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Q&A 9. How does an electric eel shock anything in an environment where everything is grounded? The conditions are so different from the usual electrical accident situation where, for example, one stands in a bathtub full of water, which is grounded through the piping, and touches a live wire. I can see how the shock passes through the body in that case, but I do not understand how the same thing happens under water. Wilmington, Delaware
When a voltage is set up between two electrodes in water, most of the current that results will flow in a straight line between the two electrodes. There will, however, be other lines of current flow that loop out into the water surrounding the electrodes. This may be understood by visualizing an electric circuit consisting of many resistors in parallel, some higher in resistance than others. There will be a certain amount of current flowing in the high resistors in spite of the fact that there are low resistance paths available. For a similar, reason current will flow in a three-dimensional pattern surrounding the electrodes in the water even though the path of lowest resistance is directly between the electrodes. If there is organic material near the electrodes it may or may not form a high resistance path with respect to the water around it (pure fresh water is not a good conductor). In any case, if the voltage between the electrodes is high enough, a shock will be registered. The electric eel acts like two electrodes with a battery voltage supply. It is somewhat misleading to think of this situation in terms of the water being part of a special electrical entity called a "ground." It is simply a conductor of a different form from the wires with which we are used to dealing.
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Q&A 10. How do electric fishes produce their electricity?
Carol Stream, Illinois
Fishes that are capable of emitting electric discharges have electric organs composed of multiple stacks of coin-shaped cells called electroplaques. The electroplaques are derived from neuromuscular tissue. Nervous stimulation of each of them produces a small electrical flow, from one face of the coin- shaped cell toward the other. Many electroplaques arranged in a column, all oriented with the innervated side up, will produce a strong net current when stimulated simultaneously. The electroplaque columns are analogous to batteries aligned in series: the more electroplaques, or batteries, in a line, the greater the voltage produced. (Up to 550 volts can be discharged by the South American electric eel, Electrophorus electricus). Also, the more stacks aligned in parallel, the greater the amperage. The ability of electroplaques to generate an electric current is due to the structure of their cell membranes and the activity of chemical pumps. Sodium ions (Na+) are actively pumped out of the cell, creating an electrochemical gradient across the cell membrane. When the resting permeability of the cell membrane is altered (initially by nervous stimulation of the electroplaque surface), there is a net flow inward of the positive sodium ions. The flow of the positive ions toward the negative interior sets up a small electric current. Interesting facts about electric capabilities in fishes are given in "Fishes with Electric Know-How," Sea Frontiers, vol. 21, no. 3, May-June 1975 and in "Living Power Plants," Sea Frontiers, vol. 3, no. 2, June 1957.
