Unit 7 Reflection

Unit 7 Reflection:  Magnetism

Magnets 

Moving charges produce a magnetic field. A domain is what we call a group of atoms whose electrons are all spinning in the same direction. When all of these domains are aligned is when an object becomes magnetized. Because all of these domains are now aligned they all point in the same direction. This is what creates the north and south poles. The charge always goes north then circles around the magnet into the south side and then up through the magnet to the north side again.  

In the picture above you can see what are called field lines. These are the magnetic charges that go throughout the magnet. As you can see when the north end meets a another north end they actually hit against each other and repel, the same goes for the south end. If you put a north and south pole next to each other the field lines go from the north side to the south side because their domains are facing in the same direction.

Horseshoe and refrigerator magnets are often called permanent magnets because of their lasting magnetic fields. However no magnet is truly a permanently magnetic, just as domains are aligned they can be unaligned. This can happen through a number of ways. Heat causes domains to be unaligned as well as simply hitting a magnet hard enough can jostle the domains. 

 Magnetizing a paperclip 

How can we magnetize a ferromagnetic material that is not magnetic, like a paper clip? First we need a magnet. A paperclips domains are all random, their all pointing in different directions and so cancel each other out. A magnet has a magnetic field because it's domains are aligned. When you bring the paperclip close the domains are attracted by the magnet and become aligned with the magnetic filed. The paperclip now has a north and south pole. The opposite pole of the paperclip is now attracted to the opposite pole of the magnet.

Earths Magnetic field 

Earth is a giant magnet. It has a north and south pole, but the geographic north pole is actually the magnetic south pole. This is why the north side on a magnet is attracted to the north pole. 

Cosmic Rays, Northern lights and Compasses. 

A charged particle moving through a magnetic field will only experience a force if it is moving perpendicular to the field. 

Cosmic Rays:  

 Every day the earth is being bombarded by particles (protons, electrons, photons) called cosmic rays. Due to earths magnetic field these particles are swept up, because they are moving perpendicularly with the field, which causes them to spiral towards the earths poles. 

Northern Lights: 

As the rays that travel along earths filed lines are brought to the poles they interact with the atmosphere creating auroras and other such things.  

Compasses: 

A compass is a tiny magnet allowed to swing around freely on a stick. Because of this it is affected by the earths magnetic field and when it is perpendicular to the field then it experiences a force that pushes it to be parallel with the filed lines and point north and south. 

 

Transformers

A transformer is a device that carries electric charges across empty space. They does by placing two coils of wire next to each other. By giving current (and an magnetic field) to one the other has a voltage induced to it. The coil that has a charge is called the primary, the coil that feels the voltage is called the secondary. It is important to note that voltage is only induce into the other coil only if the magnetic field is changing. This is why DC current will not work on a transformer, because it supplies a steady stream of current. Whereas AC switches back and forth constantly so the field is always changing. In a transformer the amount of coils (or turns) creates more voltage. If the primary has 10 turns and the secondary has 20, then the secondary would have twice as much voltage. This type of transformer is called a stepped up transformer. A transformer that has less secondary turns than the primary is called a stepped down transformer. This is how in a power line the voltage can be stepped up at the power plant and then sent long distances, then stepped down at a site closer to your house. This is similar to laptop chargers who use a step down charger in order to power your computer. Th
e wall socked may have 220 volts but the transformer in the charger pushes the voltage down to around 120 volts.

Equations 

Following these are the equations for the transformers: 

For voltage, turns and to show energy is conserved.

Primary Voltage/ No. of Primary Turns = Secondary Voltage/ No . of Secondary turns 

                                                                             or 

V(p)/Turns(p) = V(s)/ Turns  

Power is conserved equation 

PowerPrimary = PowerSecondary 

Voltage and current 

Primary Voltage X Primary Current = Secondary Voltage X Secondary Current or (VI=VI) 

Coils and Credit Cards 

We now know that when a magnet or a coil move near each other a voltage is induced. Some people found out that this could apply to how we spend money. A credit card has a strip of different magnetic charges along it (this forms the code). The credit card machine has a coil inside of it and as the magnetized strip of the card moves by it, voltage is induced which is then registered by the machine and interpreted as a code.

Motors and Generators 

Because It is much more practical to spin a coil inside a magnet, most people spin the coil to produce voltage. This arrangement of moving a coil in a stable magnetic field is called a generator. A motor is basically the exact opposite of a generator except instead of using mechanical energy to produce electrical energy, it uses electric energy to produce mechanical energy.

        A simple motor/ generator.

A simple generator/ motor.

Electric Currents and Magnetism 

As a wire carries a current, a magnetic field is produced. But if this wire moves through a magnetic field it feels a deflecting force and it will be pushed. If the wire's current changes directions than the force will also reverse. As we know the force is strongest when the current is perpendicular to the magnets. 

Heres the link to a video showing a simple motor: 

https://www.youtube.com/watch?v=O2oDaILnaoM