184_notes:perm_mag

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184_notes:perm_mag [2021/06/10 03:21] bartonmo184_notes:perm_mag [2021/07/05 21:52] (current) schram45
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 This means that within an atom there are at least 3 (more if there are more electrons/protons) sources of magnetic field simply from the way that the charged particles are moving. For each atom then, we can say that there is a net magnetic field that would point in a certain direction (typically represented by a small B-field arrow). This means that within an atom there are at least 3 (more if there are more electrons/protons) sources of magnetic field simply from the way that the charged particles are moving. For each atom then, we can say that there is a net magnetic field that would point in a certain direction (typically represented by a small B-field arrow).
 [{{184_notes:Week9_8.png?100|Net magnetic field on an atom  }}] [{{184_notes:Week9_8.png?100|Net magnetic field on an atom  }}]
-=== Non-Magnetic Materials ===+===== Non-Magnetic Materials =====
 [{{ 184_notes:Week9_9.png?200|Randomized magnetic fields sum to a net magnetic field of zero}}] [{{ 184_notes:Week9_9.png?200|Randomized magnetic fields sum to a net magnetic field of zero}}]
-If every atom has a magnetic field from its moving charges, why is //everything// not a magnet? For a non-magnetic material (like plastic, wood, glass, etc.) the direction of the magnetic field for every atom is randomized (shown in the figure). When you take the add up the magnetic field vectors from all the atoms in the material, you get a net magnetic field of zero. +If every atom has a magnetic field from its moving charges, why is //everything// not a magnet? For a non-magnetic material (like plastic, wood, glass, etc.) the direction of the magnetic field for every atom is randomized (shown in the figure to the right). When you take the add up the magnetic field vectors from all the atoms in the material, you get a net magnetic field of zero. 
  
-=== 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 =====
 [{{ 184_notes:Week9_11b.png?350|Magnetic field vectors in induced magnets with time}}] [{{ 184_notes:Week9_11b.png?350|Magnetic field vectors in induced magnets with time}}]
 There are also some materials that will become "temporary magnets" if exposed to a magnetic field. In this case the material starts out with randomized magnetic fields from its atoms. When this material is placed near a strong external magnetic field the fields from all the atoms start to align with that external field. When the external magnetic field is taken away, the magnetic fields from the atoms remain aligned for some time before returning to their randomized directions. You may have seen this when you place a nail near a strong magnet. The nail will become a magnet for a time and be able to pick up other nails; however it does not remain a magnet indefinitely.  There are also some materials that will become "temporary magnets" if exposed to a magnetic field. In this case the material starts out with randomized magnetic fields from its atoms. When this material is placed near a strong external magnetic field the fields from all the atoms start to align with that external field. When the external magnetic field is taken away, the magnetic fields from the atoms remain aligned for some time before returning to their randomized directions. You may have seen this when you place a nail near a strong magnet. The nail will become a magnet for a time and be able to pick up other nails; however it does not remain a magnet indefinitely. 
  
 For a demonstration of how you can create a temporary magnet, [[https://www.youtube.com/watch?v=1vQDlB-0mC8|you can check out this youtube video]]. For a demonstration of how you can create a temporary magnet, [[https://www.youtube.com/watch?v=1vQDlB-0mC8|you can check out this youtube video]].
-==== Magnetic Field from a Bar Magnet ====+===== Magnetic Field from a Bar Magnet =====
 [{{184_notes:magnet0873.png?200|Magnetic field modeled by iron filings around a bar magnet  }}] [{{184_notes:magnet0873.png?200|Magnetic field modeled by iron filings around a bar magnet  }}]
 One kind of magnet that we will refer to often is a bar magnet, which usually refers to a straight rectangular ferromagnet with a constant, strong magnetic field (whereas a permanent magnet can generally have more varied shapes - like a disk, u-shape, coil, etc.). Since a bar magnetic is generally a ferromagnet, this means that all the little magnetic fields from the atoms are aligned and point in the same direction, resulting in a larger, net magnetic field outside the material. If you happen to drop some iron filings around a bar magnet, the filings will align themselves with the net magnetic field of the bar magnet (shown to the side) so you can actually physically see the shape of the magnetic field. One kind of magnet that we will refer to often is a bar magnet, which usually refers to a straight rectangular ferromagnet with a constant, strong magnetic field (whereas a permanent magnet can generally have more varied shapes - like a disk, u-shape, coil, etc.). Since a bar magnetic is generally a ferromagnet, this means that all the little magnetic fields from the atoms are aligned and point in the same direction, resulting in a larger, net magnetic field outside the material. If you happen to drop some iron filings around a bar magnet, the filings will align themselves with the net magnetic field of the bar magnet (shown to the side) so you can actually physically see the shape of the magnetic field.
  
-[{{ 184_notes:Week9_12.png?350|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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  • Last modified: 2021/06/10 03:21
  • by bartonmo