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  Electricity - Magnetism Teacher's Notes  
  Magnetism
Everyone is familiar with magnetism. You know that magnets like to stick to Iron or Steel. You might also know that your cassette tape uses magnetism to store the musical information. Most people however, don't realise that magnetism is fundamentally related to electricity.
 
 
  There are several relationships between electricity and magnetism. We will explore them one by one. The first relationship we will discuss is how current flowing through a wire creates a magnetic field. A steady current creates a steady magnetic field like that created by a magnet. (We will deal with varying currents and varying magnetic fields in a later section called "Transformers".)
 
Electro-Magnet
We can create an electro-magnet by wrapping many turns of wire on a core, then causing an electrical current to flow through this coil of wire. With such a device we can create a magnetic field that can be controlled; we can turn it on and off by controlling the current. (In fact, by controlling the current gradually, instead of just on and off as shown here, we could make the magnetic field gradually become stronger or weaker.)
     
  In the animation at the left, we are controlling the current by connecting and disconnecting the battery. When the battery is not connected, there is no current flow, and therefore no magnetic field. When the battery is connected, current flows through the coil, and a magnetic field is created, as shown by the red lines. (In the real world, magnetic fields are invisible)
 
Note that the magnetic field curves around from one end of the coil to the other. The field is strongest at the ends of the coil, and gets weaker as we get farther away. We call the two ends of the coil the "poles". In fact, one is "North" and the other is "South" just like any other magnet.
 
If we use iron or steel as the core of our electro-magnet, the field at the poles will be stronger. In some sense, the magnetic field is "focussed" by the presence of these materials.
 
  The strength of the field is proportional to the amount of current flowing and to the number of turns on the coil. Each turn of wire on the coil contributes to the strength of the magnetic field. If we double the number of turns, we can make an electro-magnet twice as strong. But, this exact ratio is only true if the current flow is the same for both coils!  

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