Permanent Magnets and Electromagnets

Permanent Magnets And Electromagnets

Magnets are essential components in numerous technologies. They produce magnetic fields which can exert forces on other magnets, magnetic materials, and moving electric charges. Magnets can be broadly classified into two main types based on how they maintain their magnetic field: Permanent Magnets and Electromagnets.

Permanent Magnets

A permanent magnet is a material that retains its magnetism after being magnetised by an external magnetic field. It produces its own persistent magnetic field without any external power source.

Mechanism and Materials

Permanent magnets are typically made from hard ferromagnetic materials. As discussed in the previous section on Magnetic Properties of Materials, ferromagnetic materials are composed of microscopic regions called magnetic domains, where atomic magnetic moments are aligned. When a ferromagnetic material is placed in a strong external magnetic field, these domains align with the field, resulting in a large net magnetisation.

Hard ferromagnetic materials have a broad and large hysteresis loop (high retentivity and high coercivity). This means that once they are magnetised by a strong external field, they retain a significant amount of magnetisation (high remanence, $B_r$) even after the external field is removed ($H=0$). Furthermore, they require a large opposing magnetic intensity (high coercivity, $H_c$) to demagnetise them. This property of retaining magnetism makes them suitable for permanent magnets.

Common materials used for permanent magnets include:

Steel
Alnico (an alloy of Aluminium, Nickel, Cobalt, and Iron)
Ferrites (ceramic magnetic compounds)
Neodymium Magnets (NdFeB - Neodymium, Iron, Boron, very strong)
Samarium-Cobalt (SmCo) magnets

These materials have domain structures that are resistant to changes in magnetisation once aligned.

Characteristics of Permanent Magnets

They produce a constant magnetic field without requiring continuous electrical energy input.
The strength of their magnetic field is relatively fixed once the magnet is made, although it can decrease over time (aging) or if subjected to high temperatures or strong opposing magnetic fields.
Their polarity (North and South poles) is fixed and cannot be easily changed.
Common forms include bar magnets, horseshoe magnets, cylindrical magnets, etc.

Uses of Permanent Magnets

Permanent magnets are used in a wide variety of applications:

Compass needles.
Refrigerator doors (for sealing).
Loudspeakers and microphones.
Electric meters and some types of electric motors.
Magnetic storage media (hard disk drives, magnetic tapes - less common now).
Magnetic resonance imaging (MRI) machines (using very powerful superconducting magnets).
Magnetic separation in industry.
Toys and educational tools.

Electromagnets

An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets are usually made by winding a coil of wire around a core made of a ferromagnetic material, such as soft iron.

Mechanism and Construction

The principle of an electromagnet is based on the magnetic effect of electric current: a current-carrying coil (like a solenoid) produces a magnetic field. The magnetic field inside a long solenoid in vacuum is given by $B_0 = \mu_0 n I$, where $n$ is the number of turns per unit length and $I$ is the current.

Diagram of a simple electromagnet made of a current-carrying coil around an iron core

A simple electromagnet: A current-carrying coil wound around a soft iron core.

To make a powerful electromagnet, a core of soft ferromagnetic material is inserted inside the coil. Soft ferromagnetic materials have a narrow and small hysteresis loop (low retentivity and low coercivity). This means they are easily magnetised even by relatively weak external fields (low $H$) and, crucially, lose almost all their magnetism when the external field is removed (low $B_r$) and require very little opposing field to demagnetise ($H_c$ near zero).

When current flows through the coil, it produces a magnetic intensity $H$. This $H$ strongly magnetises the soft iron core ($M = \chi_m H$, where $\chi_m$ is very large for soft iron). The total magnetic field inside the core becomes $B = \mu H = \mu_r \mu_0 H$. Since $\mu_r$ is very large for soft iron, the total magnetic field $B$ becomes much stronger than the field $B_0 = \mu_0 H$ that the coil would produce without the core. When the current in the coil is switched off, $H$ becomes zero, and because the core is made of a soft magnetic material, its magnetisation $M$ also drops almost to zero, and the magnetic field $B$ effectively disappears.

Characteristics of Electromagnets

They produce a magnetic field only when electric current flows through the coil. The field exists only as long as the current is ON.
The strength of the magnetic field can be easily controlled by changing the current flowing through the coil. Increasing the current increases the magnetic field strength (linear relationship in the absence of saturation). The strength also depends on the number of turns in the coil and the nature of the core material.
The polarity (North and South poles) can be easily reversed by reversing the direction of the current in the coil (using the Right-Hand Rule for a solenoid).
When the current is switched off, the magnetic field essentially vanishes (very low residual magnetism in soft iron).

Uses of Electromagnets

Electromagnets are used in applications where a strong, controllable, and temporary magnetic field is required:

Electric bells.
Telephones and relays.
Lifting heavy iron and steel objects (e.g., in scrap yards).
Magnetic cranes.
Loudspeakers (in some designs).
Electric motors and generators.
Medical equipment (MRI, magnetic separation).
Particle accelerators.
Magnetic Levitation (Maglev) trains.

Comparison: Permanent Magnets vs. Electromagnets

Here's a table summarising the key differences between permanent magnets and electromagnets:

Feature	Permanent Magnet	Electromagnet
Source of Magnetism	Intrinsic property of magnetised material; aligned magnetic domains remain aligned.	Electric current flowing through a coil, usually enhanced by a ferromagnetic core.
Requirement for Field	Always present (unless demagnetised).	Present only when current flows through the coil.
Control of Field Strength	Fixed (difficult to change significantly).	Easily varied by changing the current in the coil or the number of turns.
Control of Polarity	Fixed.	Easily reversed by reversing the direction of current.
Material	Hard ferromagnetic materials (high retentivity, high coercivity).	Soft ferromagnetic materials (low retentivity, low coercivity) for the core, and a conducting coil.
Energy Consumption	No continuous energy consumption for the field itself.	Consumes electrical energy when active (to maintain current).
Applications	Compass, speakers, refrigerator doors, fixed magnets in motors.	Cranes, relays, electric bells, adjustable magnets in motors/generators, MRI.

Both permanent magnets and electromagnets are crucial components in modern technology, each suited for different applications depending on whether a constant, fixed field or a controllable, temporary field is needed.