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Hysteresis

From Academic Kids

Hysteresis is a property of systems (usually physical systems) that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state: that is, systems whose states depend on their immediate history. For instance, if you push on a piece of putty it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not immediately and not entirely. The term derives from an ancient Greek word meaning 'deficiency'. The phenomenon was identified, and the term coined, by Sir James Alfred Ewing in 1890.

Hysteresis phenomena occur in magnetic and ferromagnetic materials, as well as in the elastic and electromagnetic behavior of materials, in which a lag occurs between the application and the removal of a force or field and its subsequent effect. Electric hysteresis occurs when applying a varying electric field, and elastic hysteresis occurs in response to a varying force.

If the displacement of a system with hysteresis is plotted on a graph against the applied force, the resulting curve is in the form of a loop. In contrast, the curve for a system without hysteresis is a single, not necessarily straight, line. Although the hysteresis loop depends on the material's physical properties, there is no complete theoretical description that explains the phenomenon. The family of hysteresis loops, from the results of different applied varying voltages or forces, form a closed space in three dimensions, called the hysteroid.

Hysteresis was initially seen only as a problem, but is now considered to be of great importance in technology, and the property is used for example when constructing permanent memory for computers.

The term "hysteresis" is sometimes used in other fields, for example economics or biology. In such cases it describes a memory or lagging effect in which the order of previous events can influence the order of subsequent events.

Contents

Definition

The phenomenon of hysteresis can conceptually be explained as follows. A system can be divided into subsystems or domains, much larger than an atomic volume but still microscopic. Such domains normally occur in ferroelectric and ferromagnetic systems, since individual dipoles tend to group with each other, forming a small isotropic region. Each of the system's domains can be shown to have a metastable state. The metastable domains can in turn have two or more substates. Such a metastable state fluctates widely from domain to domain, but the average represents the configuration of lowest energy. The hysteresis is simply the sum of all domains, or the sum of all metastable states.

Magnetic hysteresis

Hysteresis is well known in ferromagnetic materials. When an external magnetic field is applied to a ferromagnet, the ferromagnet absorbs some of the external field. Even when the external field is removed, the magnet will retain some field: it has become magnetized.

Hysteresis loop:  (B) as function of  (H)
Hysteresis loop: magnetic flux density (B) as function of magnetic field strength (H)

The relationship between magnetic field strength (H) and magnetic flux density (B) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.

If the magnetic field is now reversed and increased linearily, the plotted relationship will again follow a similar curve back towards and beyond zero flux density but offset from the original curve by an amount called the remanent flux density or remanence. The cause of the offset is the fact that ferromagnetic materials will tend to retain some of the magnetism induced in them and this has to be overcome each time the magnetic field across the substance reverses.

If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. The 'thinness' of the middle bit of the S describes the amount of hysteresis.

Its practical effects might be, for example, to cause a relay to be slow to release due to the remaining magnetic field continuing to attract the armature when the applied electric current to the operating coil is removed.

This curve for a particular material influences the design of a magnetic circuit.

This is also a very important effect in magnetic tape and other magnetic storage media like hard disks. In these materials it would seem obvious to have one polarity represent a bit, say north for 1 and south for 0. However, if you want to change the storage from one to the other, the hysteresis effect requires you to know what was already there, because the needed field will be different in each case. In order to avoid this problem, recording systems first overdrive the entire system into a known state using a process known as bias. Analogue magnetic recording also uses this technique. Different materials require different biasing, which is why there is a selector for this on the front of most cassette recorders.

In order to minimize this effect and the power losses associated with it, ferromagnetic substances like Stelloy are often used. This has a low hysteresis loss.

Electrical hysteresis

Electrical hysteresis typically occurs in ferroelectric material, where domains of polarization contribute to the total polarization. Polarization is the electrical dipole moment (either C·m-2 or C·m).

Liquid-solid phase transitions

Hysteresis is also a known phenomenon in thermodynamics. Here the molecules change properties during heat transfer (via a potential field or a higher temperature increase) and it is this change that causes hysteresis.

Energy

When hysteresis occurs with extensive and intensive variables, the work done on the system is the area under the hysteresis graph.

Economics

Some economic systems show signs of hysteresis. For example, export performance is subject to strong hysteresis effects: it may take a big push (ie sizable changes in incentives) to start a country's exports, but once the transition is made, not much may be required to keep them going.

Electronics

Hysteresis can be used to filter a signal so that the output reacts slowly by taking recent history into account. For example, a thermostat controlling a heater may turn the heater on when the temperature drops below A degrees, but not turn it off until the temperature rises above B degrees. Then the on/off output of the thermostat to the heater when the temperature is between A and B depends on the history of the temperature.

Applications

Hysteresis represents states, and the characteristic curve shape is sometimes remiscent of a two-value state, also called a bistable state. The hysteresis curve really contains infinitely many states, but a simple application is to let the threshold regions (usually to the left and to the right) represent respectively the on and off states. In this way, the system can be regarded as bistable. Note that even if no external field is applied, the position of the hysteresis curve might change with time: it is not necessarily stationary, i.e. the system may not stay in the exact same state as it had previously. The system might need new energy transfer to be stationary.

The hysteresis effect can be used when connecting complex circuits with the so-called passive matrix addressing. This scheme is praised as a technique that can be used in modern nanoelectronics, electrochrome cells, memory effect, etc. In this scheme, shortcuts are made between adjacent components (see crosstalk) and the hysteresis helps to keep the components in a particular state while the other components change states. That is, one can address all rows at the same time instead of doing each individually.

In Economics, Hysteresis is used extensively in the area of Labour markets. According to theories based on hysteresis, Economic downturns (Recession) result in an individual becoming unemployed, losing his/her skills (commonly developed 'on the job', demotivated/disillusioned, and employers may use time spent in unemployment as a screen. In times of an Economic upturn or 'boom', the workers affected will not share in the prosperity, remaining Long-Term Unemployed (>52 weeks). Hysteresis has been put forward as a possible explanation for the poor unemployment performance of many economies in the 1990's. Labour market reform, and/or strong economic growth, may not therefore aid this pool of long-term unemployed, and thus specific targeted training programs are presented as a possible policy solution.

See also

External links

es:Histéresis pl:Histereza sv:Hysteres nl:Hysterese

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