Unit 7 Summary

We began this unit learning about magnetism. The source of all magnetism is moving charges. These charges, electrons, spin in completely random directions when they are not magnetized. Domains are clusters of electrons that are all spinning in the same way. They have no net direction until they are affected by a magnetic field; when magnetized, domains align so that the magnetic field can continue to flow. In each magnetic field, there are two poles- north and south. The magnetic field always comes from the north pole to the south and goes from the south to north.

It is also important to know that the poles are like charges in that like poles repel while opposite poles attract. This is because the magnetic field lines flow towards the opposite poles and repel from the ones that are like the pole they just left.

One of the big questions we asked with this section was : why do paper clips stick to magnets?

A paperclip that is not magnetized will have domains pointing in all different directions. A magnet has a magnetic field. When the magnet is close to the paperclip, the domains of the paperclip align to match the magnetic field of the magnet. The paperclip now has a north ad south pole, just like the magnet. The north pole of the paperclip is attracted to the south pole of the magnet and thus the paperclip sticks to the magnet. Now, the paperclip can act as a magnet and other paperclips stick to it, their domains also aligning to match the magnetic field.

A fun fact about magnetic poles is that the Earth's magnetic south pole is actually at the top of the Earth, which we generally consider the geographic north pole. The Earth's magnetic north pole is on the Earth's bottom, at the southern tip.

All moving charges feel a pull from a magnetic field if they are moving perpendicular to it. The Aurora Borealis, or the Northern Lights, are actually cosmic radiation- moving charges from space that got sucked into Earth's magnetic field and spiral around the field lines and into the Earth's magnetic poles. This is why we can see them in northern parts of the world- they are following the magnetic field lines into the south pole.

Any moving charge that is moving perpendicular to a magnetic field will feel a force from it. this brings us to the "right-hand rule" that explains the relationship between current, force, and magnetic fields. 

In this picture, my pointer finger represents the direction of the current of moving charges. My middle finger represents the direction of the magnetic field and my thumb shows the perpendicular direction that the charges will be caused to move in.

Electromagnetic induction is the process of inducing voltage by changing the magnetic field in loops of wire. Voltage is induced by the motion between wire and magnetic field. The more loops of wire, the greater the voltage that is induced. This is summarized in Faraday's Law, which states that:

The induced voltage in a coil is proportional to the product of its number of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within these loops.

There are three ways that voltage can be induced in a wire:

1) moving the loop of wire near a magnet

2) moving a magnet near the loop of wire

3) changing the current in a nearby loop

All of these methods involve changing the magnetic field in the loop of wire.

 This process is used in transformers. Transformers utilize a primary coil, which receive the input of electric current, and a secondary coil, which has an output of electric current. When electric current flows through the primary coil, it causes a change in the coil's magnetic field which spreads and induces voltage in the secondary coil. Voltage causes current, so the electric current is thus transferred between coils. The current must be ac for a transformer to work because this has a constantly changing current, so the magnetic field is constantly changing, which is the only way for the transfer of electricity to be constant. Transformers can either step voltage up or down, depending on the electric power required. The relationship between primary and secondary voltages is described in the equation:

The conservation of energy, which we learned in a pervious unit, comes into play here as well. The initial amount of energy must be equivalent to the ending amount as energy cannot be created. Power is the rate at which energy is transferred. Therefore, the power into the primary coil = the power out of the secondary coil. Electric power = voltage x current. Thus:

primary (Voltage x Current) =  secondary (Voltage x Current).

If the secondary coil has a greater voltage than the primary coil, it will have a weaker current, and vice versa.

Finally, we learned about generators and motors. They work in opposite ways, to put it simply. Generators have an input of mechanical energy, and through electromagnetic induction, have an output of electric energy. They are made of coils of wire and magnets (much like transformers). Motors take electric energy (the input) and through electromagnetic induction produce mechanical energy (the output). They are also made of coils of wire and magnets. The generator effect is: current as a result of motion. The motor effect is: motion as a result of current.