12. The Dynamo Process
4. A Gilbert Expt.
5. Before Gilbert
6. London in 1600
8. Oersted & Ampére
9. The Lodestone
11. The Magnetic Sun
12. Fluid Dynamos
13. Dynamo in the
14. Magnetometers and
15. Magnetic Reversals
& Moving Continents
16. The Magnetosphere
17. Magnetic Planets
Faraday even speculated (incorrectly) that the flow of the Gulf Stream in the Atlantic Ocean was somehow electrically linked to the high atmosphere, powering there an electric discharge which (in his view) was the polar aurora ("Northern Lights").
For some unusual examples of this dynamo process, see the web sites about the dynamo involving Jupiter's moon Io and about the space tether experiment on the Space Shuttle.|
Faraday's disk dynamo needs a magnetic field in order to produce an electric current. Is it possible for the current which it generated to also produce the magnetic field which the dynamo process required? That, in a nutshell, was what Larmor proposed was happening in sunspots.
At first sight this looks like a "chicken and egg" propostion: to produce a chicken, you need an egg, but to produce an egg you need a chicken--so which of these came first? Similarly here--to produce a current, you need a magnetic field, but to produce a magnetic field you need a current. Where does one begin? Actually, weak magnetic fields are always present and would be gradually amplified by the process, so this poses no obstacle.
One could, for instance, link two Faraday dynamos, each supplying the current needed to produce the other's magnetic field. They could (in principle) form a feasible self-excited dynamo, deriving its energy from whatever force was turning the disks. However, whether a fluid dynamo could mimic this behavior--e.g. swirls of fluid flow in a tank of liquid mercury--is a much harder question and took decades to resolve.
(More about dynamos in the next section and in part #14 of
Exploring Further |
The book "Michael Faraday and the Nature of Electricity" by Roberta Baxter (144 pp, Morgan Reynolds publishing, Greensboro, North Carolina, 2009) tells of Faraday's life and work. A slim book probably meant for teen-agers, it is nevertheless rather detailed and quite readable.
Optional (Mainly for Teachers)
When the shaft of a dynamo is turned by some outside force, its electrical connections can produce an electric current. Many dynamos however are reversible: if an electric current is fed into its electrical connection, the force on the current can turn the same shaft, converting the dynamo into a motor. In some models of "hybrid" automobiles, electric motors fed by a storage battery help the gasoline engine accelerate the car, but then when the car slows down, they become dynamos and feed an electric current back into the battery, saving energy which otherwise might turn into waste heat in brakes.
The Faraday disk dynamo can also reverse and become a motor, though a rather inefficient one. A tabletop demonstration of this is described in "Magnetic Force on an Electrolyte" by Marián Kireš and Zuzana Ješkova, on p. 50-51 of "The Physics Teacher," Vol. 45, January 2007.
The rotating disk here is a shallow layer of water containing copper sulfate, which conducts electricity. The solution in put in a Petri dish and placed on top of one of the poles of a vertical laboratory electromagnet; little pieces of cork floating on the fluid then trace its rotation
The electrodes are two circles of copper (they can be sawed from the ends of pipes)--a big one on the outer boundary of the solution and a small one in the middle. When the terminals of a battery of an electric power supply are connected to the two copper circles, the fluid starts rotating. Reverse the contacts and the rotation reverses too.
Questions from Users: "Why does this happen?" (electromagnetic induction)
*** Can we tell if a symmetric magnetic field rotates around its axis?
*** Dynamo theory
*** Earth--conductor or insulator?
*** Rapidly reversing magnet
*** How can an intensely hot Sun be magnetic?
*** Electric field due to electromagnetic induction
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