In simple terms, a magnetic field can be defined as a field that arises around matter due to the motion of electric charges or the presence of magnetic materials. This field exerts a force on magnetic objects, such as a compass needle. Magnetic fields typically form around magnets or wires carrying electric current. For example, the magnetic field surrounding a magnet can attract or repel another magnet or ferromagnetic materials such as iron. Magnetic fields can be visualized as magnetic field lines that extend from the north pole of a magnet to its south pole. Magnetic fields play a crucial role in many technological applications such as electronic devices, electric motors, and generators.
Definition of Magnetic Field
The magnetic field or magnetic flux density is commonly denoted by the letter B and is a vector quantity. In the SI system of units, the magnetic field is measured in tesla. This indicates that the magnetic field has both magnitude and direction. The direction of the magnetic field is defined as the direction in which the north end of a compass needle points and is visualized using magnetic field lines. These lines extend between magnetic poles: they emerge from the north pole and enter the south pole.
The magnetic field strength is denoted by the symbol H and its unit is A/m. The following relationship exists between these two quantity.
Where:
- B: Magnetic field vector [T],
- μ: Magnetic permeability [T.m/A],
- H: Magnetic field strength [A/m],
are expressed.
As an alternative to the tesla, the gauss unit is also used. The relationship between the two units is given by the following equation.
Right-Hand Rule
The right-hand rule is a method used to determine the directions of magnetic fields and electric currents. This method is used to find the direction of the magnetic field around a current-carrying wire or the direction of the magnetic force on a wire.
Application of the Right-Hand Rule
- Magnetic Field for a Straight Wire: Use your right hand and extend your thumb in the direction of the current flowing through the wire. Curl your other four fingers. The direction of your fingers indicates the direction of the magnetic field circling around the wire.
- Magnetic Field for a Solenoid (Coil): Place your right hand around the coil with your fingers curled in the direction of the current flowing through the coil. Your thumb will point in the direction of the magnetic field produced by the coil (i.e., the magnetic poles).
- Lorentz Force: A magnetic field can exert a force on a current-carrying wire or a moving charge. To determine the direction of this force, apply the right-hand rule as follows: For a straight wire or charge, point your thumb in the direction of the moving charge (or current), your index finger in the direction of the magnetic field, and your middle finger will then indicate the direction of the resulting magnetic force. (This is also known as the right-hand “three-finger rule”.)
Source of Magnetic Field
- Natural Magnets: A magnetic field is produced when the magnetic moments of atoms within a magnet align.
- Electric Current: Moving electric charges generate a magnetic field. For example, a current flowing through a straight wire creates a circular magnetic field around the wire. The Earth’s magnetic field: Motion of liquid metals within Earth’s core generates a massive magnetic field. This is a natural magnetic field that enables compasses to function.
Magnetic Force
A force can act on an electric charge or a magnetic dipole within a magnetic field. For example: A moving charge in a magnetic field experiences a force known as the Lorentz force. The magnitude of this force depends on the charge’s velocity, the strength of the magnetic field, and the angle between them. The attractive and repulsive forces between two magnets are also effects of the magnetic field.
Mathematical Expression
The magnitude and direction of the magnetic field can be calculated using the following expressions:
- Biot-Savart Law: Describes how an electric current generates a magnetic field.
- Ampere’s Law: The line integral of the magnetic field along a closed path is related to the current passing through the path.
Where:
- F: Magnetic force [N],
- q: Charge magnitude [C],
- v: Charge velocity [m/s],
- B: Magnetic field vector [T]
are expressed.
Magnetic Materials
Magnetic materials are substances that interact with magnetic fields and exhibit magnetic properties. Their characteristics are related to the motion and spin of electrons within their atomic structure. Magnetic materials are classified into different categories based on their response to magnetic fields. Below are the main types of magnetic materials and their definitions:
Diamagnetic Materials
- Definition: Materials that are weakly magnetized in the opposite direction to an applied magnetic field.
- Properties: Magnetization disappears when the magnetic field is removed. Their magnetic permeability (μ) is slightly less than 1. They have a weakly demagnetizing effect on magnetic fields.
- Examples: Copper, gold, silver, silicon, graphite.
Paramagnetic Materials
- Definition: Materials that are weakly magnetized in the same direction as an applied magnetic field.
- Properties: Magnetization disappears when the magnetic field is removed. Their magnetic permeability is slightly greater than 1. They have net magnetic moments from electrons, but these moments are randomly oriented in the absence of a magnetic field.
- Examples: Aluminum, platinum, magnesium, tungsten.
Ferromagnetic Materials
- Definition: Materials that are strongly magnetized in the presence of a magnetic field and retain their magnetization even after the field is removed.
- Properties: They can exhibit permanent magnetization even without an external magnetic field. They have high magnetic permeability. Atoms can align into magnetic regions called domains even without an external field.
- Examples: Iron, nickel, cobalt, and their alloys.
Antiferromagnetic Materials
- Definition: Materials in which the magnetic moments of neighboring atoms align in opposite directions, resulting in a net magnetic moment of zero.
- Properties: They typically exhibit weak magnetic properties when an external magnetic field is applied. The magnetic moments are completely balanced.
- Examples: Manganese oxide (MnO), nickel oxide (NiO).
Ferrimagnetic Materials
- Definition: Similar to antiferromagnetic structures, but the opposing magnetic moments are not fully balanced, resulting in a net magnetic moment.
- Properties: They can be magnetized even in the absence of an external magnetic field, similar to ferromagnetic materials. The net magnetic moment is positive but weaker than in ferromagnetic materials.
- Examples: Ferrites (e.g., magnetite – Fe₃O₄).
Superparamagnetic Materials
- Definition: Materials composed of ferromagnetic particles at the nanometer scale. Due to their small size, these particles can easily reorient under thermal energy.
- Properties: They behave like paramagnetic materials under a magnetic field. Magnetization disappears when the field is removed.
- Examples: Nanoparticle-based magnetic materials (e.g., iron oxide nanoparticles).
Magnetostrictive Materials
- Definition: Materials that change shape or dimensions when a magnetic field is applied.
- Properties: They convert magnetic energy into mechanical energy. They exhibit magnetostriction.
- Examples: Terfenol-D, iron-nickel alloys.
Applications
- Electric motors and generators rely on the ability of magnetic fields to convert energy.
- MRI (Magnetic Resonance Imaging): Strong magnetic fields are used to image the internal structure of the human body.
- In industry, electromagnets are widely used for lifting and transporting operations.
History
As early as the 13th century BCE, the Chinese used compasses. The ancient Greeks had knowledge of magnetism by 800 BCE, discovering that lodestone (Fe₃O₄) attracted iron. According to legend, the name “magnetite” derives from a shepherd named Magnes, whose shoe nails and staff tip became stuck to large pieces of lodestone while herding his flock. In 1269, Pierre de Maricourt placed a needle at various points on a spherical natural magnet and mapped the directions the needle pointed. He observed that these directions formed lines passing through two opposite points on the sphere and encircling it. He named these points the poles of the magnet. Subsequent experiments showed that every magnet, regardless of shape, has two poles: north and south. These poles exert forces on each other, similar to electric charges. The relationship between electricity and magnetism was discovered in 1819 when Danish physicist Hans Christian Oersted observed that a compass needle deflected near a current-carrying wire. Shortly afterward, André Ampere derived the quantitative laws needed to calculate the magnetic force between current-carrying conductors. In the 1820s, Michael Faraday and independently Joseph Henry demonstrated additional relationships between electric current and magnetism. Ultimately, Maxwell published all these findings and unified electricity and magnetism in his famous Maxwell’s equations.
