Electromagnetism

New Terms and Ideas

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The Domain Theory. 

In an atom, electrons are orbiting the nucleus, (as the earth orbits or revolves around the sun) and also, each electron rotates or spins on its axis. ( like the earth does). 

Because of this revolving and rotating, each electron has its own magnetic field. 
In the atoms of most elements, the magnetic fields of all the electrons in the atom cancel each other out, and the atom has no overall magnetic field. 

In just a few, they don't, and so the individual atoms behave like tiny magnets. These elements are called ferromagnetic elements, and the most common are Iron (Fe), Nickel (Ni) and Cobalt (Co). 

In a piece of iron, nickel or cobalt, groups of these atoms line up together to form small areas (about 1mm wide) called domains. 

Random

Aligned

Magnetic Fields.  A magnet can be thought of as being surrounded by a magnetic field. The field is defined as the region in which the magnetic effect can be detected. This field is strongest near the poles of the magnet.  Ferromagnetic metals are affected by this field. If iron filings are put in a magnetic field, they line up along the "lines of force" shown in the diagram above. (see also your text book.)

If two magnets arre placed near each other, they will either attract or repel each other. Unlike  poles attract each other, and like poles repel each other.

  The Earth's Magnetic Field. 

The ancient Greeks found that pieces of certain naturally occurring minerals pointed north when suspended on a string.  Since these minerals were originally found near Magnesia in Turkey, they were called magnets.They are also called lodestones.  At first people thought magnets pointed north because there was a great pile of lodestone at the North Pole.  Eventually, it was realised that the whole earth has a magnetic field, with a North and South Magnetic Pole.  Compass needles, which are themselves small magnets, line up with this field.  The magnetic North pole is not fixed in one place, but moves over time. It is not necesarily found at the same place as the geographic North pole.

 

The magnetic field of a current carrying wire. 

The field exists all along the wire. Electricians use a device which detects magnetic fields to locate wires behind walls. 
The direction of the field depends on the direction of the current. The current flow shown here is conventional flow, ie positive to negative, and not electron flow. 

The magnetic field of a current carrying coil of wire 

If a current carrying wire is wound up into a coil, then the individual circular  fields of the loops combine to produce a field like that of a bar magnet.  The direction of the field depends on the direction of the current.

The Electromagnet  The field of a solenoid is not very strong on its own. However, if a bar of iron is placed inside the coil, the domains in the bar will line up with the field of the solenoid to form a strong magnet.  An electromagnet loses its magnetic field very quickly once the current flow stops.  If the size of the current is increased, the strength of the electromagnetic field increases.  If the number of coils is increased, the strength of the electromagnetic field increases.    Electric doorbells, and magnetic recording devices are two applications of electromagnets.  How to make  an electromagnet (internet site)

Click to see it happen.

 The interaction of the fields of current carrying wires and coils and other magnets

The loud speaker Loud speakers consist of a coil that is tightly wound around the base of a speaker cone.The voice coil,as it is called, and the cone sit inside a cylindrical permanent magnet.  The speaker opposite is a model you can easily make.  An alternating (AC) current flows through the coil.  This causes the direction of the voice coil's field to change as rapidly as the current direction changes.  The voice coil is alternatively repelled and attracted to the permanent magnet. Since the voice coil is firmly attached to the cone, the cone vibrates. This causes the air near it to vibrate as well, and we hear this as sound.  The loud speaker converts electrical energy into kinetic and then sound energy. You will also find that the coil on your model gets quite hot if you try to pass too much current through it.  Show me.

The DC motor The DC motor converts electrical energy into kinetic and heat energy.  The diagram opposite is a stylised  end on view of the Motor construction kits we used in class. Only one loop of one rotor coil is shown.  Current flows up the positive brush terminal, onto the split ring commutator, around the loop and off the negative terminal.  Thus each side of the loop experiences a force in the opposite direction to the other, and the loop rotates around the shaft.  Show me. The direction of rotation depends on the direction of flow through the loops. Rotation will continue as long as current flows.  Show me. More motor links  Walter Fendt's DC motor animation

  

Credits

• pictures drawn by Sally Mack using NeoPaint and PaintBrush
• animated using GIF MovieGear
• trialled by  year 10 science students Somerville House
• copyright Somerville House 1999
• updated 14/6/99