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 magnetic field is still too weak or we need to be able to control the amount of magnetic flux that is present. So in order to produce a much stronger and more controllable magnetic field we need to use electricity.
By using coils of wire wrapped or wound around a soft magnetic material such as an iron core we can produce very strong electromagnets for use in many different types of electrical applications. This use of coils of wire 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 length of wire or cable, and as current passes along the whole of the conductor then a magnetic field is created along the whole of the conductor. The small magnetic field created around the conductor has a definite direction with both the “North” and “South” poles produced being determined by the direction of the electrical current flowing through the conductor.
Therefore, it is necessary to establish a relationship between current flowing through the conductor and the resultant magnetic field produced around it by this flow of current allowing us to define the relationship that exists between Electricity and Magnetism in the form of Electromagnetism.
We have established that when an electrical current flows through a conductor a circular electromagnetic field is produced around it with the magnetic lines of flux forming complete loops that do not cross around the whole length of the conductor. 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 center of the current carrying conductor. This is because the path length of the loops being greater the further away from the conductor resulting in weaker flux lines as shown below.
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, anticlockwise. 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 the wood screw in 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.
A magnetic field implies the existence of two poles, a north and a south. 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
The recognised direction of a magnetic field is from its north pole to its south pole. This direction can be deduced by holding the current carrying conductor in your left hand with the thumb extended pointing in the direction of the electron flow from negative to positive.
The position of the fingers laid across and around the conductor will now be pointing in the direction of the generated 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 because as we have said previously, the direction of the magnetic field depends upon the direction of current flow.
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).
Consider when a single straight piece of wire is bent into the form of a single loop as shown below. Although the electric current is flowing in the same direction through the whole length of the wire conductor, it will be flowing in opposite directions through the paper. This is because the current leaves the paper on one side and enters the paper on the other therefore a clockwise field and an anticlockwise field are produced next to each other across the sheet of paper.
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.
Lines of Force around the Loop
The current flowing through the two parallel conductors of the loop are in opposite directions as the current through the loop 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 inbetween 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. In an electrical machine this repelling of these two magnetic fields produces 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 around a straight length of current-carrying wire is very weak even with a high current passing through it. However, if several loops of the wire are wound together along the same axis producing a coil of wire, the resultant magnetic field will become even more concentrated and stronger than that of just a single loop. This produces an electromagnetic coil more commonly called a Solenoid.
Then every length of wire has the effect of electromagnetism around itself when an electrical current flows through it. The direction of the magnetic field being dependant upon the direction of the flow of current. We can increase the strength of the generated magnetic field by forming the length of wire into a coil and we will look at this effect in more detail in the next tutorial.