Electromagnetic theory is the branch of physics that studies the interaction between electric and magnetic fields. It explains how electric charges and currents produce electric and magnetic forces, and how these forces affect other charges and currents. It also describes how electromagnetic waves, such as light, radio waves, microwaves, and X-rays, are generated and propagate through space.

Electromagnetic theory is based on four fundamental equations, known as Maxwell’s equations, that relate the electric and magnetic fields to their sources and to each other. Maxwell’s equations unify the previously separate laws of electricity and magnetism, such as Coulomb’s law, Gauss’s law, Faraday’s law, and Ampere’s law. They also reveal the electromagnetic nature of light and other forms of radiation.

In this article, we will explore the basic concepts and principles of electromagnetic theory, such as electric charge, electric field, magnetic field, electric flux, magnetic flux, electric potential, current density, Lorentz force, electromagnetic induction, electromagnetic waves, polarization, reflection, refraction, interference, diffraction, and more. We will also learn how to apply electromagnetic theory to various phenomena and applications, such as electrostatics, magnetostatics, electrodynamics, circuits, antennas, transmission lines, waveguides, optical fibers, lasers, LEDs, solar cells, MRI scanners, and more.

## What is Electric Charge?

Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric field. There are two types of electric charges: positive and negative. Like charges repel each other, and opposite charges attract each other. The unit of electric charge is the coulomb ©.

Electric charge is conserved in any physical process. This means that the total amount of electric charge in a closed system remains constant. Electric charge can be transferred from one object to another by physical contact or by induction.

## What is Electric Field?

An electric field is a region of space where an electric charge experiences a force. The direction of the electric field at any point is the direction of the force that a positive test charge would feel if placed at that point. The magnitude of the electric field at any point is the ratio of the force on a test charge to the magnitude of the test charge. The unit of an electric field is newton per coulomb (N/C) or volt per meter (V/m).

An electric field can be produced by stationary or moving charges. The electric field due to a point charge is given by Coulomb’s law:

where E is the electric field magnitude at a distance r from the point charge Q, and k is a constant equal to 8.99×109 Nm$^{2$/C$}2$. The direction of the electric field is radial from the point charge.

The electric field due to a collection of charges can be obtained by vectorially adding the electric fields due to each individual charge.

The electric field can also be expressed in terms of the electric potential V, which is the work done per unit charge to move a test charge from infinity to a point in space. The relation between the electric field and the electric potential is:

where ∇ is the gradient operator that gives the direction of maximum change of a scalar function.

## What is Magnetic Field?

A magnetic field is a region of space where a moving electric charge or a magnetic material experiences a force. The direction of the magnetic field at any point is given by the right-hand rule: if you point your thumb in the direction of the current or the north pole of a magnet, your fingers will curl in the direction of the magnetic field. The magnitude of the magnetic field at any point is measured by the force on a unit north pole placed at that point. The unit of the magnetic field is the tesla (T) or newton per ampere-meter (N/A$\cdot$m).

A magnetic field can be produced by moving charges or by permanent magnets. The magnetic field due to a long straight current-carrying wire is given by Ampere’s law:

where B is the magnetic field magnitude at a distance r from the wire carrying current I, and μ0 is a constant equal to 4π×10−7 N/A$^2$. The direction of the magnetic field is tangential to a circle centered on the wire.

The magnetic field due to a permanent magnet is given by the magnetic dipole model:

where B is the magnetic field magnitude at a distance r from the magnetic dipole moment m, and × denotes the cross product. The direction of the magnetic field is perpendicular to both m and r.

The magnetic field due to a collection of magnets or currents can be obtained by vectorially adding the magnetic fields due to each individual source.

The magnetic field can also be expressed in terms of the magnetic vector potential A, which is a vector function that satisfies:

where × denotes the curl operator that gives the rotation of a vector function.

## What is Electric Flux?

Electric flux is a measure of how much electric field passes through a given area. It is defined as the dot product of the electric field and the area vector:

where ΦE is the electric flux, E is the electric field, and A is the area vector, which has a magnitude equal to the area and a direction perpendicular to the surface. The dot product means that only the component of the electric field parallel to the area vector contributes to the electric flux.

Electric flux can also be calculated by integrating the electric field over a closed surface:

where S is the closed surface, and dA is an infinitesimal area element. This integral is known as Gauss’s law for electricity, which states that the electric flux through any closed surface is proportional to the net charge enclosed by that surface:

where Q is the net charge inside the surface, and ϵ0 is a constant equal to 8.85×10−12 C$^{2$/Nm$}2$. Gauss’s law can be used to calculate the electric field due to symmetric charge distributions, such as spheres, cylinders, and planes.

## What is Magnetic Flux?

Magnetic flux is a measure of how much magnetic field passes through a given area. It is defined as the dot product of the magnetic field and the area vector:

where ΦB is the magnetic flux, B is the magnetic field, and A is the area vector, which has a magnitude equal to the area and a direction perpendicular to the surface. The dot product means that only the component of the magnetic field parallel to the area vector contributes to the magnetic flux.

Magnetic flux can also be calculated by integrating the magnetic field over a closed surface:

where S is the closed surface, and dA is an infinitesimal area element. This integral is known as Gauss’s law for magnetism, which states that the magnetic flux through any closed surface is zero:

This implies that there are no magnetic monopoles, or isolated magnetic charges, in nature. Instead, magnetic fields are always produced by dipoles or loops of current.

## What is Electromagnetic Induction?

Electromagnetic induction is the phenomenon of generating an electric current or an electric potential in a conductor due to a changing magnetic flux. It was discovered by Michael Faraday and Joseph Henry in 1831 independently.

The principle of electromagnetic induction is summarized by Faraday’s law of induction, which states that the induced electromotive force (EMF) in a loop of wire is equal to the negative rate of change of magnetic flux through the loop:

where E is the EMF, ΦB is the magnetic flux, and t is the time. The negative sign indicates that the induced EMF opposes the change in flux that causes it, according to Lenz’s law.

There are several ways to induce an EMF in a loop of wire, such as:

- Moving a magnet near the loop
- Moving the loop near a magnet
- Changing the current in another loop near the first loop
- Changing the shape or area of the loop in a magnetic field

The induced EMF can be increased by:

- Increasing the rate of change of flux
- Increasing the number of turns in the loop
- Increasing the strength of the magnetic field

The induced EMF can drive a current in a closed circuit, which can power various devices or perform work. Some examples of electromagnetic induction are:

- Generators: convert mechanical energy into electrical energy by rotating coils of wire in a magnetic field
- Transformers: transfer electrical energy from one circuit to another by varying the current in one coil, which induces an EMF in another coil
- Induction motors: convert electrical energy into mechanical energy by creating a rotating magnetic field that induces a torque on a rotor
- Induction cooktops: heat up metal pots by creating an oscillating magnetic field that induces eddy currents and Joule heating in them
- Wireless charging: charge devices without wires by creating an alternating magnetic field that induces an EMF and a current in a receiver coil
- the area and a direction perpendicular to the surface. The dot product means that only the component of the magnetic field parallel to the area vector contributes to the magnetic flux.

Magnetic flux can also be calculated by integrating the magnetic field over a closed surface:

where S is the closed surface, and dA is an infinitesimal area element. This integral is known as Gauss’s law for magnetism, which states that the magnetic flux through any closed surface is zero:

This implies that there are no magnetic monopoles, or isolated magnetic charges, in nature. Instead, magnetic fields are always produced by dipoles or loops of current.

## What is Electromagnetic Induction?

Electromagnetic induction is the phenomenon of generating an electric current or an electric potential in a conductor due to a changing magnetic flux. It was discovered by Michael Faraday and Joseph Henry in 1831 independently.

The principle of electromagnetic induction is summarized by Faraday’s law of induction, which states that the induced electromotive force (EMF) in a loop of wire is equal to the negative rate of change of magnetic flux through the loop:

where E is the EMF, ΦB is the magnetic flux, and t is the time. The negative sign indicates that the induced EMF opposes the change in flux that causes it, according to Lenz’s law.

There are several ways to induce an EMF in a loop of wire, such as:

- Moving a magnet near the loop
- Moving the loop near a magnet
- Changing the current in another loop near the first loop
- Changing the shape or area of the loop in a magnetic field

The induced EMF can be increased by:

- Increasing the rate of change of flux
- Increasing the number of turns in the loop
- Increasing the strength of the magnetic field

The induced EMF can drive a current in a closed circuit, which can power various devices or perform work. Some examples of electromagnetic induction are:

- Generators: convert mechanical energy into electrical energy by rotating coils of wire in a magnetic field
- Transformers: transfer electrical energy from one circuit to another by varying the current in one coil, which induces an EMF in another coil
- Induction motors: convert electrical energy into mechanical energy by creating a rotating magnetic field that induces a torque on a rotor
- Induction cooktops: heat up metal pots by creating an oscillating magnetic field that induces eddy currents and Joule heating in them
- Wireless charging: charge devices without wires by creating an alternating magnetic field that induces an EMF and a current in a receiver coil

## What are Electromagnetic Waves?

Electromagnetic waves are waves that consist of oscillating electric and magnetic fields. They are produced by accelerating electric charges or changing magnetic fields. They can travel through a vacuum as well as through matter. They carry energy and momentum and can exert pressure on objects.

Electromagnetic waves have different wavelengths and frequencies, which determine their properties and applications. The electromagnetic spectrum is the range of all possible wavelengths and frequencies of electromagnetic waves. It includes radio waves, microwaves, infrared waves, visible light, ultraviolet rays, X-rays, and gamma rays.

The wavelength λ and frequency f of an electromagnetic wave are related by:

where c is the speed of light in a vacuum, which is approximately 3×108 m/s. The energy E and momentum p of an electromagnetic wave is related to its frequency by:

where h is Planck’s constant, which is approximately 6.63×10−34 J$\cdot$s.

Electromagnetic waves interact with matter in different ways depending on their wavelength and frequency. Some examples are:

- Radio waves: used for communication, broadcasting, radar, and navigation
- Microwaves: used for cooking, heating, communication, and radar
- Infrared waves: used for thermal imaging, remote sensing, night vision, and communication
- Visible light: used for vision, photography, illumination, and optical instruments
- Ultraviolet rays: used for sterilization, disinfection, fluorescence, and tanning.

## Conclusion

Electromagnetic theory is a fascinating and important branch of physics that explains how electric and magnetic fields interact with each other and with matter. It is based on four fundamental equations, known as Maxwell’s equations, that describe the properties and behavior of electric and magnetic fields and their sources. The electromagnetic theory also reveals the existence and nature of electromagnetic waves, which are forms of energy that can travel through space and matter. Electromagnetic waves have different wavelengths and frequencies, which determine their characteristics and applications. The electromagnetic theory also covers the phenomenon of electromagnetic induction, which is the generation of electric current or potential in a conductor due to a changing magnetic flux. The electromagnetic theory has many practical implications and applications in various fields of science, engineering, and technology. It helps us understand and manipulate phenomena such as light, radiation, electricity, magnetism, communication, imaging, heating, and more.