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Name: Brandy
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Question:
Does DC current travel more through the center of the conductor or more on the surface of the conductor (wire)?



Replies:
Brandy -

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 center.

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 not necessary.

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 massive flow.

Larry Krengel


Hi Brandy,

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 current.

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 exterior, 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 constant density by the need to keep charge neutrality with the hard-packed metal-ions (charge +1) they wander amidst.

Jim Swenson



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