The Geiger–Marsden experiments were a landmark series of experiments by which scientists discovered that every atom contains a nucleus where all of its positive charge and most of its mass are concentrated. They deduced this by measuring how an alpha particle beam is scattered when it strikes a thin metal foil; the experiments were performed between 1908 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model"; this model was further developed by J. J. Thomson. Thomson was the scientist who discovered the electron, that it was a component of every atom. Thomson believed the atom was a sphere of positive charge throughout which the electrons were distributed, a bit like plums in a Christmas pudding; the existence of protons and neutrons was unknown at this time. They knew atoms were tiny; this model was based on classical physics.
Thomson's model was not universally accepted before Rutherford's experiments. Thomson himself was never able to develop a stable model of his concept. Japanese scientist Hantaro Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other, he proposed instead. An alpha particle is a sub-microscopic, positively charged particle of matter. According to Thomson's model, if an alpha particle were to collide with an atom, it would just fly straight through, its path being deflected by at most a fraction of a degree. At the atomic scale, the concept of "solid matter" is meaningless, so the alpha particle would not bounce off the atom like a marble, it would be affected only by the atom's electric fields, Thomson's model predicted that the electric fields in an atom are too weak to affect a passing alpha particle much. Both the negative and positive charges within the Thomson atom are spread out over the atom's entire volume. According to Coulomb's Law, the less concentrated a sphere of electric charge is, the weaker its electric field at its surface will be.
As a worked example, consider an alpha particle passing tangentially to a Thomson gold atom, where it will experience the electric field at its strongest and thus experience the maximum deflection θ. Since the electrons are light compared to the alpha particle, their influence can be neglected and the atom can be seen as a heavy sphere of positive charge. Qn = positive charge of gold atom = 79 e = 1.266×10−17 C Qα = charge of alpha particle = 2 e = 3.204×10−19 C r = radius of a gold atom = 1.44×10−10 m vα = velocity of alpha particle = 1.53×107 m/s mα = mass of alpha particle = 6.645×10−27 kg k = Coulomb's constant = 8.998×109 N·m2/C2Using classical physics, the alpha particle's lateral change in momentum Δp can be approximated using the impulse of force relationship and the Coulomb force expression: Δ p = F Δ t = k ⋅ Q α Q n r 2 ⋅ 2 r v α θ ≈ Δ p p < k ⋅ 2 Q α Q n m α r v α 2 = 8.998 ⋅ 10 9 × 2 × 3.204 ⋅ 10 − 19 × 1.266 ⋅ 10 − 17 6.645 ⋅ 10 − 27 × 1.44 ⋅ 10 − 10 × 2 θ < 0.000326 r a d The above calculation is but an approximation of what happens when an alpha particle comes near a Thomson atom, but it is clear that the deflection at most will be in the order of a small fraction of a degree.
If the alpha particle were to pass through a gold foil some 400 atoms thick and experience maximal deflection in the same direction, it would still be a small deflection. At Rutherford's behest and Marsden performed a series of experiments where they pointed a beam of alpha particles at a thin foil of metal and measured the scattering pattern by using a fluorescent screen, they spotted alpha particles bouncing off the metal foil in all directions, some right back at the source. This should have been impossible according to Thomson's model.
Mass is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. The object's mass determines the strength of its gravitational attraction to other bodies; the basic SI unit of mass is the kilogram. In physics, mass is not the same as weight though mass is determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass; this is because weight is a force, while mass is the property that determines the strength of this force. There are several distinct phenomena. Although some theorists have speculated that some of these phenomena could be independent of each other, current experiments have found no difference in results regardless of how it is measured: Inertial mass measures an object's resistance to being accelerated by a force. Active gravitational mass measures the gravitational force exerted by an object.
Passive gravitational mass measures the gravitational force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force; the inertia and the inertial mass describe the same properties of physical bodies at the qualitative and quantitative level by other words, the mass quantitatively describes the inertia. According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A body's mass determines the degree to which it generates or is affected by a gravitational field. If a first body of mass mA is placed at a distance r from a second body of mass mB, each body is subject to an attractive force Fg = GmAmB/r2, where G = 6.67×10−11 N kg−2 m2 is the "universal gravitational constant". This is sometimes referred to as gravitational mass. Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical.
The standard International System of Units unit of mass is the kilogram. The kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. However, because precise measurement of a decimeter of water at the proper temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of the international prototype kilogram of cast iron, thus became independent of the meter and the properties of water. However, the mass of the international prototype and its identical national copies have been found to be drifting over time, it is expected that the re-definition of the kilogram and several other units will occur on May 20, 2019, following a final vote by the CGPM in November 2018. The new definition will use only invariant quantities of nature: the speed of light, the caesium hyperfine frequency, the Planck constant. Other units are accepted for use in SI: the tonne is equal to 1000 kg. the electronvolt is a unit of energy, but because of the mass–energy equivalence it can be converted to a unit of mass, is used like one.
In this context, the mass has units of eV/c2. The electronvolt and its multiples, such as the MeV, are used in particle physics; the atomic mass unit is 1/12 of the mass of a carbon-12 atom 1.66×10−27 kg. The atomic mass unit is convenient for expressing the masses of molecules. Outside the SI system, other units of mass include: the slug is an Imperial unit of mass; the pound is a unit of both mass and force, used in the United States. In scientific contexts where pound and pound need to be distinguished, SI units are used instead; the Planck mass is the maximum mass of point particles. It is used in particle physics; the solar mass is defined as the mass of the Sun. It is used in astronomy to compare large masses such as stars or galaxies; the mass of a small particle may be identified by its inverse Compton wavelength. The mass of a large star or black hole may be identified with its Schwarzschild radius. In physical science, one may distinguish conceptually between at least seven different aspects of mass, or seven physical notions that involve the concept of mass.
Every experiment to date has shown these seven values to be proportional, in some cases equal, this proportionality gives rise to the abstract concept of mass. There are a number of ways mass can be measured or operationally defined: Inertial mass is a measure of an object's resistance to acceleration when a force is applied, it is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says. Active gravitational mass is a measure of the strength of an object's gravitational flux. Gravitational field can be measured by allowing a small "test object" to fall and measuring its free-fall acceleration. For example, an object in free fall near the Moon is subject to a smaller gravitational field, hence
The impact parameter b is defined as the perpendicular distance between the path of a projectile and the center of a potential field U created by an object that the projectile is approaching. It is referred to in nuclear physics and in classical mechanics; the impact parameter is related to the scattering angle θ by θ = π − 2 b ∫ r m i n ∞ d r r 2 1 − 2 − 2 U / m v ∞ 2 where v ∞ is the velocity of the projectile when it is far from the center, r m i n is its closest distance from the center. The simplest example illustrating the use of the impact parameter is in the case of scattering from a sphere. Here, the object that the projectile is approaching is a hard sphere with radius R. In the case of a hard sphere, U = 0 when r > R, U = ∞ for r ≤ R. When b > R, the projectile misses the hard sphere. We see that θ = 0; when b ≤ R, we find that b = R cos . In high-energy nuclear physics — in colliding-beam experiments — collisions may be classified according to their impact parameter. Central collisions have b ≈ 0, peripheral collisions have 0 < b < 2 R, ultraperipheral collisions have b > 2 R, where the colliding nuclei are viewed as hard spheres with radius R.
Because the color force has an short range, it cannot couple quarks that are separated by much more than one nucleon's radius. This means that final-state particle multiplicity is greatest in the most central collisions, due to the partons involved having the greatest probability of interacting in some way; this has led to charged particle multiplicity being used as a common measure of collision centrality. Because strong interactions are impossible in ultraperipheral collisions, they may be used to study electromagnetic interactions — i.e. photon-photon, photon-nucleon, or photon-nucleus interactions — with low background contamination. Because UPCs produce only two- to four final-state particles, they are relatively "clean" when compared to central collisions, which may produce hundreds of particles per event. Tests of general relativity Distance of closest approach http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/rutsca2.html
In physics, the fine-structure constant known as Sommerfeld's constant denoted by α, is a dimensionless physical constant characterizing the strength of the electromagnetic interaction between elementary charged particles. It is related to the elementary charge e, which characterizes the strength of the coupling of an elementary charged particle with the electromagnetic field, by the formula 4πε0ħcα = e2. Being a dimensionless quantity, it has the same numerical value in all systems of units, 1/137; some equivalent definitions of α in terms of other fundamental physical constants are: α = 1 4 π ε 0 e 2 ℏ c = μ 0 4 π e 2 c ℏ = k e e 2 ℏ c = c μ 0 2 R K = e 2 4 π Z 0 ℏ where: e is the elementary charge. The definition reflects the relationship between α and the permeability of free space µ0, which equals µ0 = 2hα/ce2. In the 2019 redefinition of SI base units, 4π × 1.00000000082×10−7 H⋅m−1 is the value for µ0 based upon more accurate measurements of the fine structure constant. In electrostatic cgs units, the unit of electric charge, the statcoulomb, is defined so that the Coulomb constant, ke, or the permittivity factor, 4πε0, is 1 and dimensionless.
The expression of the fine-structure constant, as found in older physics literature, becomes α = e 2 ℏ c. In natural units used in high energy physics, where ε0 = c = ħ = 1, the value of the fine-structure constant is α = e 2 4 π; as such, the fine-structure constant is just another, albeit dimensionless, quantity determining the elementary charge: e = √4πα ≈ 0.30282212 in terms of such a natural unit of charge. In atomic units, the fine structure constant is α = 1 c; the 2014 CODATA recommended value of α is α = e2/4πε0ħc = 0.0072973525664. This has a relative standard uncertainty of 0.23 parts per billion. For reasons of convenience the value of the reciprocal of the fine-structure constant is specified; the 2014 CODATA recommended value is given by α−1 = 137.035999139. While the value of α can be estimated from the values of the constants appearing in any of its definitions, the theory of quantum electrodynamics provides a way to measure α directly using the quantum Hall effect or the anomalous magnetic moment of the electron.
The theory of QED predicts a relationship between the dimensionless magnetic moment of the electron and the fine-structure constant α. The most precise value of α obtained experimentally is based on a measurement of g using a one-electron so-called "quantum cyclotron" apparatus, together with a calculation via the theory of QED that involved 12672 tenth-order Feynman diagrams: α−1 = 137.035999174. This measurement of α has a precision of 0.25 parts per billion. This value and uncertainty are about the same as the latest experimental results; the fine-structure constant, α, has several physical interpretations. Α is: The square of the ratio of the elementary charge to the Planck charge α = 2. The ratio of two energies: the energy needed to overcome the electrostatic repulsion between two electrons a distance of d apart, the energy of a single photon of wavelength λ = 2πd: α = e 2 4 π ε 0 d / h c λ = e 2 4 π ε 0 d × 2 π d h c = e 2 4 π ε 0 d × d ℏ c = e 2 4 π ε
Sir Ernest Marsden was an English-New Zealand physicist. He is recognised internationally for his contributions to science while working under Ernest Rutherford, which led to the discovery of new theories on the structure of the atom. In Marsden's work in New Zealand, he became a significant member of the scientific community, while maintaining close links to the United Kingdom. Born in Manchester, the son of Thomas Marsden and Phoebe Holden, Marsden lived in Rishton and attended Queen Elizabeth's Grammar School, where an inter-house trophy rewarding academic excellence bears his name. In 1909, as a 20-year-old student at the University of Manchester, he met and began work under Ernest Rutherford. While still an undergraduate he conducted the famous Geiger–Marsden experiment called the gold foil experiment, together with Hans Geiger under Rutherford's supervision; this experiment led to Rutherford's new theory for the structure of the atom, with a centralised concentration of mass and positive charge surrounded by empty space and a sea of orbiting negatively charged electrons.
Rutherford described this as "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back to hit you". The apparatus used in the experiment was an early version of. In 1915 he moved to Victoria University College in Wellington, New Zealand, to replace Thomas Laby as Professor of Physics. Marsden served in France during World War I as a Royal Engineer in a special sound-ranging section and earned the Military Cross. In 1922 Marsden turned from his position as Professor of Physics to bureaucracy, he was appointed Assistant Director of Education before accepting the position of Secretary of New Zealand's new Department of Scientific and Industrial Research in 1926. The new Department's focus was on assisting primary industries, Marsden worked to organise research in the area of agriculture. Marsden initiated a number of projects that kept New Zealand in touch with international developments in the field of radiation and nuclear sciences. In 1939 he pioneered the non-medical use of radioisotopes in New Zealand, conducted a series of experiments to determine the role of cobalt in animal metabolism.
With the outbreak of World War II Marsden was given the title of Director of Scientific Developments, was charged with mobilizing New Zealand's scientific manpower. During the War he worked on radar research, setting up a team to develop the radar equipment for use in the Pacific. Marsden used his scientific connections to form a team of young New Zealand Scientists who would participate in the American Manhattan Project developing the nuclear bomb, initiated the search for uranium, the raw material needed for nuclear projects, in New Zealand. Marsden had a post-war vision of a nuclear New Zealand, with scientists working on research using local nuclear reactors, developing connections with the British nuclear energy and weapons program. While this vision was not realised, in 1946 he established a team of scientists to carry out research into atomic energy and the application of nuclear science to problems in agriculture and industry. Ties between Marsden and the scientific community in Britain remained strong, in 1947 he became the DSIR's scientific liaison officer in London.
Marsden retired in 1954 and returned to Wellington, where he continued to work and travel extensively, serving on a number of committees and conducting research into environmental radioactivity. As his studies turned to the impact of fallout from radioactive bombs, Marsden came to oppose testing and the development of nuclear weapons. While Marsden had a significant role in establishing and encouraging nuclear science in New Zealand, this role of speaking out against nuclear weapons development and testing - which he only did after the British nuclear testing program was complete - is less known. In 1966, the same year France began testing nuclear bombs in the Pacific, Marsden suffered a stroke which left him confined to a wheelchair, he died at his home in Lowry Bay, Lower Hutt on the shores of Wellington Harbour in 1970. Marsden married Margaret Sutcliffe, a school teacher, in 1913, they had a son and a daughter. After Marsden's final retirement to New Zealand, Maggie died on 7 November 1956.
Two years Marsden remarried, on 26 June 1958 Joyce Winifred Chote, 30 years his junior, became his wife. She assisted him in his remaining years, joining him on his travels and supporting him during his research. Marsden's career recognitions included fellowship in the Royal Society of London in 1946, president of the Royal Society of New Zealand in 1947 and the Rutherford Memorial Lecture in 1948. In 1961 he chaired the Rutherford Jubilee Conference in Manchester, which celebrated 50 years since Rutherford's discovery of the atomic nucleus. In 1935, he was awarded the King George V Silver Jubilee Medal and appointed a Commander of the Order of the British Empire in the Silver Jubilee and King's Birthday Honours, he was appointed a Companion of the Order of St Michael and St George in the 1946 New Year Honours and a Knight Bachelor in the 1958 New Year Honours, for services to science. The Marsden Fund for basic research in New Zealand was set up in 1994. Massey University has named a major lecture theatre after him.
University of Canterbury biography of Marsden Galbreath, Ross. "Marsden, Ernest 1889–1970". Dictionary of New Zealand Biography. Ministry for Culture and Heritage. Retrieved 9 April 2011. Biography in 1966 Encyclopaedia of New Zealand Address in Transactions of the Royal S
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
Alpha particles called alpha ray or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are produced in the process of alpha decay, but may be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α; the symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are sometimes written as He2+ or 42He2+ indicating a helium ion with a +2 charge. If the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 42He. Alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles have a kinetic energy of about 5 MeV, a velocity in the vicinity of 5% the speed of light, they are a ionizing form of particle radiation, have low penetration depth. They can be stopped by the skin. However, so-called long range alpha particles from ternary fission are three times as energetic, penetrate three times as far.
As noted, the helium nuclei that form 10–12% of cosmic rays are usually of much higher energy than those produced by nuclear decay processes, are thus capable of being penetrating and able to traverse the human body and many meters of dense solid shielding, depending on their energy. To a lesser extent, this is true of high-energy helium nuclei produced by particle accelerators; when alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage. Alpha radiation is an average of about 20 times more dangerous, in experiments with inhaled alpha emitters, up to 1000 times more dangerous than an equivalent activity of beta emitting or gamma emitting radioisotopes; some science authors use alpha particles as interchangeable terms. The nomenclature is not well defined, thus not all high-velocity helium nuclei are considered by all authors to be alpha particles.
As with beta and gamma particles/rays, the name used for the particle carries some mild connotations about its production process and energy, but these are not rigorously applied. Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions, when they occur as components of cosmic rays. A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission. However, helium nuclei produced by particle accelerators are less to be referred to as "alpha particles"; the best-known source of alpha particles is alpha decay of heavier atoms. When an atom emits an alpha particle in alpha decay, the atom's mass number decreases by four due to the loss of the four nucleons in the alpha particle; the atomic number of the atom goes down by two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due to alpha decay.
Alpha particles are emitted by all of the larger radioactive nuclei such as uranium, thorium and radium, as well as the transuranic elements. Unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it; the smallest nuclei that have to date been found to be capable of alpha emission are beryllium-8 and the lightest nuclides of tellurium, with mass numbers between 104 and 109. The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray removes the excess energy. In contrast to beta decay, the fundamental interactions responsible for alpha decay are a balance between the electromagnetic force and nuclear force. Alpha decay results from the Coulomb repulsion between the alpha particle and the rest of the nucleus, which both have a positive electric charge, but, kept in check by the nuclear force. In classical physics, alpha particles do not have enough energy to escape the potential well from the strong force inside the nucleus.
However, the quantum tunnelling effect allows alphas to escape though they do not have enough energy to overcome the nuclear force. This is allowed by the wave nature of matter, which allows the alpha particle to spend some of its time in a region so far from the nucleus that the potential from the repulsive electromagnetic force has compensated for the attraction of the nuclear force. From this point, alpha particles can escape, in quantum mechanics, after a certain time, they do so. Energetic alpha particles deriving from a nuclear process are produced in the rare nuclear fission process of ternary fission. In this process, three charged particles are produced from the event instead of the normal two, with the smallest of the charged particles most being an alpha particle; such alpha particles are termed "long range alphas" since at their typical energy of 16 MeV, they are at far higher energy than is produced by alpha decay. Ternary fission happens in both neutron-induced fission (the nuclear reacti