Unit 8 Reflection

This Unit was all about magnetism and the benefits of this great discovery! On the chapter about electricity we learned that charged particles follow the Coulomb's Law - that means that the magnitude and the distance between two charges is the determinant of their attraction. When it comes to Magnetic Force the motion of the charged particle are also very important.

Every magnet has a south and a north pole, just like electrical charges, opposites  poles attract and like poles repel. It's important to remember that if there is a south pole, there is a north pole and the other way around - one cannot exist without the other. For because north and south pole can't ever exist without each other if a magnet was broken, regardless of the number of times, all pieces would have a north and south pole and therefore they would remain being magnetic. The space around a magnet is called a magnetic field.



http://www.geo.arizona.edu/xtal/nats101/s04-10.html

The earth itself has a magnetic field! This magnetic field explain why we only see the northern lights on the poles of the earth and not in the equator. Cosmic rays move towards the earth, and in the equator that movement is perpendicular to the earth's magnetic field, while in the pole it is parallel. When the rays move perpendicular there is a force of repulsion that causes the rays to be repelled by the magnetic field. When the rays move parallel there is no repulsion force, therefore the rays come in and are the lights we see!

http://www.unc.edu/depts/oceanweb/turtles/geomag.html

The earth's magnetic field is also the reason why a compass works. The key to a compass is a magnetized needle that is free to move. The needle will line up with the magnetic field of the earth and point north and south. If you want to build a compass, this is how you can do it!

The picture above shows the direction of the field outside the magnet - it goes from north to south pole. Do not assume that the direction of the field is the same inside the magnet! Inside the magnet it moves from south to north pole.  In order to understand why some things are magnetized and others are not, it is helpful to understand what domains are. Domains are a cluster of  atoms spinning in the same direction. A bar of iron, for example, is composed of many domains. The difference between a magnetized and a nonmagnetic iron bar is that the first one has its domains aligned with each other. Have you ever used one of those magnetic paper clip holders, like the one bellow?

http://dolphindentist.blogspot.com/2011/04/fun-theory-worlds-deepest-bin.html

If yes, have you ever noticed that the paper clips do not only stick to the holder but to each other?  That is because the paper clips have become magnetized, meaning that their domains are in line with the domains of the magnet in the paper clip holder. The paper clips now have a north and south pole as well and therefore are magnetized!

We also learned about electromagnets in this unit. Electromagnets are simply a current carrying coil of wire. They are really powerful and therefore are used, for example to lift up cars in a junk yard or to levitate trains!

After learning about electromagnets we learned about an important concept that is the reason why a motor works. That concept is: A current carrying wire feels a force in a magnetic field. The direction of the force can be easily determined by the right hand rule (picture in previous post). A motor works by this exact idea. There is a current carrying wire (current provided by the battery), a magnetic force due to the magnet and therefore there is a force that causes the copper loop to turn. For a more detailed explanation of motors, check out the previous post about building a simple motor.

The rest of the unit was based on electromagnetic induction! Electromagnetic induction happens when a magnet moves close to a wire (or the other way around) and therefore changes its magnetic field. The change in magnetic field results in the induction of voltage and therefore the creation of current. This concept is what explains the coils of wire on the floor in front of the traffic lights and the function of transformers. When cars drive by the wires on the floor, it acts as a magnet and therefore changes the magnetic field and induces a voltage. That induced voltage creates a current that signals the light to change.

A generator has a magnet and a coil of wire that receive the input of some type of mechanical force, the magnet creates a magnetic field that is changed when the coil of wire moves towards it. The change in magnetic field induces a voltage that creates a current. 

A transformer changes the voltage from a primary source to a secondary. The voltage is changed by using the idea of power (P = IV) and the number of turns on each side of the transformer. The number of turns (loops) in the transformer is the determinant of the output voltage. If the secondary has more turns than the primary the transformer steps up the voltage, if it has less it will step down the voltage. A transformer works on the principle that a current carrying wire feels a force in a magnetic field. With AC current there is a change in the magnetic field that induces voltage and creates current. Because of the difference in number of loops the ratio of current vs. voltage can be changed to fit the needs of an appliance. A laptop, for example, needs less current than the one coming from our outlets therefore it has a transformer that changes that voltage before it harms the computer. We learned about two equations related to transformers:

number of turns/voltage = number of turns / voltage

IV primary = IV secondary 

The first equation can let you find out how many loops of wire a transformer needs to have in order to step up/ step down a voltage. The second equation serves to find the amount of current or voltage on the transformer.



Reflection:

One of the hardest things of this unit was to remember the difference between a generator and a transformer. My teacher made the table bellow that was really helpful in understanding the difference between the two.

Besides that, the unit was simple. The idea of a compass confused me at first but after our review I understood that the needle of the compass lines up with the earth's magnetic field, or a stronger field - like on ships or big metal things that have not been moved.