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Wire Conductivity

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Wire Conductivity


name        bernice
status       student
grade        other
location     N/A

Question -   will a thick wire have more resistance than a thin wire?
---------------------------------------
Quite the opposite, it has less resistance.
What it has more of is "conductance",  which is the opposite or inverse of
resistance.

resistance is V/I     (voltage/current)
      i.e. "How many volts does it take to push thru a given current?"
conductance is I/V     (current/voltage)
    i.e. "How much current flows if you push with a certain electric
pressure?"

Clearly an identical length of thicker wire
    must allow more electrons thru
for a given pressure difference between the ends.


The "water in a pipe" analogy works for me.
electrons are like water,
voltage is like pressure,
and current is like flow rate (ie., gallons-per-minute).

The common-sense perception is that
a narrow straw has more resistance to drinking your milk-shake,
and a fat straw has less resistance.
This analogy seems accurate to me.

Jim Swenson
====================================================================
No, the behavior of the resistance is the opposite.  A thicker wire will
have less resistance than the thick.

Physical objects, such as wires,  have electrical resistance if they inhibit
electrical current.   Big numbers mean high resistance to electric current.
If the resistance is very very high, then they are called "insulators."

A thick wire is used when one wants to transmit power to a high-power
electrical device.  In this way, the power gets efficiently transmitted to
the device, and little power is wasted in the wire itself.

The only materials with zero resistance are "superconductors," which allow
current to flow without any losses, up to a limit.

There is another word called "resistivity."  Resistivity is a property of a
material itself, and doesn't depend on the shape the material is in.
Resistance depends on the shape of the material.  So for example, copper has
a resistivity of 17 nano ohm meters.  A long thin wire made of copper will
have a moderately high resistance.  A short thick wire made of copper will
have a low resistance.

Another word is "conductance."  It is the inverse of resistance.  The unit
is "mho" or inverse ohm.  Here, a big number means that current flows
easily.  People do not use this word very often, but it is used when
measuring water purity.  Water with high conductance has many impurities in
it.   Pure water has very high resistance and insulates quite well.

Bob Erck
====================================================================
No, in general it is the opposite provided other factors are the same. The
resistivity, R , of a uniformly cylindrical metal wire is: R= K x L / A
where L is the length of the wire, A is the cross-sectional area, and K is
a constant that depends on the metal and/or composition of the wire if it
is an alloy, the temperature (generally K increases with temperature for
metals). All this assumes "normal" conditions, that is, the wire isn't at a
temperature where it is superconducting, there are no phase changes in the
metal and so on.

Vince Calder
====================================================================
Hi, Bernice,

Actually its the other way around.  The bigger the wire,
the less resistance it has.  Think of the current flowing through the
wire in terms of water flowing in a pipe.  Smaller pipes have more
resistance to water flow, just like smaller wires have more electrical
resistance.

What can confuse the issue, though, is wire gage (called AWG for
American Wire Gage).  The Gage number goes down as the diameter goes
up.  So a 12 gage wire (sometimes called AWG 12) is bigger than a 14
gage wire, and it has less resistance than the 14 gage, too.

Many wire and cable manufacturers have tables posted online that
describe resistance in terms of voltage drop per foot of wire (less
drop means les resistance), and some have it expressed explicity as
ohms (the unit of resistance) per foot.  Here is a link to a webpage
with values for some different wire and a calculator where you can
enter the diameter and length and it calculates the resistance:
http://www.cirris.com/testing/resistance/wire.html

Hope this helps!

David Brandt P.E.
====================================================================
The simple answer is "No, it will have less".
This applies mostly to DC, and low frequency AC.  As signal frequency
increases, however, the current tends to ride only along the outside of the
wire, and many smaller wires will conduct better than the single large wire.
  Those many, smalller wires also have the advantage of being able to bend
and flex more easily.

Ryan B.
====================================================================

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