Magnetic Field

The magnetic field is the area around a magnet in which the effect of magnetism is felt. We use the magnetic field as a tool to describe how the magnetic force is distributed in the space around and within something magnetic in nature. In this article, let us familiarise ourselves with the magnetic field, magnetic field intensity, and its characteristics.

What is a Magnetic Field?

Magnetic Field is the region around a magnetic material or a moving electric charge within which the force of magnetism acts.

A pictorial representation of the magnetic field which describes how a magnetic force is distributed within and around a magnetic material

A magnetic field is a vector field in the neighbourhood of a magnet, electric current, or changing electric field in which magnetic forces are observable. A magnetic field is produced by moving electric charges and intrinsic magnetic moments of elementary particles associated with a fundamental quantum property known as spin. Magnetic field and electric field are both interrelated and are components of the electromagnetic force, one of the four fundamental forces of nature.

Symbol	B or H
Unit	Tesla
Base Unit	(Newton.Second)/Coulomb

History of Magnetic Field

• The research on the magnetic field began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles. He noticed that the resulting field lines crossed at two points. He named these points “poles.” After this observation, he stated that magnets always have North and South poles irrespective of how finely one slices them.
• Three centuries later, William Gilbert stated that Earth is a magnet.
• In 1750 John Mitchell, an English clergyman and philosopher, stated that magnetic poles attract and repel each other.
• In 1785, Charles-Augustin de Coulomb experimentally verified Earth’s magnetic field. In the 19th century, French mathematician and geometer Simeon Denis Poisson created the first model of the magnetic field, which he presented in 1824.
• By the 19th century, further revelations refined and challenged previously-held notions.
• In 1819, Danish physicist and chemist Hans Christian Oersted discovered that an electric current creates a magnetic field around it.
• In 1825, André-Marie Ampère proposed a model of magnetism where this force was due to perpetually flowing loops of current, instead of the dipoles of magnetic charge.
• In 1831, English scientist Faraday showed that a changing magnetic field generates an electric field. In effect, he discovered electromagnetic induction.
• Between 1861 and 1865, James Clerk Maxwell published theories on electricity and magnetism. It was known as Maxwell’s equation. These equations describe the relationship between electricity and magnetism.

Illustration of Magnetic Field

Typically, a magnetic field can be illustrated in two different ways.

• Magnetic Field Vector
• Magnetic Field Lines

Magnetic Field Vector

The magnetic field can be mathematically described as a vector field. The vector field is a set of many vectors that are drawn on a grid. In this case, each vector points in the direction that a compass would point and has a length dependent on the strength of the magnetic force.

Vector Field of a Bar Magnet

Magnetic Field Lines

Field lines is an alternative way to represent the information contained within a magnetic vector field. Magnetic field lines are imaginary lines.

Magnetic field lines are a visual tool used to represent magnetic fields. They describe the direction of the magnetic force on a north monopole at any given position

The density of the lines indicates the magnitude of the field. Taking an instance, the magnetic field is stronger and crowded near the poles of a magnet. As we move away from the poles, it is weak, and the lines become less dense.

The figure shows a magnetic field lines plot for a bar magnet

Properties of Magnetic Field Lines

• Magnetic field lines never cross each other
• The density of the field lines indicates the strength of the field
• Magnetic field lines always make closed loops
• Magnetic field lines always emerge or start from the north pole and terminate at the south pole.

Magnetic Field Intensity

Magnetic field strength is also magnetic field intensity or magnetic intensity. It is represented as vector H and is defined as the ratio of the MMF needed to create a certain Flux Density (B) within a particular material per unit length of that material. Magnetic field intensity is measured in units of amperes/metre.

It is given by the formula:

$\begin{array}{l}�=\frac{�}{�}-�\end{array}$

Where,

• B is the magnetic flux density
• M is the magnetization
• μ is the magnetic permeability

The SI unit of magnetic field intensity is Tesla. One tesla (1 T) is defined as the field intensity generating one newton of force per ampere of current per metre of conductor.

How does a Magnetic Field Originate?

The magnetic field arises when a charge is in motion. There are two basic ways to arrange for a charge to be in motion and generate a useful magnetic field. Following are the two ways:

Magnetic Field created by a Current-Carrying Conductor

Ampere suggested that a magnetic field is produced whenever an electrical charge is in motion. For our understanding, let us consider a wire through which the current is made to flow by connecting it to a battery. As the current through the conductor increases, the magnetic field increases proportionally. When we move further away from the wire, the magnetic field decreases with the distance. Ampere’s law describes this. According to the law, the equation gives the magnetic field at a distance r from a long current-carrying conductor I.

$\begin{array}{l}�=\frac{{�}_0�}{2��}\end{array}$

In the equation, µ0 is a special constant known as the permeability of free space(µ0=4π×10-7 T⋅ m/A).

Materials with higher permeability possess the ability to concentrate on magnetic fields.

The magnetic field has direction as it is a vector quantity. For conventional current flowing through a straight wire, this can be found by the right-hand rule. Imagine gripping your right hand around the wire with your thumb pointing in the current direction to use this rule. The fingers show the direction of the magnetic field, which wraps around the wire.

Motion of Electrons around the Nuclei of Atoms

Permanent magnets work based on the motion of electrons around the nuclei. We have observed that only some materials can be made into magnets, and some much stronger than others. To attain this state, some specific conditions should be met:

• Atoms have many electrons, and they are paired in such a way that the overall magnetic field cancels out. Two electrons paired this way are said to have opposite spins. From this, we understand that if we want a material to be magnetic, we need to have atoms that have one or more unpaired electrons with the same spin. Iron is a material that has four such electrons and therefore is good for making magnets out of it.
• A tiny piece of material consists of billions of atoms. If they are oriented randomly, the overall field cancels out, regardless of how many unpaired electrons the material has. The material has to be stable enough at room temperature to allow an overall preferred orientation to be established. If established permanently, then we have a permanent magnet, also known as a ferromagnet.
• Some materials become sufficiently well-ordered to be magnetic when in the presence of an external magnetic field. In the external field lines, all the electron spins up, but the alignment vanishes once the external field is removed. These kinds of materials are known as paramagnetic.

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