Temporary vs Permanent Magnets

Introduction

Key Concepts

Definition of Magnets

Magnets are materials or objects that produce a magnetic field, which exerts forces on other magnets or magnetic materials such as iron, nickel, and cobalt. This magnetic field is responsible for attracting or repelling other magnetic materials and influencing the behavior of electrically charged particles within the vicinity of the magnet.

Types of Magnets

Magnets are broadly classified into two categories: temporary magnets and permanent magnets. Each type has distinct properties and applications based on their ability to retain magnetism.

Temporary Magnets

Temporary magnets are materials that exhibit magnetic properties only when exposed to an external magnetic field. Once the external magnetic field is removed, these materials lose their magnetism.

• Examples: Soft iron, electromagnets.
• Characteristics:
  • Low coercivity: They can easily gain and lose magnetization.
  • High permeability: They can support the formation of magnetic fields easily.
  • Used in applications where magnetism needs to be turned on and off.

Permanent Magnets

Permanent magnets are materials that maintain their magnetic properties without the need for an external magnetic field. They retain their magnetism over extended periods and under normal conditions.

• Examples: Neodymium magnets, ferrite magnets, Alnico magnets.
• Characteristics:
  • High coercivity: They resist changes to their magnetization.
  • Low permeability compared to temporary magnets.
  • Used in applications requiring constant magnetic fields.

Magnetic Domains

Magnetic domains are small regions within a material where the magnetic moments of atoms are aligned in the same direction. The behavior of these domains determines whether a material behaves as a temporary or permanent magnet.

• Temporary Magnets: The domains are easily aligned and can quickly randomize when the external field is removed.
• Permanent Magnets: The domains are strongly aligned and resist randomization, maintaining overall magnetization.

Coercivity and Remanence

Coercivity refers to the resistance of a magnetic material to changes in magnetization, while remanence is the residual magnetization that remains after an external magnetic field is removed.

• Temporary Magnets:
  • Low coercivity: Easily demagnetized.
  • Low remanence: Little to no residual magnetism.
• Permanent Magnets:
  • High coercivity: Difficult to demagnetize.
  • High remanence: Significant residual magnetism.

Energy Products

The energy product of a magnet is a measure of the maximum amount of magnetic energy stored in the magnet. It is typically expressed in Mega-Gauss Oersteds (MGOe).

Permanent magnets generally have higher energy products compared to temporary magnets, making them more suitable for applications requiring strong and sustained magnetic fields.

Applications of Temporary Magnets

Temporary magnets are widely used in applications where the magnetic field needs to be controlled or switched on and off. Common applications include:

• Electromagnets: Used in electric bells, motors, and generators.
• Magnetic Latches: Employed in doors and cabinets for temporary holding.
• Scientific Equipment: Utilized in experiments requiring controllable magnetic fields.

Applications of Permanent Magnets

Permanent magnets are essential in applications that require a constant magnetic field without the need for continuous power supply. Common applications include:

• Motors and Generators: Provide the necessary magnetic flux for operation.
• Refrigerator Magnets: Simple applications for holding notes and decorations.
• Magnetic Storage Media: Used in hard drives and credit card stripes.
• Speakers and Headphones: Essential for converting electrical signals into sound.

Advantages of Temporary Magnets

• Controllability: Magnetic field can be easily turned on and off.
• Energy Efficiency: Only requires power when the magnetic field is needed.
• Flexibility: Can be used in dynamic applications where magnetism needs to change.

Advantages of Permanent Magnets

• Constant Magnetic Field: Provides a steady magnetic field without external power.
• Durability: Long-lasting magnetism with minimal maintenance.
• High Efficiency: No energy required to maintain the magnetic field.

Limitations of Temporary Magnets

• Dependence on External Field: Loses magnetism once the external field is removed.
• Energy Consumption: Continuous power may be required to maintain the magnetic field in certain applications.

Limitations of Permanent Magnets

• Cost: High-performance permanent magnets can be expensive.
• Magnetic Strength: Limited by coercivity and remanence properties.
• Temperature Sensitivity: High temperatures can demagnetize permanent magnets.

Equations and Theoretical Explanations

Understanding the behavior of temporary and permanent magnets involves fundamental electromagnetic principles. The magnetic field ($\vec{B}$) produced by a magnet is given by: $$ \vec{B} = \mu (\vec{H} + \vec{M}) $$ where:

• $\mu$ is the permeability of the material.
• $\vec{H}$ is the applied magnetic field strength.
• $\vec{M}$ is the magnetization of the material.

For permanent magnets, the magnetization $\vec{M}$ remains significant even when the external field $\vec{H}$ is removed, resulting in a sustained magnetic field. In contrast, temporary magnets have magnetization $\vec{M}$ that depends directly on the external field $\vec{H}$, diminishing once $\vec{H}$ is absent.

The coercivity ($H_c$) is an important parameter that quantifies the resistance of a magnetic material to changes in its magnetization: $$ H_c = -\frac{M_r}{\mu_0} $$ where $M_r$ is the remanent magnetization and $\mu_0$ is the permeability of free space. Permanent magnets have high coercivity values, making them resistant to demagnetizing influences.

Magnetic Hysteresis

The hysteresis curve of a magnetic material illustrates the relationship between the applied magnetic field ($H$) and the magnetization ($M$). Permanent magnets exhibit a wide hysteresis loop with high coercivity and remanence, whereas temporary magnets show narrow loops indicating low coercivity and remanence.

This hysteresis behavior explains why permanent magnets retain their magnetization while temporary magnets do not.

Comparison Table

Summary and Key Takeaways