Electromagnetism

Electromagnetism

In the last tutorial about Magnetism we looked briefly at how permanent magnets produce a magnetic field around themselves from their north pole to their south pole. While permanent magnets produce a good and sometimes very strong static magnetic field in some applications the strength of this field is still too weak or we need to be able to control the amount of magnetic flux that is present. To obtain a stronger and more controllable magnetic field we need to use electricity, with coils of wire wrapped or wound around a soft magnetic material such as an iron core. This then produces a relationship between electricity and magnetism that gives us another form of magnetism called Electromagnetism.

Electromagnetism is produced when an electrical current flows through a simple conductor such as a piece of wire or cable. A small magnetic field is created around the conductor with the direction of this magnetic field with regards to its "North" and "South" poles being determined by the direction of the current flowing through the conductor. Therefore, it is necessary to establish a relationship between current flowing in the conductor and the resultant magnetic field produced by this current flow and thereby defining the definite relationship that exists between Electricity and Magnetism in the form of Electromagnetism.

When an electrical current flows through a conductor a circular electromagnetic field is generated around it. The direction of rotation of this magnetic field is governed by the direction of the current flowing through the conductor with the corresponding magnetic field produced being stronger near to the centre of the current carrying conductor and weaker farther away from it as shown below.

Magnetic Field around a Conductor

 

A simple way to determine the direction of the magnetic field around the conductor is to consider screwing an ordinary wood screw into a sheet of paper. As the screw enters the paper the rotational action is CLOCKWISE and the only part of the screw that is visible above the paper is the screw head. If the wood screw is of the pozidriv or philips type head design,the cross on the head will be visible and it is this cross that is used to indicate current flowing "into" the paper and away from the observer.

Likewise, the action of removing the screw is the reverse, anti-clockwise. As the current enters from the top it therefore leaves the underside of the paper and the only part of the wood screw that is visible from below is the tip or point of the screw and it is this point which is used to indicate current flowing "out of" the paper and towards the observer. Then the physical action of screwing into and out of the paper indicates the direction of the current in the conductor and therefore, the direction of rotation of the electromagnetic field around it as shown below. This concept is known generally as the Right Hand Screw Action.

The Right Hand Screw Action

A magnetic field implies the existence of poles and the polarity of a current carrying conductor can be established by drawing the capital letters S and N and then adding arrow heads to the free end of the letters as shown above giving a visual representation of the magnetic field direction.

Another more familiar concept which determines both the direction of current flow and the resulting direction of the magnetic flux around the conductor is called the "Left Hand Rule".

Left Hand Rule of Electromagnetism

The direction of the magnetic field is from north pole to south pole and can be deduced by holding the current carrying conductor in your left hand with the thumb extended it will be pointing in the direction of the electron flow from negative to positive. The position of the fingers laid across the conductor will now point in the direction of the magnetic lines of force as shown.
 
If the direction of the electron flowing through the conductor is reversed, the left hand will need to be placed onto the other side of the conductor with the thumb pointing in the new direction of the electron current flow. Also as the current is reversed the direction of the magnetic field produced around the conductor will also be reversed. This "Left Hand Rule" can also be used to determine the magnetic direction of the poles in an electromagnetic coil. This time, the fingers point in the direction of the electron flow from negative to positive while the extended thumb indicating the direction of the north pole. There is a variation on this rule called the "right hand rule" which is based on so-called conventional current flow, (positive to negative).

When a single straight piece of wire is bent into the form of a single loop as shown below, the current will be flowing in opposite directions through the paper such that a clockwise field and an anticlockwise field are produced next to each other. The resulting space between these two conductors becomes an "intensified" magnetic field with the lines of force spreading out in such a way that they assume the form of a bar magnet generating a distinctive north and south pole at the point of intersection.

Electromagnetism around a Loop

Lines of Force around the Loop

The current flowing through the two parallel conductors of the loop are in opposite directions as the current exits the left hand side and returns on the right hand side. This results in the magnetic field around each conductor inside the loop being in the "SAME" direction to each other. The resulting lines of force generated by the current flowing through the loop oppose each other in the space between the two conductors where the two like poles meet thereby deforming the lines of force around each conductor as shown.

However, the distortion of the magnetic flux in between the two conductors results in an intensity of the magnetic field at the middle junction were the lines of force become closer together. The resulting interaction between the two like fields produces a mechanical force between the two conductors as they try to repel away from each other producing motion. However, as the conductors cannot move, the two magnetic fields therefore help each other by generating a north and a south pole along this line of interaction. This results in the magnetic field being strongest in the middle between the two conductors. The intensity of the magnetic field around the conductor is proportional to the distance from the conductor and by the amount of current flowing through it.

The magnetic field generated by a straight length of current-carrying wire is very weak even with a high current passing through it. However, if several loops of wire are wound together along the same axis producing a coil, the resultant magnetic field will become even more stronger than the single loop producing an electromagnetic coil more commonly called a Solenoid. Then every coil of wire uses the effect of electromagnetism when an electrical current flows through it and we will look at this effect in more detail in the next tutorial.