The Hysteresis Loop Experiment
Hysteresis is the term used to describe the loss of energy in a system over the course of time. When the displacement of a system exhibiting Hysteresis is plotted against applied force, the curve takes the shape of loop.
These systems are called bistable because a hysteresis-loop could suggest two states such as on and off. Magnetic recording media also function because of this dependence on the past. This is the reason for the phenomena of coercivity and saturation remanence, and coercivity.
The most complicated of the nonlinearities presented in this section, hysteresis is defined as a lagging in the magnetization of a ferrromagnetic material due to variations in the magnetic field applied. This happens because the magnetic domains in the material don't align perfectly when the force that is magnetizing altered. The misalignment leads to the reversal being slower than the actual reverse. The energy lost in the reverse of the magnetic field is the loss of hysteresis.
The hysteresis can be depicted graphically as a loop, commonly referred to as the B-H curve (see Figure. 4.10). The area in the centre of the loop is the energy lost due to internal friction. This characteristic is commonly found in all ferromagnetic substances and is the foundation of magnetic measurement methods.
A ferromagnetic material is typically placed inside coil wires that are supplied with an electric current. The induced magnetic field created by the current causes some or all magnetic particles to align with the magnetic field. As the atomic magnets are aligned with the field, their magnetic flux density rises, and as the polarization of the atomic magnets is restored to its initial state, the magnetic field reverts back to its initial condition. The loop or bh-curve illustrates the relationship between the magnetic field induced by the field and the density of magnetic fields.
The hysteretic behaviour of magnetic materials is very complex and diverse. Therefore, it is difficult to interpret a hysteresis curve in terms of one property of the material being studied, as the curve is a multiple-valued function (depending on how the magnetisation changes) of the field applied Ha - specimen geometry N M>.
The left-hand curve of Figure 1.6 is a representation of a variety of materials sometimes called "magnetically hard". These are substances like silicon steel that when magnetised to saturation, will retain significant amounts of the magnetic field applied (and energy) during the process of demagnetisation. These are typically used as cores in electric motors and transformers to minimize the loss of energy due to the constant reversal of the field applied in AC electrical applications.
The curve to the right of the figure shows a variety of "soft" materials such as soft iron, which loses less energy during the process of magnetisation. These are the materials that are easily demagnetised using very little reverse current and have a low coercivity (the amount of reverse external magnetising force needed to totally demagnetise the material).
The form of a hysteresis loop can be a sign of the mineralogy of the sample. Ferromagnetic minerals create narrow loops whereas antiferromagnetic minerals cause large loops. In the majority of cases, the loop of hysteresis is created in a magnetisation machine by the passage of an unmagnetised specimen through a complete cycle of an H-force magnetising followed by a demagnetising force H. The B-H curve that results is known as the magnetic hysteresis the specimen.
This set-up for experiments is a versatile training system for physics laboratories that allows one to measure the Hysteresis loss of ferromagnetic materials. The apparatus can also be used to measure the Coercive force, Retentiveness & Saturation Magnetisation of the samples.
Magnetic hysteresis measures the change in the magnetization M value of a sample between its highest value (Bmax) and its zero-field remanence value (Mzero). The shape of the loop is dependent on the mineralogy (Channell and McCabe 1994, Tauxe 1993). Ferrimagnetic minerals create smaller hysteresis loops. antiferromagnetism results in wide loops.
Each loop is accompanied by a loss of energy that is represented by a change in the curve from one point to the next. The process is described by the non-linear behaviour of a hysteretic model and there are various models of hysteresis that define specific features of these systems, e.g., the Bouc-Wen model that can be used to describe many different shapes of loops that are hysteretic.
When a magnetising current is applied to the coil, these molecules align themselves to attain the highest amount of magnetic flux density. This is known as saturation magnetisation Ms. When the magnetising coil is used these molecules align to achieve the highest level of magnetic flux, which is known as saturation magnetisation Ms. When the coil is demagnetised, the molecules disengage themselves from their alignment, and reorientate themselves in opposite directions in order to regain their original position. This causes an additional loss of energy in the form the hysteresis-like jump.
Hysteresis is a common occurrence in systems, materials or organisms that have memory. Their output is contingent on where they come from. This is because a system's rate of change (input) is influenced by its past and the input (output). This results in the presence of a delay or lag in a response. The concept is particularly important in natural phenomena, like the backlashing of gears and the elastic distortion of rubber bands, shape-memory alloys and magnetizations of ferrro magnetic materials. Hysteresis is also a feature of a variety of artificial systems such as thermostats and Schmitt triggers in electronic circuits to prevent unintentional and frequent switching.
In AC transformers the shape of the B/H curve is a major aspect in core losses. The greater the size of the hysteresis circuit, the more energy is lost. Hysteresis is not a major issue in DC transformers as the magnetic field doesn't reverse with every change in current. However, it is a significant problem and the cause of magnetic losses in cores. Hysteresis can also be integrated into bistable system such as cell biology to ensure stable digital outputs based on continuous inputs.