Electromagnetic radiation, an introduction

The energy and heat that reaches the Earth ultimately comes from the Sun, with vastly smaller amounts from other astrophysical sources (i.e., Pulsars and Quasars, and radio waves emanating from Jupiter).

The energy and heat released from the Sun comes in the form of electromagnetic (or EM) radiation, propagating outwards in all directions, and from the more sporadic solar winds.

An Overview

Electromagnetic (EM) energy, is a form of radiation, as it travels out as rays in straight lines, and is ‘electro’ ‘magnetic’ - a combination of both electric and magnetic - as it distinctly consists of two harmonic waves oscillating (‘swinging’ back and forth) perpendicular (90º) to each other - electric-waves, and magnetic-waves.

All EM waves travel at the fastest known speed in the universe - the universal speed limit: (c) 299,792,458 meters per second - known as the speed of light, as light also happens to travel at this speed limit.

The energy within EM radiation is held and transported equally in the perpendicular electric and magnetic fields propagating out into space.

This energy is measured in its irradiance or intensity. The irradiance energy from the Sun that reaches the top of Earth’s atmosphere is about 1350 Watts/m2, known as the solar constant. (Around half of this energy actually reaches the Earth’s surface).

EM energy also transports momentum (although it has no mass), which means that it is also able to exert a very tiny mechanical force on other objects: known as radiation pressure. It is this radiation pressure from photons escaping the Sun’s core, combined with thermal pressure that balances the compressive pressures from Gravity; and could also be harnessed to move a solar sail in space. This effect is virtually non-existent on Earth, however, so is not discussed any further in the main text below.

EM radiation can be understood as a wave of propagating electric and magnetic fields, and also as distinct massless energy ‘packets’ (known as photons).

EM radiation, therefore, is also understood to be particles as well as waves: known as the wave-particle duality. The energy in this packets (photons), for visible light, is very small: around 4 x 10-9 Joules - or between 1.8 electron volts (eV) for red light to 3 eV for blue light. It is important to note here that all atomic particles, such as electrons, have particle-like and wave-like aspects to their natures - not just EM radiation.

To understand this range of interrelated phenomena in more depth it is easier to first look at some of the basic concepts: electrostatics, wave-particle duality, and then magnetism; to understand them first as separate phenomena, before putting them all back together again, as part of a integrated emergent phenomena, as they are now recognised to be.

Electric Charge

There are three types of electric charge: positive, negative, and neutral. Protons have a positive charge, electrons have a negative charge, and neutrons have a neutral charge - so no charge.

Atoms and molecules that have an equal numbers of protons and electrons also have a neutral charge. Those atoms or molecules that have more electrons than protons have a net negative charge (and are known as anions), and those atoms or molecules that have less electrons than protons have a net positive charge (and are known as cations); both negatively or positively charged atoms or molecules are collectively known as ions.

Particles with the same charge repel each other, opposite charges attract, and neutral charges have no repulsion or attraction.

Even though electrons have a much smaller mass than a proton, they have the same level of charge. A proton has a charge of +e, and a electron has a charge of -e. And all electric charge is quantised - it comes in discreet units - small distinct packets of energy (meaning that half charges are not possible).

When a particle enters into the electric field (which is explained below) of a charged particle, it experiences a force - either attracting it or repelling it. This is the physical property of an electric charge. Electric charge is measured in coulomb (C) in engineering, and as elementary charge (e) in chemistry. The electric charge of a particle reduces across a distance - just like gravity; and can be calculated using Coulomb’s law: the force is proportional to the product (sum) of the charges, and inversely proportional to the square of the distance between them.

Coulombs law also shows that the electric force increases as the charge increase, and it can also be used (by chemists) to predict ionisation energy: the energy required to create an ion; in other words, the amount of energy needed for an atom or molecule, to loose an electron. As Coulomb’s Law states, that the force reduces over distance, this law shows how electrons close to the nucleus need a lot of energy to separate them from the nucleus (and those further away, less energy), and that the more protons in the nucleus (the greater the positive charge), the greater the energy required to pull electrons away from the nucleus.

Electric Current

When an electric charge is in motion it is known as electric current, with the intensity  (amplitude) usually measured in amperes.

Examples of electric current in nature can be observed in lighting, static electric discharge (i.e., by rubbing a ballon on ones head), and the solar winds effect on the Earth’s Ionosphere (creating the polar auroras).

Current can consist of any moving charged particle, such as an electron flowing through a conductive material (such as a metal), or a photon or electron through the vacuum of space, or ions (charged atoms) flowing through an electrolyte.

Creating a electric current creates a magnetic field.

This is known as Ampère's law. To the right, the image basically shows a green cable with a current (I) moving from a positive charge to a negative charge, with the generated circular magnetic field (B) produced around it, perpendicular to the current.

The relationship between electrostatics with magnetism in electric current, showed scientists that they are related phenomena, and more recently, that they are part of the same phenomena.

More about magnetism is discussed below, and in the page about the magnetosphere, however, it can be said here, that the early understanding of the relationship led to the invention of electromagnets - a technology that is used to run motors, generators, loudspeakers, and MRI machines for instance.

Electric Fields

All positively or negatively charged particles produce, and are influenced by electric fields.

An electric field is a measurable effect generated by any charged particle.

An electric field can be illustrated as a vector field (these do not really exist - it is a form of visualisation): that is to a say - a field that permeates out into space at all times, like spokes in a bicycle wheel, in all directions from the centre of a particle, to an infinite distance; and, like gravity, the  effect gets weaker the further the distance is from the particle centre.

The orange ‘equipotential lines’ represent this reduction in measurable effect over distance (closer circles have a greater potential force than circles spaced further apart).

An electric field can be calculated (and visualised) by placing a positive point test charge (which is a very small positive charge) in various places around the charged particle, which shows its’ electric field direction and magnitude.

The arrows indicate weather the electric field is repelling or attracting - outwards for positively charged particles, and inwards for negatively charged particles. For instance, when a positively charged particle interacts with a very small positive point test charge it will repel it (therefore outward pointing arrows), and when a negatively charged particle interacts with a very small positive point test charge it will attract it (therefore inward pointing arrows).

These vector arrows should not to be confused with an electric force, however, as a charged particle can only cause an electric force on (and with) another charged particle; a particle on it’s own in space has an electric field (and the vector arrows showing how it will effect - and be effected by - a positive test charge), but it does not create a force by itself.

Positively charged field lines, never cross, and always travel from positive to negative charge. As can be seen in the third image scenario, a positive charge (in this case a proton) has been attracted to a nearby negative charge (in this case an electron), and the field lines that were previously going straight outwards or inwards like wheel spokes have come and bowed and joined together; and the equipotential lines have been ‘squeezed’ in the middle and stretched on the outside. This third image becomes an electric field of a electric dipole. The fourth image shows the electric field lines for two repelling protons with the same positive charge, which, if there are no negatively charged objects around, will flow outwards for infinity, ‘in search’ for a negative charge.

The electric field combines with the magnetic field to form the electromagnetic field (this will be explained at the end of this section).

Magnetism - A Short Overview

So far, it has been illustrated that EM radiation is a propagating electric field that travels like a wave and a particle through the vacuum of space at the speed of light. However, at the beginning of this section, it was also highlighted that there are actually two waves propagating through space, together - and this second wave, not yet discussed, is the magnetic field. Magnetism is covered in more depth in the next webpage on the magnetosphere, so this section on magnetism will be brief.

Magnetic Fields

Magnets, instead of the ‘positive’ and ‘negative’ charges (monopoles) for particles, magnets have north and south dipoles. However, just like charges there is a repulsion between similar poles (like charges) and attraction between opposite poles (opposite charges). And this force also reduces over distance, and can also be calculated using Coulombs Law. Compasses can be used like the ‘test charges’ in electric fields, to understand magnetic fields, which again, look very similar (see image to the right).

The main difference in the two phenomena is that a proton (positive charge) an electron (negative charge), for example are individual elements - they are monopoles, however, it is impossible to split a magnetic into one individual south and north monopoles - and so magnets are always dipoles.

The first clear proof that both electrostatics and magnetics where part of the same phenomena was when scientists (such as Hans Christian Ørsted) found that by moving charge (making a current), a magnet would be deflected. And so: a moving electric field (a current) generates a magnetic field.

Therefore, a ring of compasses placed around a wire with a current flowing through it, made the magnets collectively point in a circle around the wire (see image in electric current section): In other words the magnetic field is perpendicular to the current, and therefore the electric field. The greater the charge, the greater the magnitude of the magnetic field, and the direction of the current determines the direction of the magnetic field.

When an electric field is moving a magnetic field is created (discussed above), and the scientist Michael Faraday proved that the opposite was also true: a moving magnetic field can generate a charge. This is called electromagnetic induction, and will be discussed more in the next page on magnetism.

Bringing it all Together

Therefore, it is currently understood that the energy that is released from the fusion reactions in the Sun, and is transported, in straight lines across space, is a combination of oscillating electric fields and magnetic fields, perpendicular to each other, in discreet ‘ packets (protons); and in a spectrum of wavelengths (discussed in the next section).

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