Introduction to electromagnetism

Hans Christian Ørsted discovered that the two were related – electric currents give rise to magnetism.

Michael Faraday discovered the converse, that magnetism could induce electric currents, and James Clerk Maxwell put the whole thing together in a unified theory of electromagnetism.

Maxwell's equations further indicated that electromagnetic waves existed, and the experiments of Heinrich Hertz confirmed this, making radio possible.

In many situations of interest to electrical engineering, it is not necessary to apply quantum theory to get correct results.

Classical physics is still an accurate approximation in most situations involving macroscopic objects.

With few exceptions, quantum theory is only necessary at the atomic scale and a simpler classical treatment can be applied.

Electrostatics deals only with stationary electric charges so magnetic fields do not arise and are not considered.

Circuit theory deals with electrical networks where the fields are largely confined around current carrying conductors.

Quantum considerations are also necessary to explain the behaviour of many electronic devices, for instance the tunnel diode.

It is given by the formula where F is the force, ke is the Coulomb constant, q1 and q2 are the magnitudes of the two charges, and r2 is the square of the distance between them.

[2] In physics, fields are entities that interact with matter and can be described mathematically by assigning a value to each point in space and time.

If the wire is straight, then the magnetic field is curled around it like the gripped fingers in the right-hand rule.

[12] Similarly, Faraday's law of induction states that a magnetic field can produce an electric current.

Together, Maxwell's equations provide a single uniform theory of the electric and magnetic fields and Maxwell's work in creating this theory has been called "the second great unification in physics" after the first great unification of Newton's law of universal gravitation.

[17] The solution to Maxwell's equations in free space (where there are no charges or currents) produces wave equations corresponding to electromagnetic waves (with both electric and magnetic components) travelling at the speed of light.

[18] The observation that these wave solutions had a wave speed exactly equal to the speed of light led Maxwell to hypothesise that light is a form of electromagnetic radiation and to posit that other electromagnetic radiation could exist with different wavelengths.

The full electromagnetic spectrum (in order of increasing frequency) consists of radio waves, microwaves, infrared radiation, visible light, ultraviolet light, X-rays and gamma rays.

[27] Silicon is an example of a semiconductors that can be used to create solar cells which become more conductive the more energy they receive from photons from the sun.

[28] Superconductors are materials that exhibit little to no resistance to the flow of electrons when cooled below a certain critical temperature.

[29] Insulators are material which are highly resistive to the flow of electrons and so are often used to cover conducting wires for safety.

[30] However, some insulators, called dielectrics, can be polarised under the influence of an external electric field so that the charges are minutely displaced forming dipoles that create a positive and negative side.

The ability of the capacitor to store electrical potential energy is measured by the capacitance which is defined as

[32] The maximum energy that can be stored by a capacitor is proportional to the capacitance and the square of the potential difference between the plates[33]

An inductor is an electronic component that stores energy in a magnetic field inside a coil of wire.

A current-carrying coil of wire induces a magnetic field according to Ampère's circuital law.

Kirchhoff's loop rule (below): Circuit theory deals with electrical networks where the fields are largely confined around current carrying conductors.

This comes from the fact that the electric field is conservative which means that no matter the path taken, the potential at a point does not change when you get back there.

[37] Rules can also tell us how to add up quantities such as the current and voltage in series and parallel circuits.

[37] For series circuits, the current remains the same for each component and the voltages and resistances add up:

For parallel circuits, the voltage remains the same for each component and the currents and resistances are related as shown: