Magnetism
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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations. See the links below.
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Magnetic Moments

Small particles like electrons, quarks, and protons have properties like mass, charge, and the intrinsic "magnetic moment." The magnetic moment describes how much like a "magnet" these particle will behave. Electrons have a magnetic moment about 1000 times large than the proton. Because of this huge difference, we will ignore the magnetic moment of protons.

Electrons exist in atoms in orbital shell. When a shell is filled by a pair of electrons, their magnetic moments cancel each other out.

But if some of those shells are filled with single electrons, then the atoms will behave more like magnet.

The most magnetic naturally occurring substances are iron, nickel and cobalt. Neodymium is used an alloy of Neodymium-Nickel-Cobalt to make the strongest magnetic material. Typically these magnets are referred to simply as, "Neodymiums."

Materials that are very magnetic are said to be Ferromagnetic. This term comes the periodic symbol for iron.

 

Magnetic Domains


When electrons get together in a substance some of their magnetic moments group together such that most will point in the same direction. This region of the same direction is called a domain. Domains vary in a substance like iron. The more domains you get to line up the stronger the magnetic property.

 

Here is a substance with various domains, These domains point in random directions. But some point in the same direction. (They are yellow.)

You would probably not consider the materials above very "magnetic." The material below has so many domains lining (in yellow,) up without any outside magnetic fields to push them around that it would be considered ferromagnetic.

In other words it is what you would call a permanent magnet.


Below is a piece of iron that is place in an external magnetic field. When hit, domains are given the energy to move in place. These domains line up with the external magnetic field. This increases the materials magnetic property.

When the domains in a ferromagnetic material line up, they remain lined up. Compare this to a paramagnetic material like aluminum or copper. In a changing external magnetic field or when charges move through the material, the domains line up. But in the absence of a changing magnetic field or an electrical current, the domains return to their random directions. paramagnetic materials do not permanently keep any ferromagnetic properties.

 

A diamagnetic material's domains line up to oppose any outside magnetic fields as long as the field is present. In the absence of the outside magnetic field the domains become random. Because the domains rotate, diamagnetic materials repel magnetic fields.

Diamagnetic materials can hover in the presence of a strong enough magnetic field. Water is a diamagnetic material. Because the frog, shown below, is mostly water, it is levitating in one of the strongest magnetic fields on the Earth.

This video can be found on YouTube at https://youtu.be/A1vyB-O5i6E

 

Cool Fact


If you break a magnet you get two smaller magnets. Each with a north and south pole.

You can keep doing this until you get down to electrons and protons. Each time you will get a smaller magnet with a north and south pole. If you could break the electrons you get a Nobel prize or a very big electric bill. ...But you would no get a smaller magnet ;-)

Monopoles are described as the existence of either a north pole or a south pole. Stable monopoles do not exist. If any one tries to sell you a monopole magnet, don't buy it because it is not real.

 

Magnetic Fields

Like electric fields, magnetic fields provide an invisible structure in space for delivering a force.

Like other fields, magnetic field lines, (refered to as b-field lines,)

  • do not touch each other and
  • are perpendicular to the suface.

Here are some characteristics specific ot b-fields

  • B-fields exist inside of the magnetic material. (Electric fields can exist inside of insulating containers but not containers made from a conductor.) This is because magnetic fields are created by the electrons inside of the materials.
  • B-fields seek the south outside of the magnetic material.

Below is a magnetic field for a bar magnet.

 

 

Attraction and Repulsion
  • The b-field lines point in the same direction, the source of the fields are repelled.
  • The b-field lines point in the opposite direction, the source of the fields are attracted.

In the image below the b-field lines point in the same direction. Therefore the two magnets are repelled from each other.

 


 

Answer the four questions below based on the picture below.

True or False

Question 1

Magnet "A" is repelled from magnet "C"

Answer

False. The b-fields between "A" and "C" point in opposite directions. This means they are attracted to each other.

 

Question 2

Magnet "A" is repelled from magnet "B"

Answer

True. "A" and "B" are repelled from each other because hte b-field that are between them point in the same direction and "likes repel."

 

Question 3

Magnet "B" is attracted to magnet "D"

Answer

False. The b-fields between "B" and "D" point in the same direction. These magnets repel each other.

 

Question 4

Magnet "D" is attracted to magnet "C"

Answer

True. The b-fields lines between "D" and C" point in opposite directions. These magnet attract each other.

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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)