The special state of space around permanent magnets and around current-carrying conductors and coils in which forces are exerted on other magnets or bodies of ferromagnetic materials is called a magnetic field. Such magnetic fields can have very different shapes and different strengths. Magnetic fields cannot be detected with our senses, they can only be recognized by their effects. This is especially true for the constantly present, relatively weak magnetic field of the earth, which is a large permanent magnet.
Magnetic fields, like other types of fields, can be characterized using field line images or field descriptive quantities. They can act on other bodies, but can also be shielded.
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The magnetic field
A magnetic field or magnetic square is the state of space around magnets in which forces are exerted on other magnets or any other bodies, especially bodies made of ferromagnetic materials. We cannot perceive a magnetic field with our sense organs, it can only be recognized by its effects. For example, iron filings align themselves in a magnetic field in a characteristic way.
Types of magnets
The phenomenon of magnetism has been known since ancient times. Magnetic ironstone occurs as a natural mineral. The forces between bodies made of corresponding material were noticed at an early stage. Electromagnetism, on the other hand, was first demonstrated in 1820 by the Danish physicist HANS CHRISTIAN OERSTED (1777-1851).
Permanent magnets are all bodies that generate an intense magnetic field without current flowing through them. Small permanent magnets are used to collect small metallic parts or to attach light objects to a magnetic board . The tips of some screwdrivers are also magnetized, which holds the metal screw at the turning slot. Permanent magnets are also used where electromagnetic induction is used to generate a current in small generators. An example of this is the bicycle dynamo .
A magnetic field exists around a permanent magnet, resulting in an alignment of iron filings.
Suitable materials for the production of permanent magnets are for example alloys of Iron and Nickel, but also various ceramic materials and neodymium. Before magnetization, the material is given the desired shape, which can be very diverse. Frequently used permanent magnets are horseshoe magnets and bar magnets as well as cylindrical magnets and magnetic discs.
Different forms of permanent magnets
The decisive difference between a non-magnetized iron rod and a magnetized one is the internal structure of the material. Basically, every substance has magnetic properties, but they can be very differently pronounced. so-called ferromagnetic materials (iron, nickel, cobalt, various alloys) are characterized by a peculiarity of their internal structure. They are made up of tiny magnetic areas of the same magnetic orientation, but usually arranged in a random pattern. These areas are called white areas or elementary magnets . Due to their random arrangement, the weak magnetic fields of the white districts compensate each other, so that there is no magnetic effect towards the outside and only a small one.
If a ferromagnetic substance is brought into an external magnetic field, the white areas align themselves along the magnetic field lines of the external field. The stronger this field is, the greater is the alignment effect. Saturation occurs at a high field strength – then all white areas in the ferromagnetic material are uniformly aligned. This process is called magnetization . If the external field is switched off, the alignment of the individual areas is maintained. The partial fields overlap to a strong magnetic field. A permanent magnet is formed. If a permanent magnet is exposed to high temperatures or strong mechanical shocks, the alignment of the white areas can change again. Demagnetization may occur.
The described effect of magnetization can be observed, for example, when small parts containing iron, such as screws or nails, are kept for a long time in the vicinity of a powerful permanent magnet. The small parts have then become magnetic themselves. The same applies to cast-iron radiators in the earth’s magnetic field.
Unmagnetized (a) and magnetized (b) iron in the model
Electromagnetism is associated with current flow: Every current-carrying conductor is surrounded by a magnetic field. This magnetic field is particularly strong around current-carrying coils with iron core. A current-carrying coil with an iron core is therefore often also referred to as an electromagnet, although here too, as with permanent magnets, there are very different designs depending on the intended use. A lifting magnet has a completely different shape than an electromagnet which is present in a door opener or in a relay.
A current-carrying coil with an iron core is an electromagnet: It attracts bodies of ferromagnetic materials.
Properties of magnets and magnetic fields
Magnets have characteristic properties which can be summarized as follows:
- Every magnet has two points where the force on a specimen is particularly large. These places are called the magnetic poles of the field, the two poles north pole and south pole. It should be noted that ceramic magnets are also made with several north and south poles. This is z.B. this is the case with ceramic magnets, which are found in bicycle dynamos.
- Poles with the same name repel each other, poles with different names attract each other..
- If a rotatably mounted sample magnet is in the field of another magnet, a torque acts on it due to the forces between the poles, which leads to an alignment of the sample magnet in the magnetic field. Thus z.B. a compass needle in the magnetic field of the earth in the direction of the field. More detailed explanations of this magnetic field are given under the heading "Earth’s magnetic field".
The following also applies to magnetic fields:
- A magnetic field has energy, which is sometimes called magnetic energy.
- Changes of a magnetic field propagate with the speed of light
Attractive or repulsive forces act between magnets
Representation of magnetic fields
Magnetic fields can be visualized similarly to electric fields and gravitational fields with the help of the model field line diagram. If you put small magnets or iron filings into a magnetic field, these small magnets or iron filings will align themselves in the magnetic field. the iron filings propagate in a certain way. If instead of the small magnets or. of the iron filings lines, one obtains a field line image.
Small magnets or iron filings align themselves in a magnetic field in a characteristic way.
Figure 7 shows different field line images of permanent magnets and electromagnets. The following agreements are valid for the field line images of magnets:
- The magnetic field lines of permanent magnets run in the outer space from the north to the south pole.
- The closer the field lines are, the stronger is the magnetic field.
- In contrast to electric field lines, magnetic field lines are closed lines. This is obvious for the field line diagram around a current-carrying conductor, but it is also valid for any other field line diagram, even if the field lines inside permanent magnets are usually not drawn. Inside a coil or inside a permanent magnet, the field lines then run from the south pole towards the north pole.
Closed lines are also called vortices. For the field lines of the magnetic field there is also no beginning and no end and therefore also no source for a magnetic field line. Therefore, a magnetic field is often characterized in the following way:
A magnetic field is a source-free vortex field.
Besides field line images, magnetic fields can also be described quantitatively by the field quantities magnetic flux density and magnetic field strength. More information about this can be found under the respective keywords.
If a closed ferromagnetic hollow body is brought into a magnetic field, then this magnetic field can no longer or hardly be detected inside the hollow space. The ferromagnetic material shields the external magnetic field almost completely. This effect is called magnetic shielding.
Magnetic shielding is due to the high permeability of ferromagnetic materials. Magnetic field lines are particularly close together in ferromagnetic materials, the field lines do not leave a closed ring of these materials, which is used, for example, to construct ring coils.
In the opposite case, the field lines of an external magnetic field easily enter bodies of ferromagnetic materials and then continue inside these bodies until they exit. As long as the object in question is ring-shaped or hollow, no magnetic field lines enter the interior of such a ferromagnetic body.
Particularly suitable for magnetic shielding are soft magnetic substances, that is, substances that can be easily magnetized and demagnetized. This is especially true for soft iron .
Magnetic shielding is used technically to keep the earth’s magnetic field, which is always present, away from highly sensitive test setups, for example, so that measurement errors in determining magnetic field strengths are ruled out.
On the other hand, magnetic shielding has a negative effect in places where, with the help of a compass it would like to determine the north direction. When wooden sailing ships were increasingly replaced by iron steamships in the last century, the ship’s compass with magnetic needle lost its traditional significance and subsequently had to be replaced by the elaborately constructed mechanical gyrocompasses.