A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed from subatomic particles to galaxies. In everyday life, the effects of magnetic fields are seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location; as such, it is an example of a vector field. The term'magnetic field' is used for two distinct but related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla, equivalent to newton per meter per ampere.
H and B differ in. In a vacuum, B and H are the same aside from units. Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, are both components of the electromagnetic force, one of the four fundamental forces of nature. Magnetic fields are used throughout modern technology in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric generators; the interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall effect; the Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass. Although magnets and magnetism were studied much earlier, the research of magnetic fields began in 1269 when French scholar Petrus Peregrinus de Maricourt mapped out the magnetic field on the surface of a spherical magnet using iron needles.
Noting that the resulting field lines crossed at two points he named those points'poles' in analogy to Earth's poles. He clearly articulated the principle that magnets always have both a north and south pole, no matter how finely one slices them. Three centuries William Gilbert of Colchester replicated Petrus Peregrinus' work and was the first to state explicitly that Earth is a magnet. Published in 1600, Gilbert's work, De Magnete, helped to establish magnetism as a science. In 1750, John Michell stated that magnetic poles attract and repel in accordance with an inverse square law. Charles-Augustin de Coulomb experimentally verified this in 1785 and stated explicitly that the north and south poles cannot be separated. Building on this force between poles, Siméon Denis Poisson created the first successful model of the magnetic field, which he presented in 1824. In this model, a magnetic H-field is produced by'magnetic poles' and magnetism is due to small pairs of north/south magnetic poles. Three discoveries in 1820 challenged this foundation of magnetism, though.
Hans Christian Ørsted demonstrated that a current-carrying wire is surrounded by a circular magnetic field. André-Marie Ampère showed that parallel wires with currents attract one another if the currents are in the same direction and repel if they are in opposite directions. Jean-Baptiste Biot and Félix Savart announced empirical results about the forces that a current-carrying long, straight wire exerted on a small magnet, determining that the forces were inversely proportional to the perpendicular distance from the wire to the magnet. Laplace deduced, but did not publish, a law of force based on the differential action of a differential section of the wire, which became known as the Biot–Savart law. Extending these experiments, Ampère published his own successful model of magnetism in 1825. In it, he showed the equivalence of electrical currents to magnets and proposed that magnetism is due to perpetually flowing loops of current instead of the dipoles of magnetic charge in Poisson's model.
This has the additional benefit of explaining. Further, Ampère derived both Ampère's force law describing the force between two currents and Ampère's law, like the Biot–Savart law described the magnetic field generated by a steady current. In this work, Ampère introduced the term electrodynamics to describe the relationship between electricity and magnetism. In 1831, Michael Faraday discovered electromagnetic induction when he found that a changing magnetic field generates an encircling electric field, he described this phenomenon in. Franz Ernst Neumann proved that, for a moving conductor in a magnetic field, induction is a consequence of Ampère's force law. In the process, he introduced the magnetic vector potential, shown to be equivalent to the underlying mechanism proposed by Faraday. In 1850, Lord Kelvin known as William Thomson, distinguished between two magnetic fields now denoted H and B; the former applied to the latter to Ampère's model and induction. Further, he derived how H and B relate to each other
Louis Georges Gouy
Louis Georges Gouy was a French physicist. He is the namesake of the Gouy balance, the Gouy–Chapman electric double layer model and the Gouy phase. Gouy was born at Vals-les-Bains, Ardèche in 1854, he became a correspondent of the Académie des sciences in 1901, a member in 1913. His principal scientific work was related to the following subjects: The propagation velocity of light waves in dispersive media. Propagation of spherical waves of small radius. Distant diffraction Electrostatics: Inductive capacity of dielectrics Surface charge Effect of the magnetic field on the discharge in rarefied gases Electrocapillarity Emission capacity of absorbent of the coloured flames Brownian motion Measurement of magnetic susceptibility of transition metal complexes with Gouy balance L. G. Gouy La Nature n°2708 du 27 février 1926 A Sella, Gouy's Balance, Chemistry World, December 2010
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets consist of wire wound into a coil. A current through the wire creates a magnetic field, concentrated in the hole in the center of the coil; the magnetic field disappears. The wire turns are wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the main advantage of an electromagnet over a permanent magnet is that the magnetic field can be changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field. Electromagnets are used as components of other electrical devices, such as motors, electromechanical solenoids, loudspeakers, hard disks, MRI machines, scientific instruments, magnetic separation equipment. Electromagnets are employed in industry for picking up and moving heavy iron objects such as scrap iron and steel.
Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic fields. British scientist William Sturgeon invented the electromagnet in 1824, his first electromagnet was a horseshoe-shaped piece of iron, wrapped with about 18 turns of bare copper wire. The iron was varnished to insulate it from the windings; when a current was passed through the coil, the iron became magnetized and attracted other pieces of iron. Sturgeon displayed its power by showing that although it only weighed seven ounces, it could lift nine pounds when the current of a single-cell battery was applied. However, Sturgeon's magnets were weak because the uninsulated wire he used could only be wrapped in a single spaced out layer around the core, limiting the number of turns. Beginning in 1830, US scientist Joseph Henry systematically improved and popularized the electromagnet. By using wire insulated by silk thread, inspired by Schweigger's use of multiple turns of wire to make a galvanometer, he was able to wind multiple layers of wire on cores, creating powerful magnets with thousands of turns of wire, including one that could support 2,063 lb.
The first major use for electromagnets was in telegraph sounders. The magnetic domain theory of how ferromagnetic cores work was first proposed in 1906 by French physicist Pierre-Ernest Weiss, the detailed modern quantum mechanical theory of ferromagnetism was worked out in the 1920s by Werner Heisenberg, Lev Landau, Felix Bloch and others. A portative electromagnet is one designed to just hold material in place. A tractive electromagnet applies a force and moves something. Electromagnets are widely used in electric and electromechanical devices, including: Motors and generators Transformers Relays Electric bells and buzzers Loudspeakers and headphones Actuators such as valves Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks MRI machines Scientific equipment such as mass spectrometers Particle accelerators Magnetic locks Magnetic separation equipment, used for separating magnetic from nonmagnetic material, for example separating ferrous metal from other material in scrap.
Industrial lifting magnets magnetic levitation, used in a maglev train or trains Induction heating for cooking and hyperthermia therapy A common tractive electromagnet is a uniformly-wound solenoid and plunger. The solenoid is a coil of wire, the plunger is made of a material such as soft iron. Applying a current to the solenoid applies a force to the plunger and may make it move; the plunger stops moving. For example, the forces are balanced; the maximum uniform pull happens. An approximation for the force F is F = C A n I / l where C is a proportionality constant, A is the cross-sectional area of the plunger, n is the number of turns in the solenoid, I is the current through the solenoid wire, l is the length of the solenoid. For units using inches, pounds force, amperes with long, solenoids, the value of C is around 0.009 to 0.010 psi. For example, a 12-inch long coil with a long plunger of 1-square inch cross section and 11,200 ampere-turns had a maximum pull of 8.75 pounds. The maximum pull is increased.
The stop becomes a magnet. An approximation for the pull P is P = A n I = + Here la is the distance between
A Kibble balance or watt balance is an electromechanical measuring instrument that measures the weight of a test object precisely by the electric current and voltage needed to produce a compensating force. It is a metrological instrument that can realize the new definition of the kilogram unit of mass based on fundamental constants, termed an electronic or electrical kilogram; the name watt balance comes from the fact that the weight of the test mass is proportional to the product of current and voltage, measured in units of watts. In June 2016, two months after the death of the inventor of the balance, Bryan Kibble, metrologists of the Consultative Committee for Units of the International Committee for Weights and Measures agreed to rename the device in his honor. Since 1889, the definition of the kilogram was based on a physical object known as the International Prototype of the Kilogram. In 2013, accuracy criteria were agreed upon by the General Conference on Weights and Measures for replacing this definition with one based on the use of a Kibble balance.
After these criteria had been achieved, the CGPM voted unanimously on November 16, 2018 to change the definition of the kilogram and several other units, effective May 20, 2019, to coincide with World Metrology Day. The Kibble balance is a more accurate version of the ampere balance, an early current measuring instrument in which the force between two current-carrying coils of wire is measured and used to calculate the magnitude of the current. In this new application, the balance will be used in the opposite sense; the balance determines the weight of the object. Thus the mass of the object is defined in terms of a current and a voltage, as described below—an "electronic kilogram." The principle, used in the Kibble balance was proposed by Bryan Kibble of the UK National Physical Laboratory in 1975 for measurement of the gyromagnetic ratio. The main weakness of the ampere balance method is that the result depends on the accuracy with which the dimensions of the coils are measured; the Kibble balance method has an extra calibration step in which the effect of the geometry of the coils is eliminated, removing the main source of uncertainty.
This extra step involves moving the force coil through a known magnetic flux at a known speed. This step was performed in 1990; the Kibble balance originating from the National Physical Laboratory was transferred to the National Research Council of Canada in 2009, where scientists from the two labs continued to refine the instrument. In 2014, NRC researchers published the most accurate measurement of the Planck constant at that time, with a relative uncertainty of 1.8×10−8. A final paper by NRC researchers was published in May 2017, presenting a measurement of Planck's constant with an uncertainty of only 9.1 parts per billion, the measurement with the least uncertainty to date. Other Kibble balance experiments are conducted in the US National Institute of Standards and Technology, the Swiss Federal Office of Metrology in Berne, the International Bureau of Weights and Measures near Paris and Laboratoire national de métrologie et d’essais in Trappes, France. A conducting wire of length L that carries an electric current I perpendicular to a magnetic field of strength B experiences a Lorentz force equal to the product of these variables.
In the Kibble balance, the current is varied so that this force counteracts the weight w of a standard mass m. This principle is derived from the ampere balance. W is given by the mass m multiplied by the local gravitational acceleration g. Thus, w = m g = B L I; the Kibble balance avoids the problems of measuring L in a second calibration step. The same wire is moved through the same magnetic field at a known speed v. By Faraday's law of induction, a potential difference U is generated across the ends of the wire, which equals BLv, thus U = B L v. The unknown product BL can be eliminated from the equations to give U I = m g v. With U, I, g, v measured, this gives an accurate value for m. Both sides of the equation have the dimensions of power, measured in watts in the International System of Units. Accurate measurements of electric current and potential difference are made in conventional electrical units, which are based on fixed "conventional values" of the Josephson constant and the von Klitzing constant, K J-90 = 2 e / h and R K-90 = h / e 2 respectively.
The current Kibble balance experiments are equivalent to measuring the value of the conventional watt in SI units. From the definition of the conventional watt, this is equivalent to measuring the value of the product KJ2RK in SI units instead of its fixed value in conventional electrical units: K J 2 R K = K J-90 2 R K-90 m g v U
A Faraday balance is a device for measuring magnetic susceptibility. Magnetic susceptibility is related to the force experienced by a substance in a magnetic field. Various practical devices are available for the measurement of susceptibility, which differ in the shape of the magnetic field and the way the force is measured. In the Faraday balance the field is homogeneous; the pole pieces of the magnet are so shaped that there is a region in which the product of the field strength and field gradient in the z direction is constant. The sample is placed in this region; the force in this case is independent of the packing of the sample and depends only on the total mass of the material present. The method is sensitive and reproducible and can be applied to single crystals; the force is measured as a weight change. An alternative method for measuring magnetic susceptibility is the Gouy balance. In this technique there is an inhomogeneous field in the central region between two poles of a permanent magnet, or an electromagnet.
The sample, in the form of a powder in a cylindrical tube, is suspended in such a way the one end lies in the centre of the field and the other is outside the magnetic field. Errors due to inefficient packing in the sample tube are difficult to eliminate
Paramagnetism is a form of magnetism whereby certain materials are weakly attracted by an externally applied magnetic field, form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, diamagnetic materials are repelled by magnetic fields and form induced magnetic fields in the direction opposite to that of the applied magnetic field. Paramagnetic materials include some compounds; the magnetic moment induced by the applied field is rather weak. It requires a sensitive analytical balance to detect the effect and modern measurements on paramagnetic materials are conducted with a SQUID magnetometer. Paramagnetism is due to the presence of unpaired electrons in the material, so all atoms with incompletely filled atomic orbitals are paramagnetic. Due to their spin, unpaired electrons have a magnetic dipole act like tiny magnets. An external magnetic field causes the electrons' spins to align parallel to the field, causing a net attraction.
Paramagnetic materials include aluminium, oxygen and iron oxide. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field because thermal motion randomizes the spin orientations, thus the total magnetization drops to zero. In the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field; this fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnetic materials is non-linear and much stronger, so that it is observed, for instance, in the attraction between a refrigerator magnet and the iron of the refrigerator itself. Constituent atoms or molecules of paramagnetic materials have permanent magnetic moments in the absence of an applied field; the permanent moment is due to the spin of unpaired electrons in atomic or molecular electron orbitals. In pure paramagnetism, the dipoles do not interact with one another and are randomly oriented in the absence of an external field due to thermal agitation, resulting in zero net magnetic moment.
When a magnetic field is applied, the dipoles will tend to align with the applied field, resulting in a net magnetic moment in the direction of the applied field. In the classical description, this alignment can be understood to occur due to a torque being provided on the magnetic moments by an applied field, which tries to align the dipoles parallel to the applied field. However, the true origins of the alignment can only be understood via the quantum-mechanical properties of spin and angular momentum. If there is sufficient energy exchange between neighbouring dipoles, they will interact, may spontaneously align or anti-align and form magnetic domains, resulting in ferromagnetism or antiferromagnetism, respectively. Paramagnetic behavior can be observed in ferromagnetic materials that are above their Curie temperature, in antiferromagnets above their Néel temperature. At these temperatures, the available thermal energy overcomes the interaction energy between the spins. In general, paramagnetic effects are quite small: the magnetic susceptibility is of the order of 10−3 to 10−5 for most paramagnets, but may be as high as 10−1 for synthetic paramagnets such as ferrofluids.
In conductive materials, the electrons are delocalized, that is, they travel through the solid more or less as free electrons. Conductivity can be understood in a band structure picture as arising from the incomplete filling of energy bands. In an ordinary nonmagnetic conductor the conduction band is identical for both spin-up and spin-down electrons; when a magnetic field is applied, the conduction band splits apart into a spin-up and a spin-down band due to the difference in magnetic potential energy for spin-up and spin-down electrons. Since the Fermi level must be identical for both bands, this means that there will be a small surplus of the type of spin in the band that moved downwards; this effect is a weak form of paramagnetism known as Pauli paramagnetism. The effect always competes with a diamagnetic response of opposite sign due to all the core electrons of the atoms. Stronger forms of magnetism require localized rather than itinerant electrons. However, in some cases a band structure can result in which there are two delocalized sub-bands with states of opposite spins that have different energies.
If one subband is preferentially filled over the other, one can have itinerant ferromagnetic order. This situation only occurs in narrow bands, which are poorly delocalized. Strong delocalization in a solid due to large overlap with neighboring wave functions means that there will be a large Fermi velocity; this is why s- and p-type metals are either Pauli-paramagnetic or as in the case of gold diamagnetic. In the latter case the diamagnetic contribution from the closed shell inner electrons wins over the weak paramagnetic term of the free electrons. Stronger magnetic effects are only observed when d or f electrons are involved; the latter are strongly localized. Moreover, the size of the magnetic
A weighing scale is a device to measure weight or mass. These are known as mass scales, weight scales, mass balance, weight balance, or scale, balance, or balance scale; the traditional scale bowls suspended at equal distances from a fulcrum. One plate holds an object of unknown mass, while known masses are added to the other plate until static equilibrium is achieved and the plates level off, which happens when the masses on the two plates are equal. A spring scale will make use of a spring of known stiffness to determine mass. Suspending a certain mass will extend the spring by a certain amount depending on the spring's stiffness; the heavier the object, the more the spring stretches, as described in Hooke's law. Other types of scale making use of different physical principles exist; some scales can be calibrated to read in units of force such as newtons instead of units of mass such as kilograms. Scales and balances are used in commerce, as many products are sold and packaged by mass; the balance scale is such a simple device that its usage far predates the evidence.
What has allowed archaeologists to link artifacts to weighing scales are the stones for determining absolute mass. The balance scale itself was used to determine relative mass long before absolute mass; the oldest evidence for the existence of weighing scales dates to c. 2400–1800 B. C. in the Indus River valley. Prior to that, no banking was performed due to lack of scales. Uniform, polished stone cubes discovered in early settlements were used as mass-setting stones in balance scales. Although the cubes bear no markings, their masses are multiples of a common denominator; the cubes are made of many different kinds of stones with varying densities. Their mass, not their size or other characteristics, was a factor in sculpting these cubes. In Egypt, scales can be traced to around 1878 B. C. but their usage extends much earlier. Carved stones bearing marks denoting mass and the Egyptian hieroglyphic symbol for gold have been discovered, which suggests that Egyptian merchants had been using an established system of mass measurement to catalog gold shipments or gold mine yields.
Although no actual scales from this era have survived, many sets of weighing stones as well as murals depicting the use of balance scales suggest widespread usage. In China, the earliest weighing balance excavated was from a tomb of the State of Chu of the Chinese Warring States Period dating back to the 3rd to 4th century BC in Mount Zuojiagong near Changsha, Hunan; the balance was made of wood and used bronze masses. Variations on the balance scale, including devices like the cheap and inaccurate bismar, began to see common usage by c. 400 B. C. by many small merchants and their customers. A plethora of scale varieties each boasting advantages and improvements over one another appear throughout recorded history, with such great inventors as Leonardo da Vinci lending a personal hand in their development. With all the advances in weighing scale design and development, all scales until the seventeenth century AD were variations on the balance scale; the standardization of the weights used – and ensuring traders used the correct weights – was a considerable preoccupation of governments throughout this time.
The original form of a balance consisted of a beam with a fulcrum at its center. For highest accuracy, the fulcrum would consist of a sharp V-shaped pivot seated in a shallower V-shaped bearing. To determine the mass of the object, a combination of reference masses was hung on one end of the beam while the object of unknown mass was hung on the other end. For high precision work, such as empirical chemistry, the center beam balance is still one of the most accurate technologies available, is used for calibrating test masses; the balance was the first mass measuring instrument invented. In its traditional form, it consists of a pivoted horizontal lever with arms of equal length – the beam – and a weighing pan suspended from each arm; the unknown mass is placed in one pan and standard masses are added to the other pan until the beam is as close to equilibrium as possible. In precision balances, a more accurate determination of the mass is given by the position of a sliding mass moved along a graduated scale.
Technically, a balance compares weight rather than mass, but, in a given gravitational field, the weight of an object is proportional to its mass, so the standard masses used with balances are labeled in units of mass. Unlike spring-based scales, balances are used for the precision measurement of mass as their accuracy is not affected by variations in the local gravitational field. A change in the strength of the gravitational field caused by moving the balance does not change the measured mass, because the moments of force on either side of the beam are affected equally. A balance will render an accurate measurement of mass at any location experiencing a constant gravity or acceleration. Precise measurements are achieved by ensuring that the balance's fulcrum is friction-free, by attaching a pointer to the beam which amplifies any deviation from a balance position. For greatest accuracy, there needs to be an allowance for the bu