Magnetism
Background
Magnetic forces are due to the interactions between magnetic fields.
All magnets have a north and south pole. A north magnetic pole is actually a north seeking pole, i.e. If it is free to rotate it will point towards the Earth’s north pole.
All permanent magnets are made of smaller magnets that we call domains. In a permanent magnet, these domains are well aligned in a certain direction which give the magnet its north and south pole. Magnetic materials such as iron aren’t exactly magnets (yet) because the domains have a random orientation. However they can become magnetized and become magnets if their domains become aligned. We will look at this in more detail down below.
For now here are some basic rules of magnetic interaction:
- Opposite poles attract
- Like poles repel
Ferromagnetism
There are certain materials (such as iron) that are always attracted to magnetic poles (north & south). These materials are called ferromagnetic materials and they have the ability to become magnetized.
Ferromagnetic materials contain what we can think of as tiny magnets called domains. In an unmagnetized piece of iron the domains are arranged randomly. However, when the iron is placed near a permanent magnet, the domains line up because they are attracted by the magnet.
Remember, opposite poles attract. The iron domains (which also have a north and south pole) will orientate themselves accordingly.
Essentially once the domains line up like this, the iron has now become a magnet itself i.e. magnetized. This is induced magnetism. However as soon as you remove the magnet, the iron domains will immediately randomize again and thus it loses its temporary magnetism.
Steel is made of iron and carbon, so it is also ferromagnetic. Compared to iron, steel is less strongly magnetized by permanent magnets but it will retain some if its induced magnetism and become a permanent magnet. It will remain magnetised until it is banged on the table or dropped (causing the domains to randomize).
Magnetic field lines
If small plotting compasses are placed around a bar magnet, the compasses show the direction of the magnetic field.
Otherwise small iron fillings can be sprinkled around the magnet and they too will line up with the magnetic field and show the field lines.
Magnetisation
Methods of magnetization
Recall above that magnetism can be induced in ferromagnetic materials. Here we will go through certain techniques that are known to work well in doing this.
Hammering in a magnetic field
Strike an iron nail squarely and sharply several times with a hammer while keeping the nail positioned in a north-south orientation. The impact of the hammer with the iron nail causes the magnetic domains within the nail to break loose from their current orientation. The Earth’s magnetic field will then reposition the domains into a new orientation parallel with the Earth’s magnetic field.
Magnetization via stroking
Stroking will align the domains in the ferromagnetic material and magnetize it.
Magnetization via electricity (direct current)
When a direct (unidirectional) current flows through a wire, a magnetic field is produced around the wire. The direction of the magnetic field depends on the direction of the current (from + to – around the circuit). The right hand grip rule can be very handy here (no pun intended):
When a direct (unidirectional) current flows through a coil of wire, a magnetic field is created around and inside the coil. The pattern of field lines outside the coil is identical to a bar magnet. Ferromagnetic materials can be magnetized by placing them inside the coil like so:
VERY IMPORTANT NOTE: Conventional current is the flow of positive charge. This means that current is always going in the direction opposite to electron flow. Therefore if you look at the diagram above, current will actually be flowing in the opposite direction of the arrows. 
This whole set up is called an electromagnet. The metal inside the coil is called a solenoid. Once the electricity is turned off, the magnetism will also switched off along with it. The magnetism can be strengthened and weakened by adjusting the voltage of the electricity.
You can actually deduce the polarity of the electromagnet by looking at the direction of the current at the ends of the solenoid.
- Clockwise = South pole
- Anticlockwise = North pole
Since the direction of current determines the polarity of the magnet, it is important to understand that the current must flow in a single direction (direct current) in order to magnetize the metal.
Methods of de-magnetization
Hammering
Physical impact on a ferromagnetic material will randomize the orientation of the domains and therefore demagnetize it.
However as discussed before, it can also be used to magnetize a material if positioned correctly and hammered within a set external magnetic field since the domains will reposition itself accordingly.
Heating
Heating a magnet will also disorientate the domains within it, and therefore cause it to lose magnetism
Alternating current
As discussed before, a flow of current in a wire creates a magnetic field which can induce magnetism in a solenoid. The polarity of the induced magnet depends on the direction of the current.
A direct current is a unidirectional current meaning the polarity of the induced magnet will always remain constant, therefore perfect for creating magnets.
An alternating current on the other hand is a type of current that constantly changes direction. This means that the induced magnetic polarity will also continuously change along with the current. This means that the north and south poles of the magnetic field is continuously changing, and thus randomizing the domain orientation in the magnet – causing it to lose magnetism.
Design and use of permanent magnets and electromagnets
Permanent magnets are designed with hard magnetic material and used for purposes where magnetism is needed over a long period of time i.e. fridge doors
Electromagnets use a solenoid to create a magnetic field. It is used for when a magnetic field needs to be turned on and off i.e. scrap metal moving.