How Does Hysteresis Affect Everyday Devices and Systems?
Hysteresis is a fundamental phenomenon observed in many physical systems, where the response of a material or system depends not only on its current state but also on its previous history. This lagging effect is common in magnetism, phase transitions, and various materials, and is generally represented through characteristic curves known as hysteresis loops. Understanding hysteresis is essential in physics and engineering, especially for optimizing systems like transformers, electromagnets, and sensors.
Definition and Fundamental Concepts
Hysteresis refers to the dependence of a system's output not only on its current input but also on the input's history. In magnetic materials, it is defined as the lag of magnetization behind the external magnetizing field. This property is inherent to all ferromagnetic substances and is critical in determining their behavior under changing magnetic fields.
The term “hysteresis” is derived from the Greek word meaning “to lag behind.” In the context of magnetism, it describes the lag of magnetization ($B$) relative to the applied magnetic field ($H$). As the magnetic field is cycled, the relationship between $B$ and $H$ forms a loop that displays this lagging behavior.
Hysteresis Loop in Magnetic Materials
The hysteresis loop is a graphical representation of the relationship between magnetic induction ($B$) and magnetizing field strength ($H$) for a ferromagnetic material. As $H$ increases, $B$ also increases and eventually reaches saturation, beyond which $B$ remains nearly constant despite further increases in $H$.
When $H$ is reduced after saturation, $B$ does not return to zero but retains a certain value known as retentivity or residual magnetism. To reduce $B$ to zero, a negative magnetizing field called the coercive force must be applied. Further cycling of $H$ results in a closed loop, known as the hysteresis loop, which is characteristic for each material.
Physical Significance of Hysteresis
The area enclosed by the hysteresis loop represents the energy lost per unit volume during one complete cycle of magnetization and demagnetization. This energy loss appears as heat in the material and is termed hysteresis loss. In applications such as transformers, minimizing hysteresis loss is important for improving energy efficiency.
Soft magnetic materials like soft iron have narrow hysteresis loops and lower hysteresis loss, making them suitable for transformer cores. Hard magnetic materials possess wider loops and higher retentivity, and are used to create permanent magnets.
Important Parameters: Retentivity and Coercivity
Retentivity is the measure of the residual magnetism retained by a material when the external magnetizing field is reduced to zero. A high retentivity indicates that the material can become a good permanent magnet.
Coercivity is the required magnetizing field, in the negative direction, to bring the residual magnetization of a material to zero. Materials with high coercivity are generally harder to demagnetize and are preferred for permanent magnet applications.
| Property | Description |
|---|---|
| Retentivity | Residual magnetism after removing the field |
| Coercivity | Reverse field to demagnetize material |
Mathematical Expression of Hysteresis Loss
The energy loss per unit volume in one cycle of magnetization is given by the area of the hysteresis loop. Quantitatively, the hysteresis loss per cycle per unit volume is:
$W = \text{Area of the } B-H \text{ curve}$
For a material subjected to $f$ cycles per second, the total hysteresis loss per second (power loss) is:
$P = f \times (\text{Area of Hysteresis Loop})$
Types of Hysteresis
Hysteresis can be classified based on whether the lagging effect depends on the rate at which the input is varied (rate-dependent) or not (rate-independent). In magnetism, most hysteresis phenomena are rate-independent, but in other systems, such as viscoelastic materials, rate dependence may be observed.
Applications and Importance
Hysteresis is important in the selection of materials for transformers, permanent magnets, and electromagnetic devices, as it affects energy efficiency and device performance. Understanding hysteresis is also vital in engineering, physical sciences, and materials research for the design of various technological systems.
For further related topics, refer to Electromagnetic Induction Revision Notes for detailed study on electromagnetic effects.
Comparison: Soft and Hard Magnetic Materials
Soft magnetic materials like soft iron are easier to magnetize and demagnetize, with smaller hysteresis loop areas and lower coercivity and retentivity. Hard magnetic materials, such as steel, exhibit larger loop areas, higher coercivity and retentivity, making them well-suited for permanent magnets.
| Soft Magnet | Hard Magnet |
|---|---|
| Easy magnetization/demagnetization | Difficult magnetization/demagnetization |
| Small hysteresis loop area | Large hysteresis loop area |
| Temporary magnet | Permanent magnet |
Other Physical Phenomena Exhibiting Hysteresis
Hysteresis is also observable in systems such as rubber elasticity, phase change materials, and piezoelectric substances. In each context, the core principle is the dependence of the system’s state on its historical stimuli, usually producing characteristic loops during input-output cycling.
For example, in thermodynamics, some materials exhibit hysteresis during heating and cooling cycles, particularly in first-order phase transitions, leading to different values during melting and solidification.
Summary of Key Points
- Hysteresis is the lag of response behind the external influence
- Represented by characteristic loops in $B-H$ curves
- Causes energy loss in magnetic and other systems
- Critical for selection of materials in devices
- Occurs in various physical, chemical, and economic systems
Studying hysteresis is essential for analyzing the performance and efficiency of electrical machines, energy storage systems, and devices relying on reversible transformations, as well as for a deeper understanding of fundamental material properties. Concepts related to energy and power can be further explored at Work, Energy And Power.
FAQs on What Is Hysteresis? Meaning, Examples, and Applications
1. What is hysteresis?
Hysteresis describes a phenomenon where the value of a physical property lags behind changes in the effect causing it. Typically, it is observed in systems like magnetism or elasticity, where the response depends on previous states rather than only current inputs.
Key points about hysteresis:
- It involves a lag or delay effect between input and output.
- Common in physics, especially magnetic materials, stress-strain behaviour, and electrical circuits.
- The system's present output depends not only on its present input but also on its history.
2. What is hysteresis in magnetism?
Hysteresis in magnetism refers to the lagging of magnetisation behind the applied magnetic field. As you cycle the magnetic field, the material retains some magnetisation even after the field is removed.
Key features:
- The process creates a hysteresis loop on a B-H (magnetisation vs. magnetic field) graph.
- Some magnetisation remains, known as residual magnetism or retentivity.
- To demagnetise the material, a reverse field called coercivity is needed.
3. Explain the hysteresis loop.
The hysteresis loop is a graphical representation showing how a material's magnetisation changes as the external magnetic field is cycled.
Main components:
- The looped curve demonstrates the lag between magnetisation (B) and the magnetic field (H).
- The width of the loop indicates the level of energy loss and magnetic hardness.
- Key points are retentivity (residual magnetism) and coercivity (reverse field needed to demagnetise).
4. What are the applications of hysteresis?
Hysteresis has several important practical applications where energy loss, lag, or memory effects are significant.
Common applications include:
- Electric transformers and electric motors (magnetic cores).
- Permanent magnets and data storage devices.
- Thermostats and various electronic switches using hysteresis to prevent rapid switching.
- Elastic materials in stress-strain cycles (rubber, polymers).
5. What causes hysteresis loss in magnetic materials?
Hysteresis loss in magnetic materials occurs due to repeated magnetisation and demagnetisation cycles. The energy is lost as heat because the magnetic domains lag behind the changing external field.
Key details:
- The area of the hysteresis loop equals the energy loss per cycle per unit volume.
- Loss increases with frequency and volume of the material.
- Materials with narrow loops (soft magnets) have lower hysteresis loss.
6. What is retentivity and coercivity in the context of hysteresis?
Retentivity is a material's ability to retain magnetisation after the external field is removed, while coercivity is the field required to demagnetise it.
Important points:
- High retentivity means strong residual magnetism.
- High coercivity indicates the need for a strong reverse field to become unmagnetised.
- These properties are crucial for designing permanent magnets and magnetic storage devices.
7. Why is hysteresis significant in transformer cores?
Hysteresis is significant in transformer cores because it causes energy loss whenever the core is magnetised and demagnetised with alternating current.
Key effects:
- The energy lost appears as heat, reducing transformer efficiency.
- Magnetic materials with narrow hysteresis loops are preferred for transformer cores to minimise losses.
- Syllabus-aligned point: Efficiency and material selection for transformers directly relate to hysteresis loss.
8. What are soft and hard magnetic materials in relation to hysteresis?
Soft magnetic materials have narrow hysteresis loops (low energy loss), while hard magnetic materials have wide loops (high retentivity and coercivity).
Differences:
- Soft magnets: Low retentivity, low coercivity, ideal for transformer cores and electromagnets.
- Hard magnets: High retentivity, high coercivity, used for permanent magnets and magnetic storage.
- The distinction is based on their hysteresis loop characteristics.
9. Is hysteresis observed only in magnetism?
No, hysteresis is seen in many fields besides magnetism, wherever a system's output lags behind input due to its history.
Examples include:
- Elastic hysteresis in stretching and relaxing materials like rubber.
- Electrical circuits with relay switches or Schmitt triggers.
- Thermal hysteresis in heating/cooling systems (like thermostats).
10. How can hysteresis loss be reduced in electrical machines?
Hysteresis loss in electrical machines can be minimised by using materials with narrow hysteresis loops and careful design.
Effective methods:
- Use soft iron or silicon steel for transformer and motor cores.
- Operate at lower frequencies whenever possible.
- Choose materials specifically engineered for low coercivity and retentivity.
