Magnetic Field: Definition, Origin, & Illustration

Magnetic field, also called a vector field, represents the magnetic influence on moving electric charges, magnetic materials, and electric currents. The magnetic fields force moving electrically charged particles in a circular or helical path and the charged particles experience a force perpendicular to their own velocity and to the magnetic field. Magnetic field can be expressed as the area around a magnet wherein the effect of magnetism is felt.

The symbol of the Magnetic Field can be denoted by B or H. It is denoted mathematically by quantities known as vectors which have direction and magnitude both. Two different vectors help represent magnetic field: Magnetic flux density (or magnetic induction) and Magnetic field strength (or magnetic field intensity) each symbolized by B and H respectively.

The unit of Magnetic Field is Telsa and its base unit is (Newton.Second)/Coulomb. Magnetic field lines are known to not cross one another. In fact, magnetic lines form closed loops, beginning from the north pole and ending at the south pole. The density of the field lines generally indicates the strength of the field.

Read More: Derivation of Lorentz Transformation

Key Terms: Electric charge, Electric Current, Magnetic Field, Electromagnetic Induction, Earth’s Magnetic Field, Magnetic Field Lines

---

What is a Magnetic Field?

[Click Here for Previous Year Questions]

Magnetic field is a field created by moving electric charges.

• Magnetic field can be defined as the area around a magnet wherein the effect of magnetism is felt.
• It is a force field that exerts a force on materials such as iron when they are placed in its vicinity.
• Magnetic fields do not require a medium to propagate; they can even propagate in a vacuum.
• The magnetic field has a bigger energy storage capacity than the electric field, which distinguishes it from the electric field and allows it to be used in every electromechanical device such as transformers, motors, and generators. 

Magnetic Field

• A magnetic field is a vector field typically found in the neighbourhood of a magnet, electric current, or changing electric field wherein the magnetic forces are detectable.
• A magnetic field can be generated by moving electric charges and intrinsic magnetic moments of elementary particles that are linked with a fundamental quantum property, otherwise called “spin”.
• Magnetic field and electric field are generally two interrelated terms and are basically the components of electromagnetic force.

---

History of a Magnetic Field

[Click Here for Sample Questions]

The history of Magnetic field is shown below:

• The magnetic field was first studied in 1269 when French scholar Petrus Peregrinus de Maricourt used iron needles to map the magnetic field on the surface of a spherical magnet.
• He saw that the resulting field lines intersected at two spots.
• These points were dubbed "poles" by him. Following this finding, he concluded that no matter how finely one cuts a magnet, it always has a North and South pole.
• William Gilbert asserted three centuries later that the Earth is a magnet.
• Magnetic poles attract and repel each other, according to John Mitchell, an English priest, and philosopher, in 1750.
• Charles-Augustin de Coulomb experimentally verified the Earth's magnetic field in 1785.
• Following this, in the nineteenth century, French mathematician and geometer Simeon Denis Poisson developed the first magnetic field model, which he published in 1824.
• By the nineteenth century, discoveries had refined and challenged previously accepted beliefs.
• Hans Christian Orsted, a Danish physicist and chemist, discovered in 1819 that an electric current generates a magnetic field around it.
• In 1825, André-Marie Ampère suggested a model of magnetism in which the force was caused by continually flowing current loops rather than magnetic charge dipoles.
• Faraday, an English scientist, demonstrated in 1831 that a changing magnetic field produced an electric field. He had discovered electromagnetic induction.
• James Clerk Maxwell published theories on electricity and magnetism between 1861 and 1865. Maxwell's equation was the name given to this equation. These equations detailed the interaction of electricity and magnetism.

Read More: Torque Current Loop

---

Illustration of Magnetic Field

[Click Here for Previous Year Questions]

Typically, a magnetic field can be presented in two ways.

• Magnetic Field Vector
• Magnetic Field Lines

Magnetic Field Vector

The magnetic field is mathematically described as a vector field. The magnetic field is thought to have both magnitude and direction. The vector field can be represented as a grid of vectors. Each vector's direction points in the direction of the compass. The length of the vector is determined by the strength of the magnetic pull.

Magnetic Field Vector

Magnetic Field Lines

Magnetic field lines are fictitious lines that encircle a magnet. The density of a field's line indicates its magnitude. The magnetic field is strongest around the South and North Poles of a magnet and weakens as it goes away from the poles.

---

Properties of Magnetic Field Lines 

[Click Here for Sample Questions]

Some of the important properties of Magnet Field Lines include:

• Magnetic field lines never cross each other.
• It follows the path with the least resistance between the opposing magnetic poles. A bar magnet's magnetic lines of force travel in closed loops from one pole to the other.
• Magnetic field lines will have the same length.
• The density of the field lines decreases as they pass from the higher permeability region to the lower permeability region.
• Lines travel from the south pole to the north pole within a material magnetic field, while in the air, they flow from the north pole to the south pole.
• The density of the magnetic field varies with distance from the pole. Their density reduces as one moves away from the pole.
• Because it has both magnitude and direction, the magnetic field is a vector quantity.

Read More:

---

How to Draw Magnetic Field Lines?

[Click Here for Previous Year Questions]

Magnetic field lines can be drawn with a compass, a bar magnet, and a piece of chart paper.

• To begin, place the paper on a drawing board. Put the bar magnet in the centre and mark it with a pencil.
• Keep the compass close to one of the magnet's poles. Make sure that there is no other magnetic material in the vicinity.
• The compass arrow can be seen pointing in many directions. Make a mark in that direction with a dot.
• Move the compass away from that spot and place it on the dot so that the base of the arrow is at the dot.
• Make a new dot in the direction the compass's arrow is now pointing.
• Repeat this method until the compass hits the opposite pole of the magnet. Connect the dots. Return to the previous position and repeat the procedures starting from a new location.
• After drawing several lines, it is clear that the lines are forming a closed loop that appears to begin at one pole of the magnet and terminate at another. This is how magnetic field lines are drawn.
• When these lines are compared to the alignment of iron flakes, the patterns are strikingly comparable. Magnetic field lines will change depending on the type of magnets used.

---

How is Magnetic Field Produced?

[Click Here for Sample Questions]

A magnetic field can be produced not just by a magnet, but also by a moving charge or electric currents. We are all aware that matter is made up of small bits known as atoms. An atom's nucleus is made up of protons and neutrons, with electrons orbiting around it.

Production of Magnetic Field 

The magnetic field is created by the spinning and circling of protons and neutrons or the nucleus of an atom. The magnetic field's direction is determined by the orbit and spin directions. The magnetic field is represented mathematically by the letter 'B.' Tesla is the name of its unit (T).

---

Magnetic Field Intensity

[Click Here for Previous Year Questions]

Magnetic field strength can also be defined as magnetic field intensity or magnetic intensity.

• Magnetic field intensity can be denoted by vector H.
• Magnetic field intensity can be defined as the ratio of the MMF required to form a certain Flux Density (B) within a specific material per unit length of the given material.
• Magnetic field intensity can be measured in units of amperes/metre.

The Magnetic Field Intensity formula can be represented as:

Here,

• B = magnetic flux density
• M = magnetization
• μ = magnetic permeability

Tesla is the SI unit of magnetic field intensity. One tesla (1 T) can be expressed as the field intensity producing one newton of force per ampere of current per metre of conductor.

How does a Magnetic Field Originate?

The magnetic field can develop when a charge is in motion. There are two methods in order to arrange a charge to be in motion, further generating a useful magnetic field.

Magnetic Lines

Magnetic Field by a Current-Carrying Conductor

Ampere implied that a magnetic field can be generated whenever an electrical charge is in motion.

• Assume a wire via which current passes by connecting it to a battery.
• As the current passing via the conductor increases, the magnetic field proportionally increases as well.
• Moving further away from the wire, the magnetic field decreases with distance.
• According to Ampere’s law, the equation yields the magnetic field at a distance r from a long current-carrying conductor I. Thus, \(B=\frac{\mu _{0}I}{2\pi r}\) (here, µ₀ = permeability of free space, µ₀ = 4π × 10^-7 T⋅m/A).
• Materials that have higher permeability are known to have the capacity to concentrate on magnetic fields.
• Since it is a vector quantity, the magnetic field is known to have a direction. For conventional current to pass via a straight wire, it can be determined by the right-hand rule. 

Motion of Electrons around Nuclei of Atoms

Permanent magnets function on the basis of the motion of electrons around the nuclei. Only some materials can be made into magnets and some are much stronger than others. In order to attain this state, some specific conditions should be met:

• Atoms possess many electrons and they are paired in such a way that the overall magnetic field negates.
• Two electrons that are paired this way, are said to have opposite spins.
• So, if the material has 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.
• Billions of atoms can be found in a tiny piece of material. The overall field nullifies, regardless of how many unpaired electrons the material possesses if they are orientated randomly.
• The material must be stable enough at room temperature so that it can allow an overall preferred orientation to get established.
• When established permanently, we have a permanent magnet, also called a ferromagnet.
• There are some materials that become well-ordered in order to be magnetic in the presence of an external magnetic field.
• The external field lines all the electron spins up, but once the external field is removed, the alignment vanishes. These types of materials are paramagnetic.

Read More: Synchrotron

---

Earth’s Magnetic Field

[Click Here for Sample Questions]

Sir William Gilbert demonstrated the earth's magnetic field for the first time in 1600. Based on his tests, he discovered that the earth had magnetic properties and a magnetic field. If a magnet is suspended freely from a thread and allowed to revolve in a horizontal plane, it will automatically align and come to rest in the north-south direction.

Hypothesis for the source of Earth’s Magnetic Field

Earth’s Magnetic Field hypothesis can be expressed by:

• The Earth's core is a hot molten liquid that contains ions. These ions circulate inside the liquid in the form of current loops, producing a magnetic field.
• The Earth rotates on its axis, and stuff on the planet is made up of charged particles. These charged particles, in the form of current loops, also revolve about the Earth's axis and are responsible for the generation of the magnetic field.
• Ionized gases make up the Earth's outer layer. When the earth rotates, the movement of ions generates an electric current, which results in the formation of a magnetic field.

---

Previous Year Questions

1. A part of a long wire carrying a current i is bent into a circle of radius … [JCECE 2010]
2. The magnetic field intensity at the centre of cubical cage … [WBJEE 2016]
3. A proton is fired through a magnetic field in a direction opposite … [JIPMER 1996]
4. The magnetic fields at two points on the axis of a circular coil … [KCET 2001]
5. A bar magnet placed in a uniform magnetic field making an angle … [NEET 2019]
6. A thin flexible wire of length L is connected to two adjacent fixed points … [KCET 2010]
7. The magnetic field at a distance r from a long wire carrying … [VITEEE 2019]
8. A 100 millihenry coil carries a current of 1 A … [NEET 1991]
9. A proton with energy of 2 MeV enters a uniform … [VITEEE 2007]
10. An electron moves in a circular path of radius … [DUET 2013]
11. A particle enters uniform constant magnetic field … [UPSEE 2016]
12. In a potentiometer experiment, the balancing length of a cell is … [VITEEE 2015]
13. Two sources of equal emf are connected … [VITEEE 2015]
14. A conducting circular loop of radius r carries … [JEE Advanced 2017]
15. Two solenoids are given - 1st has 1 turn per unit … [VITEEE 2016]

PDF on Moving Charges and Magnetism Important Questions:

---

Things to Remember

• The magnetic field is a force field that exerts a force on materials such as iron when they are placed in its vicinity. 
• Magnetic fields do not require a medium to propagate; they can even propagate in a vacuum.
• Magnetic fields such as that of Earth enable magnetic compass and other permanent magnets to get aligned in the direction of the field.
• A magnetic field is stationary around a permanent magnet or a wire carrying a steady electric current in one direction and is called a magnetostatic field.
• Its magnitude and direction remain the same at any given point.
• Whereas, the magnetic field is constantly changing its magnitude and direction around an alternating current or a fluctuating direct current. 

Also Read: