One of the properties of electricity is magnetism. All motors work based on this property of electricity and its effects.
Certain metals of the iron family can become magnetized if their atoms align such that this property is enhanced. This alignment can be due to contact with another magnet, electricity influence, being placed in a magnetic field (being in the vicinity of a magnet), or by natural events in nature.
As a result of magnetism, magnetic media can attract or reject other media by inserting attraction force or rejection forces on them.
In nature, the metallic form and salts (compounds) of the ferromagnetic materials can be found as magnets. This is the way magnetism was first recognized. Ferromagnetic materials are those that can relatively easily become magnets, and they can retain their magnetism.
Some other materials, called diamagnetic materials, can never become a magnet.
A third category is called paramagnetic materials, which can exhibit a very small degree of magnetism, but they can never retain it.
Ferromagnetic: Type of material from the iron family that is suitable for magnetization.
Diamagnetic: Materials without any significant magnetism property, such as wood and clay. They can never become a magnet.
Paramagnetic: Metals with very little (negligible) magnetic property that never can retain any magnetism after being in a magnetic field, as opposed to ferromagnetic materials.
Examples of ferromagnetic material are iron, steel, chromium, and nickel. Examples of diamagnetic material are copper, lead, mercury, silver, tin, and graphite (carbon). Examples of paramagnetic material are tungsten, aluminum, magnesium, sodium, and titanium.
A magnetic field is a part of the space around a magnet that the magnetic influence can be felt and measured. For example, there is a magnetic field between the two poles of a horseshoe magnet, as shown in Figure 1. Also, the Earth has a magnetic field, between the North Pole and the South Pole.
To elaborate on the meaning of field, consider the Earth also having a gravity field. As one moves away from the Earth, the gravity field becomes weaker until it becomes so small that it can be ignored. At that point, everything becomes weightless.
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A field can be assumed to consist of a number of imaginary lines that show the orientation of the field. For example, for a magnetic field we always consider the direction to be from the North Pole to the South Pole, and the gravity field around the Earth, it is always toward the center of Earth. For a magnetic field, these lines are called flux lines.
Magnetic field: Limited part of the space around a magnet where the magnetic effect exists.
Flux lines: Imaginary lines around a magnet indicating the direction of the magnetic effect. Stronger magnetism effect implies more flux lines through the same area.
As well as orientation, a field has strength. This strength is not constant. A magnetic field can be strong, or it can be weak at a certain point. As one moves away from the magnetic field, it becomes weaker.
For instance, the magnetic field in Figure 1 is stronger at point A than at point B. A stronger field can be assumed to have more lines (magnetic flux lines) per area; that is, the flux lines are denser.
Figure 1 Magnetic field.
Figure 2 Direction of a magnetic field inside and outside the magnet body.
As you know, a magnet can be in various forms, a yoke or a bar, for instance. No matter what the shape of a magnet is, the magnetic field direction is always from the North Pole toward the South Pole outside the magnet body along the flux lines and from south to north inside the magnet. The magnetic flux lines form closed curves. This is as shown in Figure 2.
A magnetic field can be created by electricity. The core body of such a magnet loses its magnetic property when electricity is turned off. However, many magnets do retain their magnetism. These are called permanent magnets.
Permanent magnet: Magnet with the permanent magnetic property that cannot be turned on and off or altered.
The magnetic property of a permanent magnet is due to its atomic structure. Thus, if a bar magnet is broken in the middle, each piece by itself becomes a perfect magnet having two poles, one north, and one south.
In the same way, if each of the two broken magnets is divided again into pieces, each piece becomes a magnet. Inversely, two magnets can be attached together only when their opposite poles are put together. This is why opposite poles attract and similar poles repel each other.
Heat is not good for a permanent magnet. A magnet can lose it magnetism by being heated. This changes the atomic structure of the metal.
So, if you want to destroy a magnet, you can heat it. Alternatively, if a magnet is struck hard by a hammer, it will lose its magnetism.
A ferromagnetic material can easily become a magnet.
Electricity and Magnetism
One of the properties of electricity is magnetism. When an electric current passes through a conductor, an electric field is generated around the conductor. This can be checked by placing a compass near the current carrying wire.
The compass needle aligns itself with the field generated by the electric current. This is shown in Figure 3. Each piece of the wire creates a magnetic field around it. The field lines are circles perpendicular to the wire segment; thus, depending on the point on this circle, the direction of the magnetic field changes.
The needle of a compass always stays tangent to the magnetic lines. This magnetic field is all around the wire; so, for a straight wire, the magnetic field forms a cylinder.
Figure 3 Magnetic field of a current carrying wire.
Figure 4 Right-hand rule to find the direction of the magnetic field around a wire.
Note that in Figure 3 the current direction is from positive to negative, using the conventional direction of the current. The direction of the magnetic field generated this way can always be found by the right-hand rule, as shown in Figure 4. That is, if the thumb in the right hand is aligned with the current direction, the fingers show the magnetic field direction.
The strength of this magnetic field depends on:
(1) The current in the wire and;
(2) The distance from the wire. The higher the current is, the stronger the field is.
(3) Also, the farther from the wire, the weaker the field is.
Magnetic Field of an Inductor
An inductor is a wound wire: when connected to electricity, it carries a current. In the previous section, we learned that if a straight wire carries a current, then a cylindrical magnetic field is created around it.
In the case of a coil (an inductor) the wire is circular, and normally there are a large number of loops that carry the same current. In this sense, there are many magnetic fields created because of the electric current, and they interact with each other owing to the loops in the wire. Figure 5 shows what happens in such a case.
Figure 5 Magnetic field of a coil. (a) Field direction for each individual loop. (b) Resultant magnetic field of all loops.
For each loop in the coil, the magnetic fields form a torus around the loop. Magnetic lines at two points are shown in Figure 5a.
At some points (such as A and A′), magnetic lines are in the opposite direction and cancel each other, but at some other points (such as B and B′) they combine with each other and their effects add together. The resultant magnetic field for each loop is along a line perpendicular to the loop (along with the loop axis).
Magnetic fields of all the loops, then, add together to construct the magnetic field of the coil. This field is along the coil axis, as shown in Figure 5b.
In conclusion, when a coil carries electric current, a magnetic field is generated. The direction of this magnetic field depends on the direction of the current.
Figure 6 shows a coil and the direction of its magnetic field based on the electric current direction. As noted before, the direction of the magnetic field is from north to south outside of the magnet generated by the current flowing in the coil (from + toward −).
Figure 6 Direction of the magnetic field of a coil.
Figure 7 Right-hand rule to find the direction of the magnetic field of a coil.
Moreover, the field strength depends on the current as well as other factors, such as the physical dimensions of the coil and the material of its core.
To enhance the magnetic field of a coil, a core made of a ferromagnetic material is inserted inside the coil.
The right-hand rule again can be helpful here to determine which side is the north and which side is the south, knowing the current direction. This is shown in Figure 7.
This rule states that if your right-hand fingers are in the direction of the current, then your thumb indicates the direction of the magnetic field (the thumb shows the north pole of the resulting magnetic field). Study this figure together with Figure 6. Also, note the difference between Figures 4 and 7.
The right-hand rule states that if your right-hand fingers are in the direction of the current in a coil of wires (from + toward −), then your thumb indicates the north pole of the coil magnetic field.
The magnet created by the coil in Figure 6 is not a permanent magnet. As soon as the current is turned off, the magnetic field vanishes and the magnetic effect disappears. This arrangement (a coil and a core in it) is called an electromagnet, and it has many industrial applications.
As you might imagine, it can be used for collection of (ferromagnetic) metals and for exerting force, in addition to many other applications such as opening and turning a switch or a valve on and off.
Electromagnet: A (not permanent) magnet made by a wire coil wrapped around a ferromagnetic core when carrying an electric current. The magnet can be turned on and off or its strength can be adjusted.