hysteresis (redirected from Hysteresis loss)

hysteresis (hĭs'tərē`sĭs), phenomenon in which the response of a physical system to an external influence depends not only on the present magnitude of that influence but also on the previous history of the system. Expressed mathematically, the response to the external influence is a doubled-valued function; one value applies when the influence is increasing, the other applies when the influence is decreasing. Magnetic hysteresis occurs when a permeable material like soft iron is magnetized by being subjected to an external magnetic field. The induced magnetization tends to lag behind the magnetizing force. If a field is applied to an initially unmagnetized sample and is then removed, the sample retains a residual magnetization (it has become a permanent magnet). The graph of the magnetic induction B versus the magnetic field H is called a hysteresis loop. The area of the loop is proportional to the energy dissipated as heat when the system goes through a cycle; this represents a considerable energy loss in alternating-current machinery. Thermal hysteresis occurs when the value of a given property of a body depends not only on the body's temperature but also on whether the temperature is rising or falling. An example is the dielectric constant versus temperature for certain crystals. Another kind of hysteresis is a common feature of control or cybernetic systems. A familiar example is a thermostat controlling a source of heat and set at some temperature T₀. When the room temperature falls through T₀ to some lower temperature T₁, the heating power is switched on. When the room temperature rises through T₀ to some higher temperature T₂, the power is switched off. Thus, for temperatures lower than T₁, the heat is always on; for temperatures higher than T₂, the heat is always off; but for temperatures between T₁ and T₂, the heat may be on or off (double-valued response), depending on which of the two temperatures T₁ and T₂ occurred most recently in the system's history. Unlike the previous examples, this hysteresis effect is not naturally occurring; it is designed into the control system to prevent the damage to the system that would arise from switching on and off too frequently.

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hysteresis

Lagging of the magnetization of ferromagnetic material (see ferromagnetism), such as iron, behind variations of the magnetizing field. When such a material is placed in a coil of wire carrying an electric current, the magnetic field so created forces atoms in the material to align with the field. This increases the total magnetic field to a maximum when all the atoms are aligned, though the total field lags behind the magnetizing field. When the intensity of the magnetizing field is decreased to zero, some field remains in the material, and if the magnetizing field is reversed the total magnetization also reverses but again lags behind. The complete cycle, known as a hysteresis loop, dissipates energy in the form of heat as the magnetization is reversed.

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The lag between making a change, such as increasing or decreasing power, and the response or effect of that change. It typically refers to turn-on and turn-off points in electrical, electronic and mechanical systems. For example, if a thermostat set for 70 degrees turns on when the temperature reaches 68 and turns off at 72, the hysteresis is the range from 68 to 72.