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A Primer on Electrical Terms for Electrofishing

Fisheries biologists are knowledgeable about fish, their habitat and about ways to collect fish using a variety of gear and techniques. Many are less familiar with the electrical terms which are used in fish capture using electricity. This blog is meant to explain electrical terms which are used in electrofishing classes and in the literature.

Below is a list of terms which are generally described in typical electrofishing courses as they relate to electricity in circuits, i.e. in wires, connectors and electronic components. The terms in red are the most important to know, and they usually are the ones familiar to people prior to class. The units are named for scientists and engineers who were associated with the terms. Lagniappe is a French term meaning “a little something left over” or “a little something extra.” That column can be used or ignored depending upon your interest into delving a little deeper.

Circuit terms

One thing of note here is that the symbols have not been used consistently among the fields of physics and engineering, nor have they been standardized in the electrofishing literature. And we have not done a consistent job of following the International System (SI) of identifying such symbols. What is given above is a compromise offered by some in the electrofishing community. The symbol E has been used voltage and for voltage gradient or electrical fields; here, we suggest an italic V for voltage.

Ohms Law                       Joules Law

Above are the two main laws that we ask students to learn. Ohm’s Law is for resistance as it relates to voltage and current. Joule’s Law is for power. In electrofishing, we assume purely resistive circuits, especially when we discuss current flowing through the water. Therefore, we equate power to the product of voltage x current and neglect any reactance due to inductance or capacitance. Thus, we only use the term resistance and not impedance for opposition to current flow, and we don’t mention topics such as power factors. Simpler is better. Electrical terms can be foreign to those being exposed to the science behind electrofishing. If one understands basic algebra, only the first form of Ohm’s and Joule’s laws need be remembered. The other forms can be derived from those. However, it may be easier to just memorize all three forms of Joule’s Law. All of these simple formulas are useful for electrofishing calculations.

Below is a table of corollaries of terms between circuits –where the current flows though wires, connectors and components — and electrical fields, where the current flows through water. Because the current flows through 3-dimensional water space, the field units must include some spatial component. For instance, it is no longer just voltage in volts, it becomes voltage gradient (also known as field intensity) in volts per centimeter. Electrical fields are vectors: they have both magnitude and direction.

Circuit Field terms

Again, the main terms to know are shown in red letters in the table above; they are voltage gradient, conductivity and power density, though current density is good to know, too. We can easily measure only two of the above field parameters: voltage gradient and conductivity. Current density and power density are easily calculated from voltage gradient and conductivity. See an earlier blog about ambient and specific conductivity. In electrofishing, what counts is ambient conductivity, i.e. water conductivity at the ambient temperature.

Conductivity

Conductivity is defined as current density divided by voltage gradient. A Siemen is current per voltage measured in amps per volt; it is the reciprocal of resistance in ohms. Conductivity then is in A/V/cm of water, or Siemens/cm. Because the values would be so small, it is multiplied by 1 million, and the units are generally reported in micro-Siemens per cm.

Circuits to Fields

Above is another representation of the corollary between circuit terms and field terms for the same idea. The field terms must include a spatial component because the current flows through the 3-dimensional water space and also through the 3-dimensional fish body. Below is a graphical description of the field terms as applied to a cube of water 1 cm on each side. Think of current density as discharge in a stream; it is the movement of electrical charges past a water surface. The difference in electrical potential over a 1 cm distance, in the direction of the current flow, is the voltage gradient. Power density is the power in one cubic centimeter of water. It is a measure of electrical energy per time and per water volume.

Water cube Field

The list below is of some calculations useful for converting among units. The little v symbol in the “Therefore” statement below was a square root symbol that did not translate well between software. It is just a rearrangement of Joule’s Law such that voltage gradient equals the square root of power density divided by ambient conductivity.

Power Density Current Density

This blog was intended to list and describe electrical terms related to circuits and to fields. Perhaps the best use is as a pre-requisite to an electrofishing course or just as a reference when reading journal articles about electrofishing.

 

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