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Electric Induction

Electric Induction.

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Electric Induction

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  1. Electric Induction In 1820 Hans Christian Oersted found that an electric current flowing through a wire created a magnetic field. This was the first discovery linking electricity and magnetism. British scientist Michael Faraday believed that if a current could induce a magnetic field, then a magnetic field should, inversely, induce a current. The main idea behind electric induction, as investigated by Faraday, is that an electric current can be “induced” (generated) by moving a magnet through a closed coil of wire. Also, reversing the direction in which the magnet is moved (thus reversing the magnetic field) reverses the electric flow. The results were the same whether the magnetic field moved past a stationary conductor (coil), or whether the conductor moved past a stationary magnetic field. Oersted Michael Faraday 1791-1867 British ‘natural philosopher’ http://www.bookrags.com/sciences/sciencehistory/electric-induction-wsd.html

  2. Electromagnetism The main ideas A) A current generates a magnetic field B) Changing magnetic fields generate current induced magnetic field current http://micro.magnet.fsu.edu/electromag/java/compass/index.html http://micro.magnet.fsu.edu/electromag/java/faraday2/index.html

  3. ElectromagnetismA question Compass inside a coil Battery Solid Iron ring Inside two separate coils Switch What would you expect to happen upon closing the switch in this circuit? For animation of circuit: http://micro.magnet.fsu.edu/electromag/java/faraday/

  4. Electromagnetism The solution • When the switch is closed a current passes through coil 1. That generates a magnetic field in the coil. • The iron ring, being ferromagnetic, magnetizes, carrying the magnetic field around it to coil 2. Now a change in magnetic fields has occurred in coil 2. Coil 1 Coil 3 Coil 2 • Coil 2 was under the influence of the earth’s very weak magnetic field, but is now influenced by the magnetic field generated by coil 1. The magnetic field increases momentarily, then becomes steady. According to Faraday, a current must then be generated through the wire in coil 2, but only while the magnetic field is changing. Once the field is steady, no additional current flows in coil 2. • Now, the same thing that happened in coil 1 is happening in coil 3. The momentary current created in coil 3 creates a magnetic field that orients the needle left-right. Once the current ceases, the compass returns to its original direction, responding to earths field. • You will only see the compass change again when the switch is opened. The magnetic field reduces, creating a momentary current in coil 2 but in the opposite direction.

  5. Magnetism Electron orbitals: • An electron orbital: • Is the region of space about the nucleus of an atom that an electron of that atom has a finite probability of occurring. • Is not analogous to planetary orbit! A simple view of the atom looks similar to planetary bodies and you may have pictured the electrons as orbiting around the nucleus, but to plot a path for something you need to know exactly where the object is and be able to work out exactly where it's going to be an instant later. You can't do this for electrons, so we say that they occupy regions called orbitals. • Is a region that has specific geometry based on the number of electrons “belonging” to the atom. To see 3-d images of the shapes of electron orbitals go to: http://winter.group.shef.ac.uk/orbitron/ and look specifically at the 1s, 4s, and 4d orbitals. • Another useful website about atomic orbitals and the behavior of electrons within them is : http://www.chemguide.co.uk/atoms/properties/atomorbs.html

  6. Magnetism Magnetizable materials On a macroscopic basis, we know that moving electric charges create a magnetic field. This is true also on a quantum basis: moving electrons create a magnetic field. Typically, we refer to an electron’s ’spin’ as creating the magnetic field. However, the electrons do not spin, neither about their own axes nor about the nucleus. To picture this, have a look at the typical orbitals of electrons in a chemistry book. An electron can be on one side AND the other side of a nucleus, but not in between. This weird characteristic arises from the particle-wave duality of small particles (electrons, photons, neutrons). Nonetheless, electrons create a magnetic field. Now, in materials where the electrons are paired, each electron will possess equal and opposite magnetic fields. Thus, many atoms are not inherently magnetic. For iron, nickel, cobalt, and gadolinium, there are unpaired electrons in their d-orbitals (remember s,p,d,f?). These ‘mini-magnets’ can be aligned to form a are the four ferromagnetic elements http://en.wikipedia.org/wiki/Ferromagnetic) In the presence of an external magnetic field, the electron spins will be influenced and become the same. When the magnetic field is taken off, the electrons continue spinning in that same direction because it requires energy to re-randomize them. Heating or dropping a magnet can re-randomize the electron spins and thus cause demagnetization. For a better understanding and an idea of what electron “spin” actually is go to: http://www.wonderquest.com/physics-magnetism.htm It is useful!

  7. Magnetizing materials So what exactly happens when ferromagnetic materials become ‘magnetized’? Below is an example of the process of magnetization. Imagine that the large rectangle is a piece of iron, and the smaller boxes within it are its separate electron domains. The lines going through it represent the magnetic field of the unmagnetized iron based on the random alignment (i.e. magnetization) of the unpaired electrons within those domains.

  8. Magnetizing materials Now the piece of iron has come into contact with a strong permanent magnet. The electrons, being unpaired and thus having random spins, are now lining up in accord with the magnet and are aligning themselves in the same orientation throughout the rectangle.

  9. Magnetizing materials This image shows the magnetic field lines after complete alignment of the electrons throughout the entire piece of iron. You can see that it has a distinct north and south pole with a totally uninterrupted magnetic field.

  10. Electromagnetism Eddy current When a metal conductor (picture an iron metal ring or solid piece of copper foil) is moved through a magnetic field, or a magnetic field is moved past a stationary metal conductor, a current is generated in that conductor. We know this because changes in magnetic fields generate current. This current travels in little loops called “eddies” and are thus named eddy currents. But what happens when a current flows through a continuous closed circuit? According to Faraday a magnetic field must be generated, and in this case it will be generated in the opposite direction of the magnetic field the conductor is passing through. We all know that for every action there is an equal and opposite reaction, right? It’s the same concept at work here: you expose the conductor to a force from a magnetic field and so that conductor works to oppose that force. Using the right-hand rule we can determine which direction the current must flow to generate an opposing magnetic field. Donot think of this phenomenon as cause and effect, instead view the field and conductor as a coupled system. Remember the magnet falling much more slowly through a copper tube? This was an example of eddy currents at work. The current generated by the magnet moving through the copper pipe generated its own magnetic field to oppose it, causing the magnet to travel slowly. The magnet traveling through an insulator (plastic pipe) does not generate any current, and so drops as expected. For a great illustration of what is happening here go to : http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html

  11. Magnetism Diamagnetism: levitation Quite simply, diamagnetism is the ability of an object to repel magnetic fields completely. Practically all matter on this earth is diamagnetic, even you, but this property is only exhibited in the presence of an externally applied magnetic field. Diamagnetism is the result of changes in the orbital motion of electrons due to the application of a magnetic field, which creates a magnetic force on moving electrons. This force changes the centripetal force on the electrons, causing them to either speed up or slow down in their orbital motion. This new speed modifies the magnetic moment of the electron orbital in a direction against the external field. The magnetic force of the floating object is great enough that it can completely oppose the force of gravity but small enough that the magnetic force of the object below it can suspend the object above itself. The result? Levitation! You can see more levitating objects here : http://www.hfml.ru.nl/levitate.html Levitating pyrolytic graphite http://en.wikipedia.org/wiki/Diamagnetism

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