Current Location in a Wire
Does DC current travel more through the center of the
conductor or more on the surface of the conductor (wire)?
As the frequency of electrical current goes up, it tends to travel
tends more on the surface of a conductor. DC is the ultimate low
frequency. It would tend to move through the entire conductor. So -
to answer you question - percentage-wise more is traveling through the
In some applications - such as military aircraft - 400 hz is sometimes
used (compared to 60 hz for household current). In doing this they are
able to use hollow tubing as a conductor giving them lighter weight.
The center of the conductor that is not used by the higher frequency is
A lightning bolt (real DC!) travels deeply in trees or persons struck by
the lightning. The damage done is in the portion of the object most
conductive to the current... where the most heat is generated by the
DC current will use the wire's whole cross-section evenly.
Imagine a single solid wire divided by invisibly-thin barriers
into parallel strands of equal thickness and shape.
Only at the ends are they joined together, metal-to-metal.
In this picture, each strand has equal resistance,
and equal voltage from end-to-end,
so the current in each is equal.
Only AC has a preference for a particular depth.
It prefers to be shallow, staying towards the outside.
This is a consequence of changing magnetic fields caused by the changing
If it is DC, it is not changing, so the magnetic field is steady,
and has no effect on the DC current density.
DC current only cares about resistance, not inductance or magnetism.
Weird but moot minor point:
The steady field around a wire with DC current may cause a
small voltage difference between the outside and the inside.
However the difference at one end cancels out the difference at the other.
Due to electromagnetism, parallel currents attract.
So if the metal conducts electrons, then they are squeezed inwards,
and the interior would be slightly more negative than the outside.
Contact at the starting end is made to the wire's outside.
So is contact at the finishing end.
So an electron going travelling the wire-center route may
go up a small potential step at the start,
then go down the same amount at the end.
These two steps cancel each other out.
The end-to-end voltage in the wire interior is the same as the wire
so the current densities are the same too.
Nobody even thinks about those last two paragraphs. They do not need to.
Except maybe physicists doing plasma hi-power sparks with Z-pinch.
Z-pinch is when the glow of the current in the ionized gas
spontaneously squeezes itself into a very intense sharp narrow strand,
even though it started out wide and diffuse.
It only does that if the current is very high,
and because a gas can be compressed.
In a solid metal the mobile electrons (charge -1) are forced to keep a
by the need to keep charge neutrality with the
hard-packed metal-ions (charge +1) they wander amidst.
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Update: June 2012