Magnetism explainedEffect | Principles | Permanent magnets | Power generation
Magnets represent something mysterious for many people. After all, magnetism cannot be seen, heard, smelt, tasted nor felt directly. In addition, magnets attract ferromagnetic objects as if by magic. Do you believe that an inexplicable phenomenon is at work here? This is certainly not the case! Magnetism is something that science has known about and understood for a long time. Unfortunately, passing on knowledge about magnetism to people without an affinity for science is not very easy, as magnetism cannot be touched with one's hands, nor does it lend itself to being understood more readily with the help of simple experimental setups at home. Below, you will find an explanation of magnetism that uses diagrams instead of formulae.
In an attempt to make this as understandable as possible, please forgive the somewhat imprecise descriptions. Those of you who wish to delve a little deeper into the subject will generally find suitable literature amongst the physics books written for use at senior secondary or university level. In my opinion, the books intended for use by students are frequently easier to understand than school textbooks, as the former are often suitable for self-study.
Everyone will probably know that the ends of a bar magnet are normally referred to as the south and north pole, sometimes also known as poles, akin to the earth's poles. When two magnets are brought together, the south and north pole will automatically be attracted to one another, while the same poles will repel one another without the need for further energy to be introduced. This is also how compass needles work: such needles are in fact small, light magnets that normally align themselves along the earth's magnetic field, with their alignment position providing information about the surrounding magnetic field
By placing a bar magnet on a cardboard box containing many compass needles, the effect of a magnet's strong magnetic field on its surroundings, i.e. the weak magnetic fields of compass needles can be studied. A line of compass needles can be used to render the magnetic fields "visible". This is illustrated using a picture in diagram 2.
The compass needles align themselves along the magnetic field lines, of which four have been drawn in red to illustrate this. What we are describing here are only imaginary lines, as one cannot actually see them.
As early as 1820, the physicist Oersted had already discovered that a compass needle could be deflected by wires carrying an electric current. In effect, magnetism can be generated by a flowing electric current. If one repeats the above-described experiment with a current-carrying conductor which is passed through the cardboard box holding the compass needles, one can see that a circular magnetic field is generated around the conductor (diagram 3, left side).
The left side of diagram 3 only shows those field lines in a plane perpendicular to the current-carrying conductor, as the compass needles are only positioned in a single plane. The entire cardboard box with the compass needles can be moved to and fro across the conductor, but the same diagram is always obtained. The shown relationship holds true for every point along the conductor, resulting in the magnetic field depicted on the right side of diagram 3 for a ring-shaped conductor, where the field lines drawn in red on the left diagram correspond to those on the right side. Naturally, the drawn field lines are only representative in character, as there is an infinite number of these. All of the field lines together result in the magnetic field.
Electric currents, i.e. moving electrons, give rise to magnetic fields.
he above paragraphs explain how magnetic fields can be generated with the help of electric currents. But how do permanent magnets work? These generate magnetic fields by themselves, entirely unassisted by any type of current. Actually, they also rely on electricity, but this electricity does not need to be supplied externally. The smallest magnets are namely atoms. These have at least one electron orbiting the atom's nucleus. This is shown in diagram 4 by a hydrogen atom, which only has a single electron and therefore represents an atom with the simplest structure.
This single electron operates in exactly the same manner as electrical current passing through e.g. a copper wire, and generates a magnetic field as a result of its movement. Because electrical current is nothing other than electrons moving in a single direction. Are you surprised to learn that hydrogen atoms are magnetic? Although individual hydrogen atoms generate magnetic fields, hydrogen gas as a whole is not magnetic. The reason for this is that hydrogen atoms move totally randomly and, overall, cancel out the magnetic fields of individual atoms. Seen statistically, at any given time there will be precisely as many magnets aligned in one direction as there will be magnets in the opposing direction. As is also corroborated by experience, hydrogen is not representative of a permanent magnet.
However, in certain materials (e.g. iron with small amounts of impurities) small areas form in which the magnetic fields of atoms are all oriented in the same direction. These are called Weiss' domains or elementary magnets. These can be envisaged as minuscule magnetic granules, which of course are not shaped in the beautiful regular patterns shown in diagram 5. As these Weiss' domains are initially oriented entirely randomly after the molten mass has cooled, the magnetic fields of the individual elementary magnets cancel each other out completely, as is the case with the hydrogen atoms. However, if a sufficiently large external magnetic field is applied, a number of the elementary magnets will align themselves in concordance with the external field and remain in this position, even after removal of the external field. It can best be imagined in the following way: the force necessary to realign the elementary magnets will vary, and in particular depend on the existing alignment of the elementary magnets in their original state and how they are 'hooked' with their neighboring granules. As the strength of the external field increases, so does the number of Weiss' domains that realign themselves, until a specific field strength is reached at which point all of the elementary magnets are realigned. When this has happened, a permanent magnet has been produced.
There is only a small number of materials that are ferromagnetic, i.e. expressed differently, magnetizable. Only these materials have Weiss' domains; what we are talking about is a particular, material-specific property. The conditions leading to the formation of Weiss' domains are too complex to be discussed here. The atoms of many other materials can also be realigned in an external magnetic field (such as the hydrogen atom described above), however in these materials the atoms have a propensity to return to a random orientation very quickly and readily, i.e. in response to thermal effects.
As described further above, moving electrons give rise to magnetic fields. As most effects are reversible in physics, the interesting question as to whether this is also the case here, arises. The answer can already be anticipated and is a plain "yes". But you will probably already have guessed this, considering that we have power stations.
Two bar magnets are shown with a magnetic field, in which the field lines run parallel to one another (depicted in red), between the poles of the magnets. If the distance between the magnetic poles is very small, all of the other field lines can be disregarded at first approximation because of their comparative extreme weakness. But they still exist, nevertheless.
If a conductor loop, that is to say an entirely conventional length of wire, to which a lamp has been connected, is moved from outside into the gap between the magnets, the lamp will illuminate briefly. The faster the conductor loop is moved into the magnetic field, the more brightly the lamp will illuminate. The same thing happens when the loop is withdrawn. By using this extremely simple experimental setup, one can see that electrical current is induced by the movement of a conductor through a magnetic field. The dynamo on your bicycle and the generators in power stations all operate on exactly the same principle.
A change in the magnetic field will induce an electric current in a conductor, i.e. the existing electrons in a conductor will be diverted in one direction.
We 'simulated' an approaching magnetic field in the above experiment by introducing a conductor into the magnetic field and then withdrawing it from the field again. The final question that now needs to be answered is: Is this also the case when there is no motion at all and, instead, the only change is in the strength of the magnetic field? Again, the answer to this question is "yes". When a permanent magnet is used, the only way in which the field strength can be changed is by moving the magnet away. For this reason, we need to resort to an electromagnet, i.e. a current-carrying conductor loop. To verify this, two conductor loops are brought into close proximity of one another as depicted in diagram 7, and the current flow in one conductor is changed, e.g. by switching it on or off. To enable more precise observations to be made, we shall use a current measurement device instead of a lamp this time.
The loop shown in blue is called the primary side and the loop shown in black is the secondary side. When the circuit is open, i.e. when no current is flowing, the pointer of the current measurement device (on the right side of the diagram) remains in its center position, i.e. it indicates that no current is present. If the circuit is closed on the primary side by operating the switch, one can see the pointer of the current measurement device deflect briefly to one side before returning to its center position. This signifies that electrical current was generated (or induced, to use the correct term) for a short period. The pointer deflects briefly in the opposite direction when the circuit is switched off. Electrical current is again induced, albeit one flowing in the opposite direction, during this moment. This reversal of current also happens in the experiment described further above (withdrawal of a conductor loop from the magnetic field), except it could not be seen as the lamp has no polarity and will illuminate regardless of current direction.
An electrical current will be induced in a conductor loop in response to a change in the magnetic field. In contrast, no current will be induced when a constant magnetic field exists. Electrical current can therefore be generated by the repeated movement of a conductor loop into and out of a magnetic field. This will produce a current which is said to be positive for movement into the magnetic field and negative for movement out of the field, i.e. in effect the current direction will change continually. This type of current is called alternating current. This is the sole principle of operation of a generator in an electricity generating plant.
