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Band theory | physics 31/01/2020

https://www.britannica.com/science/band-theory Teori Pita Energi

Band theory | physics Band theory, in solid-state physics, theoretical model describing the states of electrons, in solid materials, that can have values of energy only within certain specific ranges. The behaviour of an electron in a solid (and hence its energy) is related to the behaviour of all other particles around

17/01/2020

Rutherford's alpha particle scattering experiment changed the way we think of atoms.

Before the experiment the best model of the atom was known as the Thomson or "plum pudding" model. The atom was believed to consist of a positive material "pudding" with negative "plums" distributed throughout.

Thomson or plum pudding model
Rutherford directed beams of alpha particles (which are the nuclei of helium atoms and hence positively charged) at thin gold foil to test this model and noted how the alpha particles scattered from the foil.

Rutherford Alpha Particle Scattering Experiment
Rutherford made 3 observations:

Most of the fast, highly charged alpha particles went whizzing straight through undeflected. This was the expected result for all of the particles if the plum pudding model was correct.
Some of the alpha particles were deflected back through large angles. This was not expected.
A very small number of alpha particles were deflected backwards! This was definitely not as expected. Rutherford later remarked "It was as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back at you!"
To explain these results a new model of the atom was needed.

orbiting negative electron
In this model the positive material is concentrated in a small but massive (lot of mass - not size) region called the nucleus. The negative particles (electrons) must be around the outside preventing one atom from trespassing on its neighbours space to complete this model. The diagram below will help you to understand the results of the experiment.

Try the experiment for yourself and note how the results can be explained using this new model:

Install Flash

17/01/2020

What Are the 4 Atomic Models?
The atom is the most basic unit of any element that still maintains the properties of that element. Because atoms are far too small to see, their structure has always been something of a mystery. For thousands of years, philosophers and scientists have proposed theories concerning the make-up of this mysterious particle, with increasing degrees of sophistication. Although there were many models, four main ones have led to our current concept of the atom.

The Plum Pudding Model
The so-called plum pudding model was proposed by the scientist J.J. Thomson in 1904. This model was conceived after Thomson's discovery of the electron as a discrete particle, but before it was understood that the atom had a central nucleus. In this model, the atom is a ball of positive charge -- the pudding -- in which the electrons -- the plums -- are located. The electrons rotate in defined circular paths within the positive blob that makes up the majority of the atom.

Planetary Model
This theory was proposed by the Nobel Prize winning chemist Ernest Rutherford in 1911 and is sometimes called the Rutherford model. Based on experiments that showed the atom appeared to contain a small core of positive charge, Rutherford postulated that the atom consisted of a small, dense and positively charged nucleus, around which electrons orbited in circular rings. This model was one of the first to propose the odd idea that atoms are mostly made up of empty space through which the electrons move.

Bohr Model
The Bohr model was devised by Neils Bohr, a physicist from Denmark who received the Nobel prize for his work on the atom. In some ways it is a more sophisticated enhancement of the Rutherford model. Bohr proposed, as did Rutherford, that the atom had a small, positive nucleus where most of its mass resided. He stated that the electrons orbited around this nucleus like planets around the sun. The main improvement of Bohr's model was that the electrons were confined to set orbits around the nucleus, each having a specific energy level, which explained experimental observations such as electromagnetic radiation.

Electron Cloud Model
The electron cloud model is currently the most sophisticated and widely accepted model of the atom. It retains the concept of the nucleus from Bohr and Rutherford's models, but introduces a different definition of the motion of electrons around the nucleus. The movement of electrons around the nucleus in this model is defined by regions where there is a greater probability of finding the electron at any given moment. These regions of probability around the nucleus are associated with specific energy levels and take on a variety of odd shapes as the energy of the electrons increase.



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17/01/2020

Atomic theory
From Wikipedia, the free encyclopedia

"Atomic model" redirects here. For the unrelated term in mathematical logic, see Atomic model (mathematical logic).
This article is about the historical models of the atom. For a history of the study of how atoms combine to form molecules, see History of molecular theory.

The current theoretical model of the atom involves a dense nucleus surrounded by a probabilistic "cloud" of electrons
In chemistry and physics, atomic theory is a scientific theory of the nature of matter, which states that matter is composed of discrete units called atoms. It began as a philosophical concept in ancient Greece and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of atoms.

The word atom comes from the Ancient Greek adjective atomos, meaning "indivisible".[1] 19th century chemists began using the term in connection with the growing number of irreducible chemical elements. Around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "uncuttable atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments, such as neutron stars, extreme temperature and pressure prevents atoms from existing at all.

Since atoms were found to be divisible, physicists later invented the term "elementary particles" to describe the "uncuttable", though not indestructible, parts of an atom. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.

Contents
1 History
1.1 Philosophical atomism
1.2 John Dalton
1.3 Avogadro
1.4 Brownian Motion
1.5 Discovery of subatomic particles
1.6 Discovery of the nucleus
1.7 First steps toward a quantum physical model of the atom
1.8 Discovery of isotopes
1.9 Discovery of nuclear particles
1.10 Quantum physical models of the atom
2 See also
3 Footnotes
3.1 Bibliography
4 Further reading
5 External links
History
Philosophical atomism
Main article: Atomism
The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India. The word "atom" (Greek: ἄτομος; atomos), meaning "uncuttable", was coined by the Pre-Socratic Greek philosophers Leucippus and his pupil Democritus (c. 460 – c. 370 BC).[2][3][4][5] Democritus taught that atoms were infinite in number, uncreated, and eternal, and that the qualities of an object result from the kind of atoms that compose it.[3][4][5] Democritus's atomism was refined and elaborated by the later Greek philosopher Epicurus (341 – 270 BC), and by the Roman Epicurean poet Lucretius (c. 99 – c. 55 BC).[4][5] During the Early Middle Ages, atomism was mostly forgotten in western Europe.[4] During the 12th century, atomism became known again in western Europe through references to it in the newly-rediscovered writings of Aristotle.[4]

In the 14th century, the rediscovery of major works describing atomist teachings, including Lucretius's De rerum natura and Diogenes Laërtius's Lives and Opinions of Eminent Philosophers, led to increased scholarly attention on the subject.[4] Nonetheless, because atomism was associated with the philosophy of Epicureanism, which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable by most European philosophers.[4] The French Catholic priest Pierre Gassendi (1592 – 1655) revived Epicurean atomism with modifications, arguing that atoms were created by God and, though extremely numerous, are not infinite.[4][5] Gassendi's modified theory of atoms was popularized in France by the physician François Bernier (1620 – 1688) and in England by the natural philosopher Walter Charleton (1619 – 1707).[4] The chemist Robert Boyle (1627 – 1691) and the physicist Isaac Newton (1642 – 1727) both defended atomism and, by the end of the 17th century, it had become accepted by portions of the scientific community.[4]

John Dalton
Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. The first was the law of conservation of mass, closely associated with the work of Antoine Lavoisier, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products).[6] The second was the law of definite proportions. First established by the French chemist Joseph Louis Proust in 1799,[7] this law states that if a compound is broken down into its constituent chemical elements, then the masses of the constituents will always have the same proportions by weight, regardless of the quantity or source of the original substance.

John Dalton studied and expanded upon this previous work and defended a new idea, later known as the law of multiple proportions: if the same two elements can be combined to form a number of different compounds, then the ratios of the masses of the two elements in their various compounds will be represented by small whole numbers. For example, Proust had studied tin oxides and found that there is one type of tin oxide that is 88.1% tin and 11.9% oxygen and another type that is 78.7% tin and 21.3% oxygen (these are tin(II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found that an atomic theory of matter could elegantly explain this law, as well as Proust's law of definite proportions. For example, in the case of Proust's tin oxides, one tin atom will combine with either one or two oxygen atoms to form either the first or the second oxide of tin.[8]

Dalton believed atomic theory could explain why water absorbed different gases in different proportions - for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen.[9] Dalton hypothesized this was due to the differences in mass and complexity of the gases' respective particles. Indeed, carbon dioxide molecules (CO2) are heavier and larger than nitrogen molecules (N2).

Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).
In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss there exactly how he obtained these figures.[9] The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of Thomson's textbook, A System of Chemistry. Finally, Dalton published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and 1810.

Dalton estimated the atomic weights according to the mass ratios in which they combined, with the hydrogen atom taken as unity. However, Dalton did not conceive that with some elements atoms exist in molecules—e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest compound between any two elements is always one atom of each (so he thought water was HO, not H2O).[10] This, in addition to the crudity of his equipment, flawed his results. For instance, in 1803 he believed that oxygen atoms were 5.5 times heavier than hydrogen atoms, because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water was HO. Adopting better data, in 1806 he concluded that the atomic weight of oxygen must actually be 7 rather than 5.5, and he retained this weight for the rest of his life. Others at this time had already concluded that the oxygen atom must weigh 8 relative to hydrogen equals 1, if one assumes Dalton's formula for the water molecule (HO), or 16 if one assumes the modern water formula (H2O).[11]

Avogadro
The flaw in Dalton's theory was corrected in principle in 1811 by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect the volume that it occupies).[12] Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance: since two liters of hydrogen will react with just one liter of oxygen to produce two liters of water v***r (at constant pressure and temperature), it meant a single oxygen molecule splits in two in order to form two particles of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and made a clear distinction between molecules and atoms.

Brownian Motion
In 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it.[13] This model was validated experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory).

Discovery of subatomic particles
Main articles: Electron and Plum pudding model

The cathode rays (blue) were emitted from the cathode, sharpened to a beam by the slits, then deflected as they passed between the two electrified plates.
Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays.[14]

A Crookes tube is a sealed glass container in which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being a form of light, were composed of very light negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists). He measured the mass-to-charge ratio and discovered it was 1800 times smaller than that of hydrogen, the smallest atom. These corpuscles were a particle unlike any other previously known.

Thomson suggested that atoms were divisible, and that the corpuscles were their building blocks.[15] To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model[16] as the electrons were embedded in the positive charge like raisins in a plum pudding (although in Thomson's model they were not stationary).

Discovery of the nucleus
Main article: Rutherford model

The Geiger-Marsden experiment
Left: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.
Right: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.
Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center.

In the Geiger–Marsden experiment, Hans Geiger and Ernest Marsden (colleagues of Rutherford working at his behest) shot alpha particles at thin sheets of metal and measured their deflection through the use of a fluorescent screen.[17] Given the very small mass of the electrons, the high momentum of the alpha particles, and the low concentration of the positive charge of the plum pudding model, the experimenters expected all the alpha particles to pass through the metal foil without significant deflection. To their astonishment, a small fraction of the alpha particles experienced heavy deflection. Rutherford concluded that the positive charge of the atom must be concentrated in a very tiny volume to produce an electric field sufficiently intense to deflect the alpha particles so strongly.

This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge. Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection.[18]

First steps toward a quantum physical model of the atom
Main article: Bohr model
The planetary model of the atom had two significant shortcomings. The first is that, unlike planets orbiting a sun, electrons are charged particles. An accelerating electric charge is known to emit electromagnetic waves according to the Larmor formula in classical electromagnetism. An orbiting charge should steadily lose energy and spiral toward the nucleus, colliding with it in a small fraction of a second. The second problem was that the planetary model could not explain the highly peaked emission and absorption spectra of atoms that were observed.

The Bohr model of the atom
Quantum theory revolutionized physics at the beginning of the 20th century, when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in discrete amounts known as quanta (singular, quantum). In 1913, Niels Bohr incorporated this idea into his Bohr model of the atom, in which an electron could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy.[19] Under this model an electron could not spiral into the nucleus because it could not lose energy in a continuous manner; instead, it could only make instantaneous "quantum leaps" between the fixed energy levels.[19] When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra).[19]

Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't predict those of multielectron atoms. Worse still, as spectrographic technology improved, additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In 1916, Arnold Sommerfeld added elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.

Discovery of isotopes
Main article: Isotope
While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one element at each position on the periodic table.[20] The term isotope was coined by Margaret Todd as a suitable name for these elements.

That same year, J.J. Thomson conducted an experiment in which he channeled a stream of neon ions through magnetic and electric fields, striking a photographic plate at the other end. He observed two glowing patches on the plate, which suggested two different deflection trajectories. Thomson concluded this was because some of the neon ions had a different mass.[21] The nature of this differing mass would later be explained by the discovery of neutrons in 1932.

Discovery of nuclear particles
Main articles: Atomic nucleus and Discovery of the neutron
In 1917 Rutherford bombarded nitrogen gas with alpha particles and observed hydrogen nuclei being emitted from the gas (Rutherford recognized these, because he had previously obtained them bombarding hydrogen with alpha particles, and observing hydrogen nuclei in the products). Rutherford concluded that the hydrogen nuclei emerged from the nuclei of the nitrogen atoms themselves (in effect, he had split a nitrogen).[22]

From his own work and the work of his students Bohr and Henry Moseley, Rutherford knew that the positive charge of any atom could always be equated to that of an integer number of hydrogen nuclei. This, coupled with the atomic mass of many elements being roughly equivalent to an integer number of hydrogen atoms - then assumed to be the lightest particles - led him to conclude that hydrogen nuclei were singular particles and a basic constituent of all atomic nuclei. He named such particles protons. Further experimentation by Rutherford found that the nuclear mass of most atoms exceeded that of the protons it possessed; he speculated that this surplus mass was composed of previously-unknown neutrally charged particles, which were tentatively dubbed "neutrons".

In 1928, Walter Bothe observed that beryllium emitted a highly penetrating, electrically neutral radiation when bombarded with alpha particles. It was later discovered that this radiation could knock hydrogen atoms out of paraffin wax. Initially it was thought to be high-energy gamma radiation, since gamma radiation had a similar effect on electrons in metals, but James Chadwick found that the ionization effect was too strong for it to be due to electromagnetic radiation, so long as energy and momentum were conserved in the interaction. In 1932, Chadwick exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium radiation", and by measuring the energies of the recoiling charged particles, he deduced that the radiation was actually composed of electrically neutral particles which could not be massless like the gamma ray, but instead were required to have a mass similar to that of a proton. Chadwick now claimed these particles as Rutherford's neutrons.[23] For his discovery of the neutron, Chadwick received the Nobel Prize in 1935.

Quantum physical models of the atom
Main article: Atomic orbital

The five filled atomic orbitals of a neon atom separated and arranged in order of increasing energy from left to right, with the last three orbitals being equal in energy. Each orbital holds up to two electrons, which most probably exist in the zones represented by the colored bubbles. Each electron is equally present in both orbital zones, shown here by color only to highlight the different wave phase.
In 1924, Louis de Broglie proposed that all moving particles—particularly subatomic particles such as electrons—exhibit a degree of wave-like behavior. Erwin Schrödinger, fascinated by this idea, explored whether or not the movement of an electron in an atom could be better explained as a wave rather than as a particle. Schrödinger's equation, published in 1926,[24] describes an electron as a wavefunction instead of as a point particle. This approach elegantly predicted many of the spectral phenomena that Bohr's model failed to explain. Although this concept was mathematically convenient, it was difficult to visualize, and faced opposition.[25] One of its critics, Max Born, proposed instead that Schrödinger's wavefunction described not the electron but rather all its possible states, and thus could be used to calculate the probability of finding an electron at any given location around the nucleus.[26] This reconciled the two opposing theories of particle versus wave electrons and the idea of wave–particle duality was introduced. This theory stated that the electron may exhibit the properties of both a wave and a particle. For example, it can be refracted like a wave, and has mass like a particle.[27]

A consequence of describing electrons as waveforms is that it is mathematically impossible to simultaneously derive the position and momentum of an electron. This became known as the Heisenberg uncertainty principle after the theoretical physicist Werner Heisenberg, who first described it and published it in 1927.[28] This invalidated Bohr's model, with its neat, clearly defined circular orbits. The modern model of the atom describes the positions of electrons in an atom in terms of probabilities. An electron can potentially be found at any distance from the nucleus, but, depending on its energy level, exists more frequently in certain regions around the nucleus than others; this pattern is referred to as its atomic orbital. The orbitals come in a variety of shapes-sphere, dumbbell, torus, etc.-with the nucleus in the middle.[29]

17/01/2020

Atomos was proposed by Democritus; he lived around 400-300 BC. Atomos means the smallest parts of matters.

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Electricity and magnetism
Electricity and magnetism are manifestations of a single underlying electromagnetic force. Electromagnetism is a branch of physical science that describes the interactions of electricity and magnetism, both as separate phenomena and as a singular electromagnetic force. Amagnetic field is created by a moving electric current and a magnetic field can induce movement of charges (electric current). The rules of electromagnetism also explain geomagnetic and electromagnetic phenomena by explaining how charged particles of atoms interact.

Before the advent of technology, electromagnetism was perhaps most strongly experienced in the form of lightning , and electromagnetic radiation in the form of light. Ancient man kindled fires that he thought were kept alive in trees struck by lightning. Magnetism has long been employed for navigation in the compass. In fact, it is known that Earth's magnetic poles have exchanged positions in the past.

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Some of the rules of electrostatics, the study of electric charges at rest, were first noted by the ancient Romans, who observed the way a brushed comb would attract particles. It is now known that electric charges occur in two different forms, positive charges and negative charges. Like charges repel each other, and differing types attract.

The force that attract positive charges to negative charges weakens with distance, but is intrinsically very strong—up to 40 times stronger than the pull of gravity at the surface of the earth. This fact can easily be demonstrated by a small magnet that can hold or suspend an object. The small magnet exerts a force at least equal to the pull of gravity from the entire Earth.

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The fact that unlike charges attract means that most of this force is normally neutralized and not seen in full strength. The negative charge is generally carried by the atom's electrons, while the positive resides with the protons inside the atomic nucleus. Other less known particles can also carry charge. When the electrons of a material are not tightly bound to the atom's nucleus, they can move from atom to atom and the substance, called a conductor, can conduct electricity. Conversely, when the electron binding is strong, the material resists electron flow and is an insulator.

When electrons are weakly bound to the atomic nucleus, the result is a semiconductor, often used in the electronics industry. It was not initially known if the electric current carriers were positive or negative, and this initial ignorance gave rise to the convention that current flows from the positive terminal to the negative. In reality we now know that the electrons actually flow from the negative to the positive.

Electromagnetism is the theory of a unified expression of an underlying force, the electromagnetic force. This is seen in the movement of electric charge, that gives rise to magnetism (the electric current in a wire being found to deflect a compass needle), and it was Scottish physicist James Clerk Maxwell (1831–1879), who published a unifying theory of electricity and magnetism in 1865. The theory arose from former specialized work by German mathematician Carl Fredrich Gauss (1777–1855), French physicist Charles Augustin de Coulomb (1736–1806), French scientist André Marie Ampère

(1775–1836), English physicist Michael Faraday (1791–1867), American scientist and statesman Benjamin Franklin (1706–1790), and German physicist and mathematician Georg Simon Ohm (1789–1854). However, one factor that did not contradict the experiments was added to the equations by Maxwell to ensure the conservation of charge. This was done on the theoretical grounds that charge should be a conserved quantity, and this addition led to the prediction of a wave phenomena with a certain anticipated velocity. Light, with the expected velocity, was found to be an example of this electro-magnetic radiation.

Light had formerly been thought of as consisting of particles (photons) by Newton, but the theory of light as particles was unable to explain the wave nature of light (diffraction and the like). In reality, light displays both wave and particle properties. The resolution to this duality lies in quantum theory , where light is neither particles nor wave, but both. It propagates as a wave without the need of a medium and interacts in the manner of a particle. This is the basic nature of quantum theory.
Classical electromagnetism, useful as it is, contains contradictions (acausality) that make it incomplete and drive one to consider its extension to the area of quantum physics , where electromagnetism, of all the fundamental forces of nature, it is perhaps the best understood.

There is much symmetry between electricity and magnetism. It is possible for electricity to give rise to magnetism, and symmetrically for magnetism to give rise to electricity (as in the exchanges within an electric transformer). It is an exchange of just this kind that constitutes electromagnetic waves. These waves, although they don't need a medium of propagation, are slowed when traveling through a transparent substance.

Electromagnetic waves differ from each other only in amplitude, frequency, and orientation (polarization). Laser beams are particular in being very coherent, that is, the radiation is of one frequency, and the waves coordinated in motion and direction. This permits a highly concentrated beam that is used not only for its cutting abilities, but also in electronic data storage, such as in CD-ROMs.

The differing frequency forms are given a variety of names, from radio waves at very low frequencies through light itself, to the high frequency x rays and gamma rays.

The unification of electricity and magnetism allows a deeper understanding of physical science, and much effort has been put into further unifying the four forces of nature (e.g., the electromagnetic, weak, strong, and gravitational forces. The weak force has now been unified with electromagnetism, called the electroweak force. There are research programs attempting to collect data that may lead to a unification of the strong force with the electroweak force in a grand unified theory, but the inclusion of gravity remains an open problem.

Maxwell's theory is in fact in contradiction with Newtonian mechanics, and in trying to find the resolution to this conflict, Einstein was lead to his theory of special relativity. Maxwell's equations withstood the conflict, but it was Newtonian mechanics that were corrected by relativistic mechanics. These corrections are most necessary at velocities, close to the speed of light.

Paradoxically, magnetism is a counter example to the frequent claims that relativistic effects are not noticeable for low velocities. The moving charges that compose an electric current in a wire might typically only be traveling at several feet per second (walking speed), and the resulting Lorentz contraction of special relativity is indeed minute. However, the electrostatic forces at balance in the wire are of such great magnitude, that this small contraction of the moving (negative) charges exposes a residue force of real world magnitude, namely the magnetic force. It is in exactly this way that the magnetic force derives from the electric. Special relativity is indeed hidden in Maxwell's equations, which were known before special relativity was understood or separately formulated by Einstein.

Electricity at high voltages can carry energy across extended distances with little loss. Magnetism derived from that electricity can then power vast motors. But electromagnetism can also be employed in a more delicate fashion as a means of communication, either with wires (as in the telephone), or without them (as in radio communication). It also drives motors and provides current for electronic and computing devices.

See also Aurora Borealis and Aurora Australialis; Earth, interior structure; Electromagnetic spectrum; Ferromagnetic; Quantum electrodynamics (QED); Quantum theory and mechanics

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