Electromagnets

We now know from previous lessons that a conductor carrying a flat current creates a circular magnetic field around itself at all points along its length, and that the direction of rotation of this magnetic field depends on the direction of the current passing through the conductor, the Left Hand Rule.

In the final lesson on electromagnetism, we found that if we bend the conductor into a single loop, the current will flow in opposite directions throughout the cycle, creating an area that is clockwise and counterclockwise. Electromagnetism uses this principle with several separate cycles that are magnetically combined to produce a single coil.

Electromagnets are basically wire coils that act like rod magnets with a pronounced north and south pole when an electric current passes through the coil. The static magnetic field produced by each coil cycle is collected with its neighbor with a concentrated combined magnetic field, such as the single wire loop that we looked at in the last lesson in the center of the coil. The resulting static magnetic field, which has an arctic at one end and a south pole at the other, is uniform and much stronger in the center of the coil than on the outside.

Force Lines Around Electromagnet

Force Lines Around Electromagnet

The magnetic field produced by this is stretched in the form of a rod magnet that gives a pronounced north and south pole, proportional to the amount of current flowing in the flux coil. If additional layers of wire flowing the same current onto the same coil are wrapped, the magnetic field power will increase.

Therefore, it can be seen that the amount of flux present in any magnetic circuit is directly proportional to the current passing through it and the number of wire windings inside the coil. This relationship is called Magneto Motivated Force or m.m.f. and is defined as follows:

Magneto Motivated Force

Magnetomotor force is expressed as a current flowing from an N-turn coil. Therefore, the magnetic field power of an electromagnet is determined by the amperage turns of the coil, and with more wire rotation in the coil, the power of the magnetic field becomes so great.

Magnetic Power of Electromagnet

Now that we know that two adjacent conductors carry current, magnetic fields are established according to the flow direction of the current. The resulting interaction of the two areas is in the same way that a mechanical force is experienced by the two conductors.

When the current flows in the same direction (on the same side of the coil), the area between the two conductors is weak, causing a gravitational pull, as shown above. Likewise, as the current flows in opposite directions, the area between them intensifies and conductors are pushed.

The density of this area around the conductor is proportional to the distance at which the strongest point is next to the conductor and gradually weakens as it moves away from the conductor. In the case of a single flat conductor, the flowing current and the distance from it are the factors that govern the density of the area.

Therefore, the formula for calculating the "Magnetic Field Power" of a conductor carrying long straight currents, sometimes called "Magneting Force", is derived from the current flowing through it and the distance from it.

Magnetic Field Power for Electromagnets

The power of the H – magnetic field in amperage-rotation/meter, At/m
N – the number of turns of the coil is
the current flowing from the coil in amps, A
L – the length of the coil in meters, m
To summarize later, the strength or density of the magnetic field of a coil depends on the following factors:

  • The number of turns of the wire inside the coil.
  • The amount of current flowing through the coil.
  • Core material type.

The magnetic field power of the electromagnet also depends on the type of core material used. Because the main purpose of the nucleus is to concentrate magnetic flux in a well-defined and predictable path. Until now, only air-core (hollow) coils have been considered. However, the addition of other materials to the core (coil center) has a huge control over the power of the magnetic field.

If the material is not magnetic, for example, wood, it can be considered free space for computational purposes, as they have very low permeability values. However, if the core material is made of Ferromamanyetic material such as iron, nickel, cobalt or any mixture of their alloys, a significant difference in the flux density around the coil will be observed.

Ferromamanyetic materials are materials that can be magnetized and are usually made of soft iron, steel or various nickel alloys. The introduction of such material into a magnetic circuit has the effect of intensifying the magnetic flux and making it denser and denser, strengthening the magnetic field created by the current in the coil.

We can prove this by wrapping a wire coil around a large soft iron nail and connecting it to a battery as shown. This simple class experiment allows us to get a large amount of clips or pins, and by adding more rotation to the coil we can make the electromagnet more powerful. This degree of density of the magnetic field, either by a hollow air core or by the addition of ferromagnetic materials to the nucleus, is called Magnetic Permeability.

Permeability of Electromagnets

If the electromagnet uses cores from different materials of the same physical dimensions, the power of the magnet will vary according to the core material used. This change in magnetic force is due to the number of flux lines passing through the central core. If the magnetic material has a high permeability, then the flux lines can be easily formed and pass through the central core, a measure of permeability (μ) and the ease with which this nucleus can be magnetized.

The numerical constant given for the permeability of a vacuum is given as follows: μo = 4.π.10-7 H/m and the relative permeability of the free space (vacuum) is usually given as one. It is this value that is used as a reference in all calculations related to permeability, and all materials have their own permeability values.

The problem with using only the permeability of different iron, steel or alloy cores is that the relevant calculations can be very large and therefore it is more convenient to define the materials according to their relative permeability.

Relative Permeability is and is given as a product of the permeability of μo empty space with symbol μr, μ (absolute permeability).

Relative Permeability

Materials with slightly less permeability than those of empty space (vacuum) and weak to magnetic fields, with negative sensitivity, are called Diamagnetic by nature, such as water, copper, silver and gold. These materials, which are slightly more permeable than those of empty space and are very little drawn by a magnetic field themselves, are called Paramagnetic by nature, such as gases, magnesium and tantalum.

Electromagnetism Sample

The absolute permeability of a soft iron core is given as 80 milli-henries/m (80.10-3). Let's calculate the equivalent relative permeability value.

When ferromamagnetic materials are used in the nucleus, the use of relative permeability to define field strength gives a better idea of the strength of the magnetic field for the different types of materials used. For example, a vacuum and the relative permeability of air are one and around 500 for an iron core, so we can say that the field power of an iron core is 500 times stronger than an equivalent hollow air coil, and this relationship is much greater. Easier to understand than 0.628×10-3 H/m, (500.4.π.10-7).

Air permeability can be only one, while the permeability of some ferrite and permalloy materials can be 10,000 or more. However, there are limits to the amount of magnetic field force that can be obtained from a single coil, as the nucleus becomes densely saturated as the magnetic flux increases.