Magnetism Unit Summary

Magnets and Magnetism:

Magnets:

The source of all magnetism is moving charges. These charges are spinning and moving inside an object and they congregate in groups called domains. The reason magnets are magnetized is because all of their domains are aligned. Domains are groups of moving charges that are grouped because they are all spinning in the same direction. When all of the domains align in the same direction, the object is magnetized. 

This is what the domains look like in a magnetized and not magnetized object:

 When an object is magnetized, it has a north and a south pole. The way you can tell which is which is by looking at the direction of the domains. In a magnet, the field lines(which indicate the direction the domains are facing) run South to North always. Here is a picture of field lines and poles in a magnet. 

If you were to place a permanent magnet very close to a table covered in tiny magnet fragments, the magnets would be attracted to the magnet and the result would reveal an actual visual of these field lines. It would look like this: You can see the magnets aligning with the domains of the permanent magnet and you can see the actual field lines. 

The poles of a magnet and the field lines help us to explain their attraction or repulsion. 

Opposite poles attract because when the the North Pole of one magnet is next to the South Pole of another, it is oriented so that the field lines of both magnets are going in the same direction. This is a helpful representation of this: (the arrows above the magnets indicate that they are attracted. Don't be confused, they do not indicate the direction of field lines). 

Similarly, Like Poles repel because when two like poles are situated next to each other as shown above, the field lines run in opposite directions. This makes it so that when they get close, the fields bump heads instead of cohering. 

Moving Charges Feel a Force in a magnetic field:

Another thing we learned about moving charges is that when they move perpendicular to a magnetic field, the feel a Force. 

There are two good examples of this in action. One is the northern lights. The northern lights are cosmic rays that are able to enter earth's atmosphere. Why do they only occur at the poles? Consider this image of the Earth's field lines:

When the charges of the cosmic rays are moving perpendicular to the magnetic field of the earth, as they would need to do to enter the earth at the equator, the charges feel a Force and are forced back into space. However, when the charges try to enter at the poles, they are moving parallel to the magnetic field. You can imagine a charge entering along side the lines at the top. This phenomenon that occurs at the poles is exclusive to the poles because anywhere else, the charges move perpendicular and are forced away. 

Another thing that we may apply this concept to is motors. A motor uses the force felt by charges in a magnetic field to create a torque. 

A motor consists of a battery which provides current to a current carrying wire that you would place near a magnet. The charges in the wire feel a force from the magnetic field of the magnet, and, if you strategically allow the force to be exerted, you can create a torque and spin the wire. 

A version of a simple motor like this looks like this: The loop of wire is scraped so that the force felt from the magnetic field is used to turn the loop rather that just shake it. 

By using the current carrying wire and magnet to move the loop of wire, a motor changes Electromagnetic Energy into Mechanic Energy. 

Electromagnetic Induction:

Electromagnetic Induction is a very important topic that we learned about this unit. Electromagnetic Induction uses a change in a magnetic field to induce a Voltage which causes a current. This is the converse of the way a motor works. Whereas a motor turned Electromagnetic Energy into Mechanic Energy, Electromagnetic Induction uses Mechanical Energy and turns it into Electromagnetic Energy. 

A common and useful example of where this is employed is at stoplights. At stoplights, there is a coil of wire in the asphalt. Your car has a magnetic field and when you drive over the coil of water, it changes the magnetic field (this is using Mechanical Energy). The change in the magnetic field induces a voltage and that voltage causes a current (this is the Electromagnetic Energy). The current caused is sent to the stoplight so that it knows you are waiting for a green light.  

You may recognize the loops on the road, they look like this:

The second and most important implementation of Electromagnetic Induction is in Transformers. You may recognize a transformer as that box on your computer charger, or the structure between power lines. The Transformer is used to either step-up or step-down voltage. For instance, a wall socket always has 120V, however your computer may need less (like 10V) that that so the transformer will step-down. 

The transformer relays on the same concept: Transformers consist of coils of wire that have AC current in them. This current has charges moving back and forth which changes the magnetic field. The change in magnetic field induces a voltage.

Here is where the process changes a little. The Transformer consists of two coils of wire, each with a different number of loops. Faraday's law is a law that states that the number of loops is directly proportional to the voltage, so if you want to step-up the Voltage you have to increase the number of loops in the Second coil and vise versa for step-down. The first coil(generally the one that hooks up to the outlet) is called the Primary. The one connected to the device is called the Secondary. 

So, when the voltage is induced in the Primary, by proximity it induces a voltage in the Secondary but because of the different number of loops, the voltage is either smaller or larger than before. In this system, Power is conserved so we use this equation to help us find Voltage and number of loops in each:  

#loops in 1/Voltage in 1 = #loops in 2/Voltage in 2

That is our Unit on Magnetism, Thanks for reading!