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184_notes:perm_mag [2021/06/16 18:45] – [Non-Magnetic Materials] bartonmo184_notes:perm_mag [2021/07/05 21:52] (current) schram45
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 ===== Permanent Magnets ===== ===== Permanent Magnets =====
 [{{184_notes:Week9_10.png?200|Magnetic fields from atoms point in the same direction in permanent magnets  }}] [{{184_notes:Week9_10.png?200|Magnetic fields from atoms point in the same direction in permanent magnets  }}]
-In contrast to non-magnetic materials, permanent magnets (also called [[https://en.wikipedia.org/wiki/Ferromagnetism|ferromagnets]]) are materials where the magnetic fields from all the atoms point in (roughly) the same direction. This is usually accomplished by putting the material in a very strong magnetic field (typically from a large current rather than another ferromagnet). It is possible for the field from each of the atoms to start to randomize (or un-align themselves) but this generally takes a very long time for a permanent magnet. (There are also other kinds of magnetic materials besides ferromagnets, such as [[https://en.wikipedia.org/wiki/Diamagnetism|diamagnets]], [[https://en.wikipedia.org/wiki/Paramagnetism|paramagnets]], and [[https://en.wikipedia.org/wiki/Antiferromagnetism|antiferromagnets]] - but we won't talk about those in this class.)+In contrast to non-magnetic materials, permanent magnets (also called [[https://en.wikipedia.org/wiki/Ferromagnetism|ferromagnets]]) are materials where the magnetic fields from all the atoms point in (roughly) the same direction (seen in the figure above). This is usually accomplished by putting the material in a very strong magnetic field (typically from a large current rather than another ferromagnet). It is possible for the field from each of the atoms to start to randomize (or un-align themselves) but this generally takes a very long time for a permanent magnet. (There are also other kinds of magnetic materials besides ferromagnets, such as [[https://en.wikipedia.org/wiki/Diamagnetism|diamagnets]], [[https://en.wikipedia.org/wiki/Paramagnetism|paramagnets]], and [[https://en.wikipedia.org/wiki/Antiferromagnetism|antiferromagnets]] - but we won't talk about those in this class.)
  
 ===== Induced Magnets ===== ===== Induced Magnets =====
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 [{{ 184_notes:Week9_12.png?400|Magnetic vector field about a permanent bar magnet}}] [{{ 184_notes:Week9_12.png?400|Magnetic vector field about a permanent bar magnet}}]
-Since the magnetic field is a vector field, it has both a magnitude and a direction at every location in space, which we can represent with an arrow at various points around the magnet. Closer to the poles of the magnet, the field is much stronger, which we represent with longer arrows. By convention, we say that the direction of the magnetic field points away from the north pole and in towards the south pole outside of the magnet (note that the magnetic fields from the atoms inside the magnet point from the south pole to the north pole). So in lieu of carrying iron filings around with us, we represent the magnetic field of bar magnet as shown in the figure.  (You may notice that these magnetic field lines look very similar to those of an [[184_notes:pc_force|electric dipole]].)+Since the magnetic field is a vector field, //it has both a magnitude and a direction at every location in space//, which we can represent with an arrow at various points around the magnet. Closer to the poles of the magnet, the field is much stronger, which we represent with longer arrows. By convention, we say that the direction of the magnetic field points away from the north pole and in towards the south pole outside of the magnet (note that the magnetic fields from the atoms inside the magnet point from the south pole to the north pole). So in lieu of carrying iron filings around with us, we represent the magnetic field of bar magnet as shown in the figure.  (You may notice that these magnetic field lines look very similar to those of an [[184_notes:pc_force|electric dipole]].)
  
 ===== Measuring Earth's Magnetic Field and other Magnetic Fields ===== ===== Measuring Earth's Magnetic Field and other Magnetic Fields =====
 One of the most important magnetic fields (that we actually model as a giant bar magnet) is [[https://en.wikipedia.org/wiki/Earth%27s_magnetic_field One of the most important magnetic fields (that we actually model as a giant bar magnet) is [[https://en.wikipedia.org/wiki/Earth%27s_magnetic_field
-|Earth's magnetic field]]. Geophysicists believe that the source of the Earth's magnetic field are currents generated in conductive material in the Earth's core. This is particularly important when we are trying to measure a magnetic field, since all measurements are taken on Earth and will (in some way) have to include Earth's field. We have already mentioned that Earth's magnetic field is relatively small at ~$10^{-5}T$. If the magnetic field that you are measuring is large, then Earth's magnetic field may be negligible; however, if it is small, you may need to consider Earth's field more carefully in your considerations+|Earth's magnetic field]]. Geophysicists believe that the source of the Earth's magnetic field are currents generated in conductive material in the Earth's core. This is particularly important when we are trying to measure a magnetic field, since all measurements are taken on Earth and will (in some way) have to include Earth's field. We have already mentioned that Earth's magnetic field is relatively small at ~$10^{-5}T$. If the magnetic field that you are measuring is large, then Earth's magnetic field may be negligible; however, if it is small, you may need to consider Earth's field more carefully in your assumptions
  
 [{{  184_notes:Week9_13.png?200|Direction of earth's magnetic field}}] [{{  184_notes:Week9_13.png?200|Direction of earth's magnetic field}}]
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 Since Earth's field is relatively constant, we can actually use Earth's magnetic field to measure the magnetic field from other sources. So say we have a current that runs perpendicularly to Earth's B-field (shown as out of the page/screen in the figure). If we consider a point above the wire (Point P), the Earth's magnetic field will (always) point toward the geographic north. However, the current will produce a B-field at Point P that points to the west (or $-\hat{x}$) using the right hand rule (point your fingers out of the page then curl them toward Point P - your thumb should be pointing to the left). So if we put the compass at Point P, we would expect it to point in the direction of the //**net**// magnetic field (or to the north west in this case). The example below will go into more detail about how we can use that to calculate the size of the magnetic field from the wire. Since Earth's field is relatively constant, we can actually use Earth's magnetic field to measure the magnetic field from other sources. So say we have a current that runs perpendicularly to Earth's B-field (shown as out of the page/screen in the figure). If we consider a point above the wire (Point P), the Earth's magnetic field will (always) point toward the geographic north. However, the current will produce a B-field at Point P that points to the west (or $-\hat{x}$) using the right hand rule (point your fingers out of the page then curl them toward Point P - your thumb should be pointing to the left). So if we put the compass at Point P, we would expect it to point in the direction of the //**net**// magnetic field (or to the north west in this case). The example below will go into more detail about how we can use that to calculate the size of the magnetic field from the wire.
 ==== Examples ==== ==== Examples ====
-[[:184_notes:examples:Week9_earth_field|Using the Earth's Magnetic Field for Measurements]]+  * [[:184_notes:examples:Week9_earth_field|Using the Earth's Magnetic Field for Measurements]] 
 +    * Video Example: Using Earth's Magnetic Field for Measurements 
 +{{youtube>0kc-otyFZY0?large}}
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