Unit # 7 Summary

During this unit we covered five main topics:
1. Magnetic fields and poles
2. Magnetic domains
3. Currents and magnetism
4. Motors and generators
5. Transformers

Magnetic Fields and Poles

Magnetism is a force caused my moving particles. Moving charges are the source of all magnetism.

Magnetic fields is the area that is effected by the magnet.

Magnetic fields have a direction.

Permanent magnet's field goes out through the north side of the magnet and into the south. As shown in the following picture

The reason why like poles repel and opposite poles attract is because of their magnetic fields. As shown in the following.

The earth also has magnetic poles. However, what we consider to be geographical north is actually magnetic south and geographical south is magnetic north. 

Magnetic Domains 

A magnetic domain is within amagnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction.

Domains can be changed. For example, paper clips. 

On their own, paper clips are not magnetic. However, when they get near the magnet, their domains will align with the magnetic field of the permanent magnet. 

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. 

The formulas you need for equations about transformers are 

# of Primary Turns/Primary Voltage

 =

 # of Secondary Turns/Secondary Voltage

Power Primary = Power Secondary

because Power = Current * Voltage,

IV primary = IV secondary

A transformer is simply made with two coils of wire. 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.