UNIT 7 REVIEW

In this Unit, we covered these major points:

1. Magnetic Fields and Poles
2. Magnetic Domains
3. Electric Currents with Magnetism
4. Motors and Generators
5. Transformers

Magnetic Fields and Poles

Magnetism is the force due to the movement of charged particles.

Regions called magnetic poles give rise to these magnetic forces.  

Magnetic fields are caused by MOVING charges. 

The direction of magnetic fields in a permanent magnet are released from the North side and enter through the south side:

  
We can use magnetic field lines to show why like poles repel and opposite poles attract like this:

As shown in the image above, the like poles' magnetic field lines are going toward each other, causing them to bounce off and repel.  However with the opposite poles, the lines are both going in the same direction, and they act together.

The Earth has magnetic poles as well as geographic poles.  This can be confusing because the geographically north end of Earth is actually it's magnetic south pole

A compass works because of Earth's magnetic poles. A compass needle is really just a magnet that is free to spin. The South pole of the compass needle is attracted to the North pole of the earth, which is the geographical South Pole. The North end of the needle is attracted to the south pole of the Earth which is geographically the North Pole. 

What causes the Northern Lights?

Cosmic rays released from outer space are directed toward the Earth's equator.  However, once the rays come into contact with Earth's magnetic fields, they spin and follow the fields all the way to the poles, where they create waves of light.

Magnetic Domains

Magnetic Domains are clustered of aligned atoms.

For instance, all Iron has magnetic domains.  However, not all Iron is magnetic.  This is because the domains may not be aligned. A domain is a group of atoms whose electrons are spinning in the same direction.

A ferromagnetic material is one that is highly susceptible to magnetism, such as a paper clip or a nail.

This is how we can magnetize a ferromagnetic paper clip:

If the paper clip is jostled, the domains will come out of alignment.

A Permanent Magnet can be weakened when it's domains are unaligned as well.  This can happen when the magnet is dropped or heated.

Electric Currents and Magnetism

Magnetic Force on Current-Carrying Wires

When a wire is carrying a current, it has a magnetic field around it.  Use the "right hand rule" to find the direction of that magnetic field:

An electromagnet is a current carrying coil of wire. 

The strength of an electromagnet is increased by the number of turns in the coil.  This also increases the voltage of the wire, increasing the current, therefore strengthening the magnet.

If the magnetic field is continuously changing, this is how a generator works. 

Motors and Generators

Motors and generators have almost identical construction but opposite roles. Motors turn electrical energy into mechanical energy, whereas generators turn mechanical energy into electrical energy. In a motor or generator, a current runs through a coil, which feels a force because it is within a magnetic field. It feels this force because it is MOVING and all moving charges feel a force in a magnetic field. The force felt by the wire causes a torque, causing the coil to spin.  This spin produces usable mechanical energy.

DC (direct current) cannot be used for a transformer because the current it produces only moves in one direction whereas the AC (alternating current) continuously changes the direction of the current, which causes the change in the magnetic field. If a DC current were used, the generator would only work briefly when turned on ant turn off, where as an AC current constantly goes back and forth. 

Transformers

Transformers are used to increase or decrease voltage through electromagnetic induction  The small box connected to your computer charger is actually a transformer. 

A transformer is simply made with two coils of wire. There is a primary coil and a secondary coil. The primary coil is connected to the power source so it is the input and the secondary coil is the output.  Whenever the primary switch is opened or closed, voltage is induced to the secondary switch.

The number of turns in the wire is directly proportional to the voltage induced. The more turns in the wire, the more voltage there will be. The less turns the less voltage there will be. If the secondary has more turns than the primary, it is a step up because the secondary will produce more voltage than the primary. If the secondary has less turns than the primary it will be a step down because the secondary produces less voltage than the primary.

This equation shows the relationship between primary and secondary coils:

# of Primary Turns/Primary Voltage

 =

 # of Secondary Turns/Secondary Voltage

the powers are equal:

Power Primary = Power Secondary

because Power = Current * Voltage,

IV primary = IV secondary

This unit, the section I had the most difficulty with was generators.  I didn't understand why the coil moved. I finally understood it when i realized current is a moving charge and moving charges feel a force in a magnetic field.  Besides that, induction was a little complicated.