Electromagnetism

Electromagnetism is the theory of electricity, magnetism and light that was developed in the 19th century by Michael Faraday and put into mathematical language by James Clerk Maxwell. 

Electricity - Electricity is the presence of either a positive or negative electric charge. This produces an electric field. When the electric charges move, they produce an electric current, which produces a magnetic field. Electric fields are produced by electric charges at rest.

Magnetism - A magnetic field will arise from the presence of an electric current and the magnetic moments (which is a quantity that determines the torque, or, rotational force, that it will experience in a magnetic field) of elementary particles. Magnetic fields are produced by electric currents or electric charges in motion. Any device that produces a magnetic field is known as a magnet. 

James Clerk Maxwell, in 1873, published his: "Treatise on Electricity and Magnetism". This was a two volume treatise on electromagnetism and it described the interactions of positive and negative charges. This work included various ideas and implications about the nature of this force that were all verified experimentally.

Maxwell's equations:

• Gauss' law for electricity: Unlike electric charges attract and like electrical charges repel. This is the relationship between a static electric field and the electric charges that produced the field. The static electric field points away from positive charges and towards the negative charges. An electric flux, is the measure of the flow of an electric field in a given area. According to Gauss' law, the electric flux that leaves a volume is proportional to the electric charge on the inside of it. This law is used to calculate electric fields around charged objects.

• Gauss' law for magnetism: This law describes sources of magnetism. It also shows how magnetic fields will be closed loops. Magnetic poles, attract and repel, in a manner similar to that of electric charges, as described in Gauss' law. For every south magnetic pole, there must be a corresponding north magnetic pole, and vice-versa. Magnetism is not caused by "magnetic charges", however, by the presence of a magnetic dipole. They resemble positive and negative "charges", however, are bound together and must include each other. According to this law, the magnetic flux moving inward toward the south pole will equal the flux moving outward from the north pole. The net flux is always zero. A magnetic monopole, is a hypothetical magnet with only one pole. These are not allowed by Gauss' law for magnetism. There will also be zero magnetic flux through a closed surface.

• Faraday's law (of induction): Michael Faraday, is typically credited with the discovery of induction in 1831. Induction is the phenomenon where a magnetic field, will produce an electric current. Faraday discovered that varying magnetic fields could create electric fields. Moving magnets near a circuit would cause electricity to flow through that circuit! However, sadly, Michael Faraday lacked a formal education and could not put his ideas into the language of mathematics. Thus, Maxwell will create a mathematical summary of Faraday's work. This, will be known as the Maxwell-Faraday law, as it is a generalization of Faraday's law. A magnetic field, will accompany an electric field, and vice versa. An example of this relationship would be: a moving bar magnet, produces a magnetic field, which can thus, produce an electric field at a wire. The induced voltage will be proportional to the magnetic flux that produced it. Changing magnetic fields can produce an electric field.

• Ampere's circuital law (with Maxwell's addition): This is the law that is used to calculate magnetic fields. The discovery that electric currents produce magnetic fields was made by Hans Christian Orsted in 1820 who was experimenting with a compass and some electric currents. 3 months after this discovery, Orsted published his ideas. Prior to this discovery, it was believed that electricity and magnetism were completely separate forces. This was a revelation. Ampere continued to pursue this idea and formulated his law: that electric currents generate magnetic fields. Ampere demonstrated that two wires with electric currents can attract or repel, like magnets do. James Clerk Maxwell will contribute to this work even further. This magnetic field would be proportional to the electric current and to the displacement current. The displacement current is the rate at which the electric field changes. Maxwell's idea was that there are two ways that a magnetic field can be produced:

1) By an electric current.

2) Changing electric fields.

The implications of this proposal was that, changing magnetic fields, can produce electric fields, and vice-versa. This led to his proposal that there could be propagating electromagnetic waves or electromagnetic radiation, as it will become known. If that wasn't enough, when he calculated the speed of the electromagnetic wave, it was, exactly the speed of light. This led to the proposal, which is now known to be true, that visible light, is a portion of the spectrum of electromagnetic radiation. 

James Clerk Maxwell, had not only unified electricity and magnetism, however, also, electromagnetism with optics. This was the second great unification in physics, since Isaac Newton's proposal that the laws that keep the Moon in orbit around the Earth are the same laws that cause an apple to fall to the ground.

Throughout the 19th century, scientists began to discover types of electromagnetic radiation beyond visible light. What was uncovered were radio waves, infrared waves, ultraviolet waves, X-rays and gamma rays. A new understanding of radiation itself was reached!

In 1800, William Herschel, began to suspect, via experiment, that different colors may have different temperatures associated with them. Herschel had been moving a thermometer through light split by a prism. What he noticed was that temperatures increased noticeably at the red end of the spectrum compared with the blue. He decided the apparently empty region where the spectrum's red light fades into invisibility. This turned out to be the hottest region of all. The highest temperature was beyond red. Herschel had discovered infrared radiation. He called this radiation that could not be seen "calorific rays."

In 1801, Johann Wilhelm Ritter, investigated how different colors of light effected silver salts. He was working on the other end of the spectrum. What he found was that they darkened more rapidly in blue light than in red. Ritter tested the reaction of the salts beyond the violet end of the spectrum. He found it was stronger still. What he found he called, "chemical rays" and they were later re-named "ultraviolet radiation."

The physics of Herschel and Ritter's day had little to say about radiations invisible to the human eye. These results will not be put into their proper light until the work of James Clerk Maxwell. Maxwell's discovery linked electric and magnetic effects into a single phenomenon of electromagnetism. Maxwell described light as a "transverse wave." This wave had interlinked electric and magnetic elements. These waves traveled at the speed of light. This was the first indication that there was an entire spectrum of electromagnetic radiation.

• Radio waves (1 mm - 100 km): The longest of the radio waves moving toward the Earth are absorbed in the ionosphere. Medium sized radio waves may hit the surface of the Earth. The smaller radio waves may be absorbed by carbon dioxide in the atmosphere and water vapor.

• Microwaves (1 mm - 1 m): Microwaves are used for communication and in microwave ovens. 

• Infrared (750 nm - 1 mm): Infrared rays are emitted by warm objects. These are also mostly absorbed in the earth's atmosphere by carbon dioxide and water vapor.

• Optical window (390 nm - 750 nm): Animal eyesight has evolved to be able to detect this portion of the spectrum of electromagnetic radiation. 

• Ultraviolet (10 nm - 400 nm): Most of the UV rays from the Sun are stopped by the earth's O-zone layer.

• X-rays (.01 nm - 10 nm): X-rays can carry large amounts of harmful energy. The Earth's atmosphere is used to protect us from them.

• Gamma rays (less than .02 nm): Even shorter than the X-rays are the gamma rays that have an even greater potential to cause damage. Luckily, our Earth has a protective atmosphere.