Electricity and Magnetism
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Wikipedia-Article "Electricity"
Electricity is a general term applied to phenomena involving a fundamental property of matter called an electric charge
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Related concepts
In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms.
- Electric charge: a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
- Electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.
- Electric potential the potential energy per unit charge associated with a static (time-invariant) electric field.
- Electric current: a movement or flow of electrically charged particles.
- Electrical energy: energy made available by the flow of electric charge through a conductor or from the forces between charged particles.
- Electric power: The rate at which electric energy is converted into another form, such as light, heat, or mechanical energy (or converted from another form into electric energy).
History
Ancient
According to Thales of Miletus, writing circa 600 BCE, a form of electricity was known to the Ancient Greeks who found that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. The Greeks noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump.
The origin of the word "electricity" is from the Greek word ēlektron, a word the ancient Greeks used for both "amber" and "electrum".
An object found in Iraq in 1938, dated to about 250 BCE and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating.
Modern
In 1600 the English scientist William Gilbert returned to the subject in De Magnete, and coined the modern Latin word electricus from ηλεκτρον (elektron), the Greek word for "amber", which soon gave rise to the English words electric and electricity. He was followed in 1660 by Otto von Guericke, who is regarded as having invented an early electrostatic generator. Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators; and C. F. Du Fay, who first identified the two types of electricity that would later be called positive and negative. The Leyden jar, a type of capacitor for electrical energy in large quantities, was invented at Leiden University by Pieter van Musschenbroek in 1745. William Watson, experimenting with the Leyden jar, discovered in 1747 that a discharge of static electricity was equivalent to an electric current.
In June, 1752, Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite during a thunderstorm. Following these experiments he invented a lightning rod and established the link between lightning and electricity. If Franklin did fly a kite in a storm, he did not do it the way it is often described (as it would have been dramatic but fatal). It was either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who created the convention of positive and negative electricity.
Franklin's observations aided later scientists such as Michael Faraday, Luigi Galvani, Alessandro Volta, André-Marie Ampère, and Georg Simon Ohm whose work provided the basis for modern electrical technology. The work of Faraday, Volta, Ampere, and Ohm is honored by society, in that fundamental units of electrical measurement are named after them.
Volta worked with chemicals and discovered that chemical reactions could be used to create positively charged anodes and negatively charged cathodes. When a conductor was attached between these, the difference in the electrical potential (also known as voltage) drives a current between them through the conductor. The potential difference between two points is measured in units of volts in recognition of Volta's work.
The invention of the electric telegraph showed that commercial and practical use could be made of electrical phenomena. By the end of the 19th century electrical engineering became a distinct profession, separate from the physicist or inventor. The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, inventor of the polyphase induction motor; Samuel Morse, inventor of the telegraph; Antonio Meucci, an inventor of the telephone; Thomas Edison inventor of the phonograph and a practical incandescent light bulb; George Westinghouse, inventor of the electric locomotive; Charles Steinmetz, theoretician of alternating current; Alexander Graham Bell, another inventor of the telephone and founder of a successful telephone business.
The rapid advance of electrical technology in the latter 19th and early 20th centuries lead to commercial rivalry such as the so-called War of the Currents), between Edison's direct-current system or Westinghouse's alternating-current method. Often concurrent research in widely scattered locations lead to multiple claims to the invention of a device or system.
Electric charge
- For more details on this topic, see Electric charge.
Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge." There are two types of charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law.
Electric field
- For more details on this topic, see Electric field.
The concept of electric field was introduced by Michael Faraday. The electrical field force acts between two charges, in the same way that the gravitational field force acts between two masses. However, electric field is a little bit different. Gravitational force depends on the mass of two bodies, whereas electric force depends on the electric charge of two bodies. While gravity can only pull two masses together, the electric force can be an attractive or repulsive force. The criteria for the direction of the forces between two charged bodies is generally proposed as follows:
1) Both charges are of same sign (i.e. both charges are positive) in which case there will be a repulsive force between the two.
2) The charges are opposite in which case there will be an attractive force between the two bodies.
The magnitude of the force is dependent upon the distance between the two bodies, and varies inversely with the square of the distance between them. The magnitude of the force is also directly proportional to the product of the unsigned magnitude of the two charges. The most common experience with electric charge in everyday life is that of static cling - when two particular types of materials are rubbed together, they tend to stick together, at least for a while. This phenomenon occurs because of the exchange of charges between the two materials-- one becomes positively charged while the other becomes negatively charged, and because of their opposite signs there will be a force of attraction between them. Another common experience with electric charge is when one rubs his or her shoes on the carpet while walking and touches a doorknob experiencing a shock. When you rub your feet along the carpet your body acquires a charge, and when you bring your finger close to the doorknob the charges spark off of your finger to discharge.
Electric potential
- For more details on this topic, see Electric potential.
The electric potential difference between two points is defined as the work done per unit charge (against electrical forces) in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradients indicate the direction of heat flows. Similarly, there is an electric potential at every point in space, and its gradient in the electric field indicates where charges move.
Electric current
- For more details on this topic, see Current (electricity).
The electric charge which occurs naturally within conductors can be forced to flow, while the charges within insulators are locked in place and cannot be moved. Devices that use charge flow principles in materials are called electronic devices. A flow of electric charge is called an electric current. A direct current (DC) is a unidirectional flow; alternating current (AC) is a flow whose time average is zero, but whose energy capability (RMS level) is not zero. With AC the electric current repeatedly changes direction. Electric current is measured in Amperes
Ohm's Law is an important relationship describing the behaviour of electric currents: See also: electrical conduction
For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the conditions, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.
Electrical energy
- For more details on this topic, see Electrical energy.
Electrical energy is the flow of electrons or ions. When electrons are flowing through a wire or through hundreds of feet of air in the case of lightning it is because they are being forced to do so by an electrical field. A force is exerted on the electrons and they move. Work is done on the charged particles. A force is pushing them through a distance. More properly, they are moving from outer orbitals of one atom to another, being pushed by the electromotive force. While the electrons are in motion they contain kinetic energy. Consequently, atomic level electricity is a form of kinetic energy.
Electric power
- For more details on this topic, see Electric power.
Electric power is the capacity of the circuit for performing work in a particular amount of time. When a charge moves in a conductor, work is done by that charge. Devices can be made which convert this work into heat (Electric arc furnaces), light (light bulbs and Fluorescent lamps), or motion, i.e. kinetic energy (electric motors).
The unit for all forms of power is the watt (symbol: W). In practice, however, this is generally reserved for the real power component. Apparent power is conventionally expressed in volt-amperes (VA) since it is the simple multiple of rms voltage and current. The unit for reactive power is given the special name "VAR", which stands for volt-amperes-reactive.
SI electricity units
| SI electromagnetic units
|
||||
|---|---|---|---|---|
| Symbol | Name of Quantity | Derived Units | Base Units | |
| I | Current | ampere (SI base unit) | A | A |
| q | Electric charge, Quantity of electricity | coulomb | C | A·s |
| V | Potential difference | volt | V | J/C = kg·m2·s−3·A−1 |
| R, Z | Resistance, Impedance, Reactance | ohm | Ω | V/A = kg·m2·s−3·A−2 |
| ρ | Resistivity | ohm metre | Ω·m | kg·m3·s−3·A−2 |
| P | Power, Electrical | watt | W | V·A = kg·m2·s−3 |
| C | Capacitance | farad | F | C/V = kg−1·m−2·A2·s4 |
| Elastance | reciprocal farad | F−1 | V/C = kg·m2·A−2·s−4 | |
| ε | Permittivity | farad per metre | F/m | kg−1·m−3·A2·s4 |
| χe | Electric susceptibility | (dimensionless) | - | - |
| Conductance, Admittance, Susceptance | siemens | S | Ω−1 = kg−1·m−2·s3·A2 | |
| σ | Conductivity | siemens per metre | S/m | kg−1·m−3·s3·A2 |
| H | Magnetic field, magnetic field intensity | ampere per metre | A/m | A·m−1 |
| Φm | Magnetic flux | weber | Wb | V·s = kg·m2·s−2·A−1 |
| B | Magnetic flux density, magnetic induction, magnetic field strength | tesla | T | Wb/m2 = kg·s−2·A−1 |
| Reluctance | ampere-turns per weber | A/Wb | kg−1·m−2·s2·A2 | |
| L | Inductance | henry | H | Wb/A = V·s/A = kg·m2·s−2·A−2 |
| μ | Permeability | henry per metre | H/m | kg·m·s−2·A−2 |
| χm | Magnetic susceptibility | (dimensionless) | - | - |
See also
Devices
Engineering
Safety
Electrical phenomena in nature
- Matter: — since atoms and molecules are held together by electric forces.
- Lightning: electrical discharges in the atmosphere.
- The Earth's magnetic field — created by electric currents circulating in the planet's core.
- Sometimes due to solar flares, a phenomenon known as a power surge can be created.
- Piezoelectricity: the ability of certain crystals to generate a voltage in response to applied mechanical stress.
- Triboelectricity: electric charge taken on by contact or friction between two different materials.
- Bioelectromagnetism: electrical phenomena within living organisms.
- Bioelectricity — Many animals are sensitive to electric fields, some (e.g., sharks) more than others (e.g., people). Most also generate their own electric fields.
- Gymnotiformes, such as the electric eel, deliberately generate strong fields to detect or stun their prey.
- Neurons in the nervous system transmit information by electrical impulses known as action potentials.
External links
- What is electricity?
- Merriam-Webster: Electricity
- Tyndall: Faraday as Discovery: Identity of Electricities
- US Energy Department Statistics
- How to save on your electricity bills
- Electricity around the world
- A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin, Created and Collected by Robert A. Morse (2004)
- Understanding Electricity and some Electronics in 10 minutes(Steve Rose, Maui)
- Electricity Misconceptions
Wikipedia-Article "Magnetism"
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For magnetism as a neurological sign, see Magnetism (neurological sign).
In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials.
Magnetic materials
Some well known materials that exhibit easily detectable magnetic properties are iron, some steels, and the mineral lodestone; however, all materials are influenced to one degree or another by the presence of a magnetic field, although in most cases the influence is too small to detect without special equipment.
Physics of magnetism
Magnetic forces are fundamental forces that arise due to the movement of electrical charge. Maxwell's equations describe the origin and behavior of the fields that govern these forces (see also the Biot-Savart law). Thus, magnetism is seen whenever electrically charged particles are in motion. This can arise either from movement of electrons in an electric current, resulting in "electromagnetism", or from the quantum-mechanical orbital motion (there is no orbital motion of electrons around the nucleus like planets around the sun, but there is an "effective electron velocity") and spin of electrons, resulting in what are known as "permanent magnets".
Charged particle in a magnetic field
When a charged particle moves through a magnetic field B, it feels a force F given by the cross product:
where
is the electric charge of the particle- is the velocity vector of the particle
- is the magnetic field
Because this is a cross product, the force is perpendicular to both the motion of the particle and the magnetic field. It follows that the magnetic force does no work on the particle; it may change the direction of the particle's movement, but it cannot cause it to speed up or slow down.
Magnetic dipoles
Normally, magnetic fields are seen as dipoles, having a "South pole" and a "North pole"; terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe.
A magnetic field contains energy, and physical systems stabilize into the configuration with the lowest energy. Therefore, when placed in a magnetic field, a magnetic dipole tends to align itself in opposed polarity to that field, thereby canceling the net field strength as much as possible and lowering the energy stored in that field to a minimum. For instance, two identical bar magnets normally line up North to South resulting in no net magnetic field, and resist any attempts to reorient them to point in the same direction. The energy required to reorient them in that configuration is then stored in the resulting magnetic field, which is double the strength of the field of each individual magnet. (This is, of course, why a magnet used as a compass interacts with the Earth's magnetic field to indicate North and South).
Magnetic monopoles
Contrary to normal experience, some theoretical physics models predict the existence of magnetic monopoles. Paul Dirac observed in 1931 that, because electricity and magnetism show a certain symmetry, just as quantum theory predicts that individual positive or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. In practice, however, although charged particles like protons and electrons can be easily isolated as individual electrical charges, magnetic south and north poles do not appear in isolation. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain why the observed elementary particles carry charges that are multiples of the charge of the electron.
In modern elementary particle theory, the quantization of charge is realized in a spontaneous breakdown of a non-abelian gauge symmetry. It should be noted that the monopoles predicted in certain grand unified theories are different from the one originally thought of by Dirac. These monopoles, unlike that of elementary particles are solitons, namely localised energy packets. These monopoles, if they exist at all, contradict cosmological observations. A solution to this monopole problem in cosmology gave rise to the currently interesting idea of inflation.
Atomic magnetic dipoles
The physical cause of the magnetism of objects, as distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale from the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus; this motion can be considered as a current loop, resulting in an orbital dipole magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called the spin dipole magnetic moment (although current quantum mechanical theory states that electrons neither physically spin, nor orbit the nucleus).
The overall magnetic moment of the atom is the net sum of all of the magnetic moments of the individual electrons. Because of the tendency of magnetic dipoles to oppose each other to reduce the net energy, in an atom the opposing magnetic moments of some pairs of electrons cancel each other, both in orbital motion and in spin magnetic moments. Thus, in the case of an atom with a completely filled electron shell or subshell, the magnetic moments normally completely cancel each other out and only atoms with partially-filled electron shells have a magnetic moment, whose strength depends on the number of unpaired electrons.
The differences in configuration of the electrons in various elements thus determine the nature and magnitude of the atomic magnetic moments, which in turn determine the differing magnetic properties of various materials. Several forms of magnetic behavior have been observed in different materials, including:
Magnetars, stars with extremely powerful magnetic fields, are also known to exist.
Types of magnets
Electromagnets
Electromagnets are useful in cases where a magnet must be switched on or off; for instance, large cranes to lift junked automobiles.
For the case of electric current moving through a wire, the resulting field is directed according to the "right hand rule." If the right hand is used as a model, and the thumb of the right hand points along the wire from positive towards the negative side ("conventional current", the reverse of the direction of actual movement of electrons), then the magnetic field will wrap around the wire in the direction indicated by the fingers of the right hand. As can be seen geometrically, if a loop or helix of wire is formed such that the current is traveling in a circle, then all of the field lines in the center of the loop are directed in the same direction, resulting in a magnetic dipole whose strength depends on the current around the loop, or the current in the helix multiplied by the number of turns of wire. In the case of such a loop, if the fingers of the right hand are directed in the direction of conventional current flow (i.e. positive to negative, the opposite direction to the actual flow of electrons), the thumb will point in the direction corresponding to the North pole of the dipole.
Permanent Magnets
Magnetic metallic elements
Many materials have unpaired electron spins, but the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed "magnetic"). Due to the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt, and nickel, as well the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring (ferro)magnets were used in the first experiments with magnetism. Technology has expanded the availability of magnetic materials to include various manmade products, all based, however, on naturally magnetic elements.
Composites
Ceramic or ferrite
Ceramic, or ferrite, magnets are made of a sintered composite of powdered iron oxide and barium/strontium carbonate ceramic. Due to the low cost of the materials and manufacturing methods, inexpensive magnets (or nonmagnetized ferromagnetic cores, for use in electronic component such as radio antennas, for example) of various shapes can be easily mass produced. The resulting magnets are noncorroding, but brittle and must be treated like other ceramics.
Alnico
Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal.
Injection molded
Injection molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.
Flexible
Flexible magnets are similar to injection molded magnets, using a flexible resin or binder such as vinyl, and produced in flat strips or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.
Rare earth magnets
'Rare earth' (lanthanoid) elements have a partially occupied f electron shell (which can accommodate up to 14 electrons.) The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore these elements are used in compact high-strength magnets where their higher price is not a factor.
Samarium cobalt
Samarium cobalt magnets are highly resistant to oxidation, with higher magnetic strength and temperature resistance than alnico or ceramic materials. Sintered samarium cobalt magnets are brittle and prone to chipping and cracking and may fracture when subjected to thermal shock.
Neodymium iron boron (NIB)
Neodymium iron boron (NdFeB) magnets have the highest magnetic field strength, but are inferior to samarium cobalt in resistance to oxidation and temperature. This type of magnet is expensive, due to both the cost of raw materials and licensing of the patents involved. This high cost limits their use to applications where such high strengths from a compact magnet are critical. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion and thermal protection where required.
Single Molecule Magnets (SMMs) and Single Chain Magnets (SCMs)
In the nineties it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a "domain" level and theoretically could provide a far denser storage medium than conventional magnets. In this direction research on monolayers of SMMs is currently under way. Very briefly, the two main attributes of an SMM are:
- a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.
- a negative value of the anisotropy of the zero field splitting (D)
Most SMM's contain manganese, but can also be found with vanadium, iron, nickel and cobalt clusters. More recently it has been found that some chain systems can also display a magnetization which persists for long times at relatively higher temperatures. These systems have been called Single Chain Magnets.
Nano-Structured Magnets
Some nano-structured materials exhibit energy waves called magnons that coalesce into a common ground state in the manner of a Bose-Einstein condensate.
See results from NIST published April 2005, [1] or [2]
SI magnetism units
| SI electromagnetic units
|
||||
|---|---|---|---|---|
| Symbol | Name of Quantity | Derived Units | Base Units | |
| I | Current | ampere (SI base unit) | A | A |
| q | Electric charge, Quantity of electricity | coulomb | C | A·s |
| V | Potential difference | volt | V | J/C = kg·m2·s−3·A−1 |
| R, Z | Resistance, Impedance, Reactance | ohm | Ω | V/A = kg·m2·s−3·A−2 |
| ρ | Resistivity | ohm metre | Ω·m | kg·m3·s−3·A−2 |
| P | Power, Electrical | watt | W | V·A = kg·m2·s−3 |
| C | Capacitance | farad | F | C/V = kg−1·m−2·A2·s4 |
| Elastance | reciprocal farad | F−1 | V/C = kg·m2·A−2·s−4 | |
| ε | Permittivity | farad per metre | F/m | kg−1·m−3·A2·s4 |
| χe | Electric susceptibility | (dimensionless) | - | - |
| Conductance, Admittance, Susceptance | siemens | S | Ω−1 = kg−1·m−2·s3·A2 | |
| σ | Conductivity | siemens per metre | S/m | kg−1·m−3·s3·A2 |
| H | Magnetic field, magnetic field intensity | ampere per metre | A/m | A·m−1 |
| Φm | Magnetic flux | weber | Wb | V·s = kg·m2·s−2·A−1 |
| B | Magnetic flux density, magnetic induction, magnetic field strength | tesla | T | Wb/m2 = kg·s−2·A−1 |
| Reluctance | ampere-turns per weber | A/Wb | kg−1·m−2·s2·A2 | |
| L | Inductance | henry | H | Wb/A = V·s/A = kg·m2·s−2·A−2 |
| μ | Permeability | henry per metre | H/m | kg·m·s−2·A−2 |
| χm | Magnetic susceptibility | (dimensionless) | - | - |
Other magnetism units
See also
- Plastic magnet
- Magnet
- Electromagnetism
- Magnetic field
- Magnet therapy
- Magnetic circuit
- Michael Faraday
- James Clerk Maxwell
External links
- http://www.wondermagnet.com/magfaq.html Info on electromagnetism and homemade projects & experiments.
References
- Griffiths, David J. (1998). Introduction to Electrodynamics (3rd ed.), Prentice Hall. ISBN 013805326X.
- Tipler, Paul (2004). Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.), W. H. Freeman. ISBN 0716708108.
- ^ Nanomagnets Bend The Rules. URL accessed on November 14, 2005.
- ^ Nanomagnets bend the rules. URL accessed on November 14, 2005.