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Magnetic energy is the energy that is stored in a current-bearing coil in the form of its magnetic field. It is the result of the work that the current has to perform in opposition to the induced voltage (Faraday’s law of induction). Conversely, this magnetic energy is released again in the form of electric current when the magnetic field collapses. Magnetic energy is also stored in a magnetized material. It is equivalent to the work that must be expended in order to align the magnetic dipoles of this material in an external magnetic field. In ferromagnetic materials, the magnetic dipoles align themselves in small zones (“Weiss domains), even without an external magnetic field. If the Weiss domains are now aligned by an external magnetic field, a permanent magnet is produced. Incidentally: If a permanent magnet is heated above a critical temperature, it loses its magnetization. The magnetic energy is released as additional heat at this so-called Curie temperature. Information and ideas: A simple experiment on magnetization: If you pass a permanent magnet over an iron nail, it magnetizes the nail. What work has to be expended for this, apart from the friction work? Is the permanent magnet or its magnetic energy “used up in the process?
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If a metallic disc moves through a static magnetic field whose field lines are perpendicular to the direction of movement, voltages are induced in the disc and eddy currents form. According to Lenz’s law, the induced current generates a magnetic field to oppose the change in magnetic field that produced the current. The braking effect is examined at the edges of the static magnetic field. At the position where the disc enters the external magnetic field, the current tries to maintain the state “without the magnetic field”. Therefore, the current is directed such that its induced magnetic field offsets the external magnetic field. The fields oppose each other (in the graphic, the induced field is directed “outward”), resulting in magnetic repulsion. The resulting force counteracts the direction of movement, slowing down the disc. The situation is reversed at the position where the disc exits the magnetic field. The induced current tries to maintain the state “with the magnetic field”. Therefore, it is directed such that its induced magnetic field strengthens the external magnetic field (in the graphic, the induced field is directed “inward”). This results in magnetic attraction. The resulting force once more counteracts the direction of movement, slowing down the disc again. Information and ideas: This principle is also applied to eddy current sorting of residual waste containing aluminum.
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The sun is the main supplier of energy to the earth, providing about 99.98% of the total energy contribution to the earth’s climate. The vast potential offered by the technical use of solar energy as a source of energy is shown clearly by the fact that the current world energy consumption is only 0.006% of the solar energy received. The diagram provides an overview of the quantities of energy given off by the sun and arriving on the earth. It should be noted that 100% of the energy radiated by the sun is ultimately given off again by the earth back into space. The energy balance of the earth is equal at all levels from the surface of the earth into outer space. But watch out: A tiny fragment of the radiated energy is stored by photosynthesis (approx. 0.1%) or by human activities (approx. 0.005%) and remains on the earth for a longer period of time. Information and ideas: To make the illustration clearer, the size ratios of the sun and the earth are not shown to scale. It is taken into account that ultimately 100% of the radiated energy is reflected back into outer space.
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A wind power plant converts the kinetic energy of air masses to electric energy. This information module explains how this works. There is particularly great potential in the use of wind energy on the open ocean (offshore wind energy). High-voltage direct-current transmission (HVDC) is used for this purpose. Since power generated from wind energy is not available around the clock, energy stores are needed. The statistics show the share of wind power in the power mix of individual countries. By the way: an upwind power plant (solar chimney) is not a wind power plant, but a solar thermal power plant. Information and ideas: Suitable as an introduction to a field trip to a wind power plant. Using the following sources: ABB AG; Fraunhofer IWES; WindEurope; GWEC
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The three-bladed wind turbine with horizontal rotation axis shown here is the most common design for large wind power plants. The wind turbine consists of a rotor and a nacelle (engine housing), which are installed on a high tower. How it works: The anemometer measures the wind velocity. The data is sent to a monitoring computer, which controls the turbine and operates the yaw motor, which orients the wind turbine. When the turbine is in the optimum position relative to the wind, the wind applies a torque to the rotor blades: The wind turbine rotates (approximately 20 rotations/min) and, along with it, the drive shaft. The gearset converts the rotational speed of the rotor to the rotational speed needed for the generator (in Europe: 1,500 rpm or 3,000 rpm; in the United States: 1,800 rpm or 3,600 rpm). The generator generates the electric power, which is conducted down to the base of the wind turbine via cables. There it is fed into the grid. Under optimum wind conditions, the efficiency of a wind turbine is around 40–51 percent. (The theoretical maximum limit is around 59.3 percent, but this is practically unattainable.) By the way: the brake prevents the wind turbine from rotating, for example, in violent storms or when it requires maintenance. (There are also wind turbines without gearsets; see the description in the “Generator for a wind turbine” medium.) Information and ideas: What are the advantages of a three-bladed wind turbine over a one-, two-, or four-bladed turbine? It should be noted that wind turbines with a vertical rotation axis also exist (Savonius and Darrieus turbines). When are these designs used?
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