Ohm’s law states that E = I X R — volts (electromagnetic force) is equal to amps (intensity) times ohms (resistance). When the equation is used together with the power law — volts times amps equal watts — it is possible to derive several formulas, solving for various unknowns. All of this is conveniently organized in the Ohm’s law wheel.
For the most part, Ohm’s law is relevant for alternating as well as direct current, either on an instantaneous basis or for any ongoing flow of electrical energy through a conductor or load. However, there are some specific frequency-dependent phenomena that arise when the voltage fluctuates. Generally they become more pronounced as the frequency rises. These effects include capacitive and inductive reactance. To define them in the Ohm’s law formulation, volt-amps is used rather than ohms. Instead of resistance, the overall metric is known as impedance.
High frequencies also give rise to what is known as the skin effect, which is simply a consequence of self-inductance. This effect, which opposes the flow of current, is most pronounced along the center-line of a conductor and is caused by proximity to adjacent current flow. It is the region of greatest impedance, which manifests as a gradient that diminishes toward the surface of the conductor. For this reason, the surface or skin of the wire is more conductive. There is nothing mysterious about the skin effect, but it is a powerful parameter that should be considered when the frequency, current, distance traveled or conductor material warrants.
The skin efffect is one of the reasons that each phase of a high-voltage transmission line is often subdivided into separate, smaller conductors. They are grouped close together because, being at the same potential, there is no danger of arcing.
It is also the reason that ground electrode conductors that connect to lightning rods and run down the sides of buildings are composed of braided strands surrounding a hollow core. Lightning, with a waveform that has a very fast rise time, resembles high-frequency electrical energy. So it is essential to mitigate the skin effect in a conductor meant to conduct during a lightning strike.
When conditions warrant, tubular conductors can be used to good effect, saving labor and materials in large installations. Skin depth, the region of greatest conductance, is proportional to the material’s resistivity. For this reason, it is of greatest consequence in a good conductor such as copper.
Skin effect was first described in detail by the brilliant electrical engineer, theoretician and experimenter Oliver Heaviside. It can be derived from the equation for AC current density, J, in a conductor which drops exponentially from its value at the surface JS according to the depth d from the surface. In the equation, δ is called the skin depth. The skin depth is thus defined as the depth below the surface of the conductor at which the current density has fallen to 1/e (about 0.37) of JS. It is normally approximated as the accompanying equation where ρ = resistivity of the conductor; ω = angular frequency of current = 2π × frequency; μr = relative magnetic permeability of the conductor; μ0 = the permeability of free space.
Donald Roberts says
Seems to me during a lighting storm a set of jumper cables attach to front and rear bumper would give the lighting a place to travel since the lighting is looking for ground..at sea we would take jumper cables attach to the guide cables holding the mast and lower over the side into the water…