MadSci Network: Earth Sciences

Re: What is the official name of the 'skin effect'?

Area: Earth Sciences
Posted By: William Beaty, Engineer / Interactive exhibit designer
Date: Sun Apr 20 12:19:30 1997
Area of science: Earth Sciences
ID: 859870895.Es

Hi Kent!

The "skin effect" is called... The Skin Effect. Perhaps you are thinking of "Faraday Shielding", which is an entirely different process? The Skin Effect applies only to changing electric currents, including AC power transmission, radio signals, brief impulses, and even light. There is no skin effect for continuing currents, if you have DC, then the charge flows throughout the entire wire. Skin effect depends on conductivity of the metal and on frequency of the current. The "skin depth" for poor conductors (brass, water, etc.) is much larger than for good ones (copper, silver, etc.) Also, the "skin depth" for low frequency is larger than for high. Lightning is an impulse current, so the Skin Effect usually does cause much of the current to be on the outer surface of a metal cage. However, "hot" lightning, the type that sets fires, lasts for half a second (sometimes up to 1.5 sec!), and for our purposes can be considered to be DC rather than impulse. If your car is struck by a long-lasting "hot" lightning bolt, it will still act as protection mostly because the metal shell is a far better conductor than the air inside. The path for current will be through the metal car body. If you were barefoot, and if the upholstry was wet, and if you somehow soaked the ceiling and placed your hand upon it, then a tiny portion of the total current *would* pass through you (and probably be deadly).

"Faraday Shielding" is an entirely different effect. When a conductive object is given a DC charge imbalance, the imbalance appears only on a molecule-thick skin, and the electrostatic fields only appear outside the object. Faraday shielding during a lightning strike only prevents your body from taking on an imbalanced charge. Your hair would not stand up, and there would be no corona discharges off your fingertips. But these effects aren't dangerous. Faraday shielding doesn't deal with currents, and its the current through your flesh which makes lightning dangerous.

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