In the illustration above, the domains are lined up. The strength of each domain is added to its neighbor and the material becomes a magnet.
To Kill a Magnet
Heat it. That will allow domains to shift around.
Hit it. Sudden impacts will scramble the domains. This is why refrigerator magnets weaken over time as they repeatedly fall off the door.
Materials that are capable of being permanent magnets. Being capable does not mean that they MUST be magnets. Iron nails are typically not magnetic but can become magnetized.
Earth’s Magnetic Field
Caused by the rotation of the FeNi core.
The north end of a magnetic compass points N
The magnet in the core is reversed.
The Earth’s Core is “reversed” from what you might think it should be.
Quick story: The northern direction has been known to be special since ancient times since it sits under the only star in the sky that does not move: Polaris- The North Star.
Also, going back to antiquity, the discovery of the magnetic compass gave us a needle that pointed North and that needle was labeled with an “N”. After studying magnets, we realized that opposites attract and likes repel. The last piece of the puzzle was the discovery that the Earth’s core created magnetism which influenced the direction that magnets point.
If you put all these things together, you realize that the magnet at the core of the Earth is actually reversed from what you think it should be! The south end of the Earth’s internal magnet points up so that the N end of compasses are attracted and point towards North.
N & S poles are always paired.
If you break a magnet in half, you will still have a N and a S. You can keep breaking the magnet in half but each half will still have a N and a S. This is because the magnetism ultimately comes from the spin of the electrons and you cannot break the magnet down small enough to split the electron in half to separate the two ends of an electron.
Dipole: a paired set of poles.
Monopole: a magnet with a single pole (impossible)
Lines that show the direction of the field.
More lines or more densely packed lines = stronger field.
Lines go from N to S
In a bar magnet, the lines will stretch from the North Pole to the South Pole in a wide looping “butterfly” pattern. The lines are closest and therefore strongest near the poles.
Note: as the lines loop around, near “the equator” the lines are running the opposite way.
In a horseshoe magnet, the lines still flow from North to South. Between the arms of the horseshoe, the field is much more uniform: lines are parallel and evenly spaced.
Temporary vs Permanent magnet
A permanent magnet is one that gets its magnetism from the alignments of the domains. They cannot be easily turned on and off or reversed.
An electromagnet is caused by electric current through a wire.
Passing electricity through a wire creates a magnetic field around the wire.
If you coil the wire, the field spirals around the wire.
More wires = more flux lines = stronger magnet.
Inserting an iron rod concentrates the magnetism and makes a stronger magnet.
The Left-Hand Rule
Relationship between Moving Charges (current), Magnetism, and Force.
All three happen in three “mutually perpendicular” directions.
Magnetic field: Four fingers are the filed lines
Electric current: thumb points teh direction of current flow
Force: the direction that you palm faces.
Using any two will give you the third.
For example: FORCing a wire through a magnetic field will give you electric current.
Passing current through a wire within a magnetic field will create force.
Electric generators and electric motors are the same device only used in opposite ways: putting electricity into a motor will create motion. Putting motion into a motor will generate electricity.