulator. There are a number of mechanisms that cause
electromagnetic and radio frequency interference. Let's consider each one
in detail:

1. Near field magnetic interference. This type of interference is caused by
lower frequency magnetic fields. Typically this is produced by transformers
or high current carrying wires. The interference is called near field (or
near zone) because the field strength drops off quickly as a function of
distance. Insulators have little or no effect on magnetic fields, so there
will be little, if any change in interference. Even still, magnetic
interference can be significantly reduced by simply twisting a pair of
conductors and minimizing the loop area.

2. Near field electric interference or capacitive coupling. This type of
interference is caused by varying electric fields. This is often produced
by varying voltages in a nearby conductor. The effect is small, but can be
significant if high impedance amplifiers and small signals are involved.
This is also a near field effect, so it also drops off quickly with
distance. Insulators with a high permittivity will attenuate the electric
field in proportion to the permittivity of the material. The relative
permittivity of distilled water is quite high, around 80. This means that
the electric field in the water will be 80 times smaller than the electric
field outside the water. This is a big change, but it probably will not
have a large effect on the over EMI/RFI if the wires are shielded and the
shield is connected to the appropriate ground. Here's a link to a web page
describing the effects of insulators in capacitors:

3. Far field electromagnetic interference. This type of interference is
caused by radio waves generated by an antenna or an unshielded wire
carrying high frequencies currents. The electric field in this type of
interference will also be attenuated like case #2. If you are talking about
speaker wire, my guess is that you are interested in audio frequencies. The
wavelength of these frequencies is very long, on the order of thousands of
meters. For wires that are much shorter than this distance, there will be
little effect. Of course, higher frequencies are a different story. Here's
a link to a web site that describes the effect of microwaves in water:
It is possible to shield against
RFI, but this depends heavily on the frequency.

In summary, the water will have little or no effect on EMI/RFI in most
cases. If there is any effect, it will reduce the electric field component
only. Any signal in the wire might will probably not be significantly affected.

I hope that answers your question.

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