The Characteristics of Magnetic Fields
What Is a Magnet and a Magnetic Field?
A magnet is an object that has a magnetic field strong enough to influence other materials. The molecules in a magnet are aligned to all face one way, which gives the magnet its magnetic field. Sometimes the molecules can align permanently, making a permanent magnet. Temporary magnets’ molecules only line up for a period of time before losing their magnetism. The length of the time they are aligned varies.
Magnetic fields are everywhere; anything that uses a magnet generates one. Switching on the light or television produces a magnetic field of some sort, and most metals (ferromagnetic metals) do as well.
The magnetic field of a magnet can be likened to lines of magnetic flux (magnetic flux is basically the amount of magnetic field an object has). The iron filings experiment demonstrates lines of magnetic flux. When you place a card over a magnet, then gently sprinkle iron filings onto the card, tapping the card will cause the iron filings to arrange themselves into lines that follow the field of the magnet underneath. The lines may not be very distinctive, depending on the strength of the magnet, but they will be clear enough to notice the pattern that they follow.
What Direction Does Magnetic Flux Flow?
A magnetic flux ‘flows’ from pole to pole; from south pole to north pole within a material, and from north pole to south pole in air. The flux seeks the path with least resistance between the poles, which is why they form close loops from pole to pole. The lines of force are all of the same value, and they never cross each other, which explains why the loops get further away from the magnet. Because the distance between the loops and the magnet increases, the density decreases, so the magnetic field gets weaker the further away from the magnet it gets. The size of a magnet doesn’t have an effect on the magnetic field strength of a magnet, but it does on the flux density of it. A larger magnet would have a larger dimensional area and volume, so the loops would be more spread out when flowing from pole to pole. A smaller magnet, however, would have a smaller area and volume so the loops would be more concentrated.
What Causes Poles to Attract or Repel One Another?
If two magnets are placed with their ends facing each other, one of two things can happen: they either attract or repel each other. This depends on which poles are facing each other. If like poles are facing each other, for example north-north, then the lines of flux are flowing in opposite directions, towards each other, making them push each other away, or repel. It’s like when two negative particles or two positive particles are being forced together—the electrostatic force makes them push away from one another.
Because the lines of flux flow from one pole, around the magnet and back into the magnet via the other pole, when opposite poles of two magnets face each other, the flux seeks the path which has the least amount of resistance, which would therefore be the opposite pole facing it. The magnets, therefore, attract one another.
Flux Density and Magnetic Field Strength
Flux density is the magnetic flux per unit cross-sectional area of the magnet. The intensity of the magnetic flux density is affected by the intensity of the magnetic field, the quantities of the substance, and the intervening media between the source of the magnetic field and the substance. The relationship between flux density and magnetic field strength is therefore written as:
B = µH
In this equation, B is the flux density, H is the magnetic field strength, and µ is the magnetic permeability of a material. When produced in a full B/H curve, it is apparent that the direction in which H is applied affects the graph. The shape made as a result is known as a hysteresis loop. The maximum permeability is the point where the slope of the B/H curve for the unmagnetized material is the greatest. This point is often taken as the point where a straight line from the origin is tangent to the B/H curve.
When values B and H are zero, the material is completely demagnetised. As the values increase, the graph curves steadily until it reaches a point where the increase in magnetic field strength has a negligible effect on the flux density. The point at which the value for B levels out is called a saturation point, meaning that the material has reached its magnetic saturation.
As H changes direction, B doesn’t immediately fall to zero. The material preserves some of the magnetic flux that it had gained, known as residual magnetism. When B finally reaches zero, all the material’s magnetism has been lost. The force required to remove all of the material’s residual magnetism is known as the coercive force.
Because H is now going in the opposite direction, another saturation point is reached. And when H is applied in the original direction again, B reaches zero in the same way as before, completing the hysteresis loop.
There is a considerable variation in the hysteresis loops of different materials. Softer ferromagnetic materials, such as silicon steel and annealed iron, have smaller coercive forces than that of hard ferromagnetic materials, therefore giving the graph a much narrower loop. They are easily magnetised and demagnetised and can be used in transformers and other devices in which you want to waste the least amount of electric power heating the core as possible. Hard ferromagnetic materials, such as alnico and iron, have much larger coercive forces, making them more difficult to be demagnetised. This is because they are permanent magnets since their molecules remain aligned permanently. Hard ferromagnetic materials are therefore useful in electromagnets since they will not lose their magnetism.