Background
Water is more electrically conductive than air. Electrical resistivity is a property of a material and not of the specific dimensions of an object. It is measured in ohm-meters or W m. The ohm(W) is the unit of resistance named after Ohm.
Georg Simon Ohm (1787 - 1854). Born in Erlangen, Germany, his later work as a physicist resulted in the 1827 discovery of the mathematical law of electric-current called "Ohm's Law."
The ohm, a unit of electrical resistance, is equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.. If you know the resistivity of a material, multiply by its length, and divide by its area, this will give you the resistance.
Air is 20 billion times more resistive than seawater. Also, the higher the temperature or salinity, the lower the resistivity. Because of this, the ability to detect electrical signals is fairly common among aquatic organisms. Most animals produce small amounts of electricity, for example, through small muscle contractions while breathing. Sharks and rays are able to detect this electrical activity, and use this ability to find their prey. Other fish use electric pulses to communicate. Especially in dark waters where the sense of sight may not be useful, these tiny electrical impulses may be the only clue that another animal is around. However, because the electrical field rapidly attenuates, an electric sixth sense can never replace vision or sound as a means for sensing the environment at greater distances.
Electricity and its behavior in water also present some aquatic animals with still other possibilities. The electric eel uses a strong pulse of electricity to stun or even kill its prey or to warn away an intruder. The shark detects the electrical field created when seawater (a conductor) flows in the earth’s magnetic field. This ability allows the shark to orient and navigate in the absence of visual cues.
Animals
We have chosen sharks and electric eels for the exhibits on electricity.
Sharks have fascinated scientists for years. The shark has changed little in more than 300 million years in the earth’s oceans, and they possess finely-tuned, sharp senses and unique sense organs like the ampullae of Lorenzini. Although they are considered to possess a low level of intelligence by many experts, they are legendary hunters of the seas.
Classified as fish, sharks are among the oldest creatures on our planet. Sharks differ from most fish because their skeletons consist of soft, flexible cartilage instead of bones. While the cartilage enhances a shark’s swimming ability, it offers little support for the shark’s internal organs out of water. Just a few minutes out of water will give a shark a fatal injury.
Sharks must also swim continuously to stay afloat because they don’t have the air-filled swim bladder found in other types of fish. Most top-swimming sharks must also continue moving to obtain the oxygen necessary to stay alive.
As many as 62 species of shark roam the eastern waters of North America. Some stay in the high seas, far from shore, while others live close to the bottom, but many inhabit shallow waters in estuaries and close to shore.
A shark can navigate across the ocean using the earth's magnetic field.
As it swims through the water it senses the tiny voltages that are generated.
Electric Eel
Electric Eel is
found in the Amazon Basin, in marshy areas or stagnant arms of rivers, areas where other
fishes find it difficult to live because of the deficiency of dissolved oxygen.
The electric eel eats other fish, killing them with electric shocks of up to 600 volts.
Electric eels can fatally electrocute a horse. Their vital organs are located immediately
behind the head; the other 7/8 of its body is tail, containing the electrically generating
organ. This organ is composed of 5000-6000 electrocytes, and is arranged like a dry
battery. The head acts as the positive pole of the battery, the tail as the negative pole.
When the eel is at rest there is no generation of electricity, but when it starts to move
it emits electrical impulses at the rate of about 25/sec. During intense feeding
discharges of up to 50/sec have been recorded.
These discharges aid in locating food and navigation, as well as the killing of prey.
Small animals within range are killed outright, while large mammals may become dazed and
drown. A human can withstand one discharge, but would not survive several.
As in other electric fish, low-amplitude electric discharges may function in communication, although this has not been well studied in this species. The species is not currently threatened or endangered, but may be in the future due to habitat destruction.
Exhibits
SciTech has three exhibits that explore how aquatic animals use electricity to survive and thrive in their environments: Shark Navigation and its companion exhibit, Swinging Generator, and Electric Eel.
Swinging Generator demonstrates the physics principle that moving magnets generate an electric current. This simple physics concept is of profound importance to the shark’s ability to find its way in a dark and sometimes featureless ocean.
Concept: Faraday effect
Objective: Students will be introduced to the simple and important physics principle known as the Faraday effect. Students will later relate this principle to the shark’s ability to navigate.
How to use: Swing the magnet past the coil. Watch the meter as the magnet swings. Notice that an electric current is being generated.
Vary the
parameters of the exhibit by:
Explanation: Unseen lines of magnetic force spread outward from the magnet. Some of the lines of force go through the coil. Current is made in the coil as the number of lines going though it changes. The faster the number of lines changes, the more current is generated. This is known as the Faraday effect. Michael Faraday, English physicist 1791, 1867
Physics-Biology Connection: Sharks use this same physics principle to navigate in underwater darkness. Although sharks have excellent sight, they can’t rely on sight in dark or murky waters. They need another, reliable way to find their way in the ocean. Their electric sense allows them to take readings like our meter, so they know what direction they are swimming.
Shark Navigation
Shark Navigation allows students to manipulate a model shark across a magnetic field (representing the Earth’s magnetic field) to show the Faraday effect in action.
Concepts: Faraday effect, ampullae of Lorenzini
Objective: Students will make the connection between shark navigation and the Faraday effect.
How to use: Move the model shark at the end of the white pole in different directions. Watch the colored lights on the oval picture of the shark at the top of the pole. Notice that the lights flash when the shark moves across the magnetic field, but not when it moves in the same direction as the magnetic field.
Explanation: There are magnets in this exhibit representing the earth’s magnetic field. An electrical field is created when the shark moves across the earth’s magnetic field. In the exhibit, the lights flash when electricity is generated. This is known as the Faraday effect.
Sharks sense these weak electrical fields through passages in their skin called ampullae of Lorenzini. The cells in their ampullae are so sensitive they can detect electric fields as small as 0.01 microvolts per centimeter! The colored lights on the oval shark picture are a visual way to see this electric sense. The fact that water is a good conductor of electricity makes it possible for sharks to possess this sixth electrical sense.
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1.liver 2.Spiral valve 3.Heart 4.Pancreas 5.Stomach 6.Eye 7.Nostril 8.Ampullae of Lorenzini |
9.Teeth 10.Pectoral Fin 11.Pelvic Fin 12.Anal Fin 13.Caudal or Tail Fin 14.Second Dorsal Fin 15.First Dorsal Fin 16.Gills |
Physics-Biology Connection: A shark uses this ability like a compass to orient and navigate in the absence of visual cues. Besides navigation, sharks use their amazing electric sense to find prey, or food. Most animals conduct small amounts of electricity when they use their muscles, for example, as they breathe. Sharks sense these tiny currents and know exactly where their prey is.
Electric Eel presents students with an opportunity to compare the
electrical resistivity of air and water. An electric eel is like a bunch of
batteries all hooked together. The eel generates a voltage which is conducted by the water
and stuns or kills its prey.
Concepts: Electrocytes, Electrical resistivity
Objective: Students will learn that water conducts electricity much better than air.
How to use: Use the probes to touch the body of the
model electric eel in the upper tank (air). Notice the
reading on the meter. Use the probes to touch the plates in
the lower tank (water). Notice the reading on the meter.
Students can vary the distance at which the probes touch the electric eel (air) and the
plates (water).
Explanation: An electric eel’s body has thousands of tiny battery-like cells called electrocytes, which produce electric current. The electrocytes are arranged end to end like the batteries in a flashlight. In the upper tank of air, the probes must touch the eel itself to take a reading. In the lower tank of water, the probes only need to touch the copper plates leading to the water to take a reading. This is due to water’s low resistivity to electricity. The eel emits a pulse of electricity. The graph shows how long the pulse lasts.
Physics-Biology Connection: Electric eels use their environment to help them survive. Taking advantage of water’s excellent conductive properties, the electric eel sends out a pulse of electricity to stun an enemy, or to kill its prey. An older, longer eel (with more electrocytes) can generate a much stronger electric pulse than can a younger, shorter eel.
Electric eels can fatally electrocute a horse. The vital organs are located immediately behind the head - the other 7/8 of its body is tail, containing the electrically generating organ. This organ is composed of 5000-6000 elements, arranged like a dry battery. The head acts as the positive pole of the battery, the tail as the negative pole. When the eel is at rest there is no generation of electricity, but when it starts to move it emits electrical impulses at the rate of about 25/sec. During intense feeding discharges of up to 50/sec have been recorded.
These discharges aid in locating food and navigation, as well as the killing of prey.
Small animals within range are killed outright, while large mammals may become dazed and
drown. A human can withstand one discharge, but would not survive several.
rev. May 30, 2000. E.M.