FUNDAMENTAL FORCES OF NATURE Familiar Forces Tension Forces Ask a student to hold one end of a piece of string in their hand, while you pull on the other end. Test your strength on a Newton spring balance. The tension (stretching) force is along the string and away from the support point. Compression Forces Push (gently) against the palm of someone's hand with a ruler. The compression (squashing) force is along the ruler and towards the support point. When a ruler is flexed so that it curves downward at its midpoint, the timber fibres on the ‘inside’ of the curve will be in compression. The fibres on the ‘outside’ of the curve will be in tension. The same thing happens in a concrete bridge or the lintel over a window or door, even though it is not obvious to the eye. That’s why it’s necessary to place reinforcing steel bars in the section of a concrete beam which is in tension, since concrete is weak in tension but reasonably strong in compression. Friction Forces Everyone is familiar with how difficult it is to walk on icy surfaces. Most people, at some time or other, have slipped at the kitchen sink because of water spillage. Many have experienced a nasty fright when the car in which they were travelling skidded. Try pushing the computer mouse pad along the table. Friction is a contact force between surfaces whose critical importance becomes obvious only when it’s absent. Reaction Forces When you push against a wall, the wall pushes back. When a lift travels from the top storey of a tall building, you experience a mild version of weightlessness, as the upward reaction exerted by the lift floor on you is momentarily reduced. On the other hand, you experience a momentary weight increase when the lift takes off from the ground. Seatbelts are worn in cars at all times and in aeroplanes at take-off and landing to provide reaction forces against the forces arising from accelerations. The Four Fundamental Forces of Nature The Gravitational Force When a baby starts to play by dropping objects out of its pram, it has begun its journey as an experimental physicist. Familiarity hides the wondrous and unusual nature of this force from our close scrutiny. This force intrigued the ancient Greeks, who claimed that heavier objects fell towards the ground faster than lighter ones. It is claimed that Galileo showed by experiment that two objects, regardless of their weights, would hit the ground simultaneously if dropped from identical heights. A careful reading of Galileo's experiments shows that he was well aware of the effects of air resistance on falling objects. Based on the classical wisdom of the Greek Philosophers, especially Plato and Aristotle, the Earth was the place where change occurred. In contrast the Heavens were eternal and unchanging. When Newton observed the 'apple fall from the tree', he had a brilliant insight. In his own words, 'I began to think of gravity extending to ye orb of the moon ', Newton proceeded to show by calculation that the gravitational force which caused the apple to fall to the ground was the same as the force that caused the moon to accelerate towards the Earth. He showed, on the basis of known measurements, that , where is the acceleration experienced by the moon due to the Earth's gravitational pull and g is the acceleration due to gravity at the Earth's surface. He then proceeded to compute the ratio on the basis of the 'inverse square law' and obtained the same answer. The hammer blow to the classical view was his derivation of the elliptical orbits of the planets around the Sun from the same inverse square law of gravitational force. Every particle of matter in the universe attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of their distances apart. In symbols F is the gravitational force; G is the universal gravitational constant; m 1 and m 2 are the particle masses and d is the distance between their centres of mass. Newton believed the force was proportional to the mass of each particle, because the force on a falling body is proportional to its mass. This relationship is known as Newton’s law of gravitation. The law applies to particles or objects whose dimensions are very small compared with the other distances involved. Newton was able to show that even an object as big as the Earth could be viewed as a uniform sphere with all the mass concentrated at its centre point. The gravitational force is a very small force. It is a very difficult force to detect between two 1 kg masses 1 m apart. In this case it is in fact numerically equal to G , with a value of N. Some appreciation of just how tiny this is can be gauged by comparison with the force on a falling apple, which is roughly 1 N. It is so small that it can be ignored inside atoms. However, it dominates everyday life due to the close proximity of the huge mass of the Earth and because it is only attractive. Its range is infinite. Newton's law of gravitation explains how a body falls and how the planets move around the Sun, but leaves unexplained why these events happen as they do. The gravitational force pulls objects towards each other, even though they are not in physical contact. Modern physics interprets this action at a distance as arising from an exchange of particles between the objects experiencing the force. In the case of the gravitational force the exchange particle is called the graviton . The graviton is postulated to exist, but has not been discovered. The gravitational force is a fundamental force because it operates between any two elementary particles. The Electromagnetic Force Experiments show that, sometimes, after any two different materials are rubbed together they exert forces on each other. Each has acquired an 'electric charge'. Furthermore, experiments show that there are two kinds of charge. The two kinds tend to cancel one another out and in this respect are opposite. Hence one kind is called positive and the other kind is called negative. Polythene rubbed with wool acquires a negative charge , whereas perspex (cellulose acetate) rubbed with wool acquires a positive charge. The force between two point charges is proportional to the product of the charges and is inversely proportional to the square of their distance apart. In symbols where F is the force, Q 1 and Q 2 are the charges and d is the separation distance. for air or vacuum. This relationship is called Coulomb’s law. Moving charges experience a force in a magnetic field and also create (induce) magnetic fields. The combined effect (if applicable) of the magnetic force and the coulomb electrostatic force is called the electromagnetic force. We do not directly experience the strength of the electrostatic force as individuals. The delicate balance between the negative electrons and the positive protons in our constituent atoms prevents such an experience. Suppose however that 0.1% of someone's electrons were transferred to someone else. The consequent force of attraction that these people would feel at a distance of 1 m apart can be found by applying Coulomb’s law. For simplicity, suppose the mass of each person is 50 kg and that each person is composed entirely of C-12 atoms. Now 12 grams of carbon contains electrons [No. of electrons in a carbon atom × No. of carbon atoms in one mole]. Hence the total number of electrons in each person is . Thus the number of electrons moved from one person to the other is . The force of attraction is N. This force is approximately equal to a thousandth part of the weight of the earth. It is also instructive to compare the eleectrostatic and gravitational attractions between a proton (charge +e and mass kg) and an electron (charge -e C and mass kg) placed 1 metre apart. F e = electric attraction N F g = gravitational attraction N Hence . The electromagnetic force acts between all charged particles. Its range is infinite. It is the force that binds atoms and molecules together. It is responsible for tension, compression, friction and reaction forces at the atomic level. Like gravity, it acts at a distance, with the photon acting as the exchange particle. The Strong Nuclear Force This is the very strong attractive force between nucleons, which holds the atomic nucleus together against the repulsive electrostatic forces between protons. It is also called the strong interaction. Its existence was confirmed by the discovery of the neutron. The strong force acts over a very short range. If its effects went much outside the nuclear surface, it would not be possible to explain Rutherford's alpha-particle scattering experiment solely in terms of electrostatic repulsion. In the range of internucleon separation of about 1 to 3 fm it is strongly attractive, but more or less disappears beyond 3 fm. (m) [1 fm = 1 femtometre] At distances of less than 1 fm the force must be sufficiently repulsive to prevent the nucleus collapsing. The strong nuclear force acts at a distance, as was the case with the gravitational and electrostatic forces. Imagine the nucleons as a group of dancers. If they form a ring by interlocking hands around their waists, they can continue to dance quite comfortably provided they stay within limits. If they try to pull apart, the 'force' holding them together gets stronger; if they get too close together they can no longer dance comfortably. The Weak Nuclear Force (The Weak Interaction) In 1930, on the basis of energy and momentum conservation, Pauli proposed the existence of a third particle to explain the range of energies shown by the electrons in beta emission. He offered a crate of champagne to the first person to prove the existence of this particle, which was christened the neutrino by the Italian physicist Enrico Fermi in a jocular response to a journalist's question about Chadwick's discovery of the neutron. The neutrino proved extremely elusive. Cowan and Reines finally found it in 1956. Its existence implies that there is a fourth distinct force in nature. Its interaction with matter is so rare and tenuous that this interaction cannot be explained in terms of any of the other three fundamental forces. This fourth force is called the weak nuclear force or the weak interaction. It is intermediate in strength between the gravitational and electromagnetic forces. It has a range of less than 10 -2 fm. This force also acts at a distance. This weak interaction, or force, is involved when a neutron decays to a proton, electron, and an antineutrino in the process called beta decay. Comparison of the Four Fundamental Forces Force Relative Strength Range Action Gravitational 1 all particles Weak Nuclear 10 23 10 -18 m all particles Electromagnetic 10 36 charged particles Strong Nuclear 10 38 10 -15 m protons, neutrons
Introduction to particle physics
Introduction to Particle Physics
What are the Elementary Constituents of Matter? Particle Physicists Aim to answer What are the forces that control their behaviour at the most basic level?
Conservation of energy and momentum in nuclear reactions
<ul><li>In Nuclear Reactions momentum and mass-energy is conserved – for a closed system the total momentum and energy of the particles present after the reaction is equal to the total momentum and energy of the particles before the reaction </li></ul><ul><li>In the case where an alpha particle is released from an unstable nucleus the momentum of the alpha particle and the new nucleus is the same as the momentum of the original unstable nucleus </li></ul>Conservation Laws
Neutrino must be present to account for conservation of energy and momentum <ul><li>Large variations in the emission velocities of the particle seemed to indicate that both energy and momentum were not conserved. </li></ul><ul><li>This led to the proposal by Wolfgang Pauli of another particle, the neutrino, being emitted in decay to carry away the missing mass and momentum. </li></ul><ul><li>The neutrino (little neutral one) was discovered in 1956. </li></ul>Wolfgang Pauli __
Calculate the energy released in the reaction 1.008665 u 1.007825 u 0.0005486 u 1 u = 1 J = __ kg eV
<ul><li>First artificial splitting of nucleus </li></ul><ul><li>First transmutation using artificially accelerated particles </li></ul><ul><li>First experimental verification of E = mc 2 </li></ul><ul><li>Irish Nobel Prize </li></ul><ul><li>E.T.S. Walton 1951 </li></ul>Cockroft and Walton Ernest Walton John Cockcroft
History of search for basic building blocks of nature <ul><li>Ancient Greeks: </li></ul><ul><li>Earth, Air, Fire, Wate r </li></ul><ul><li>By 1900, nearly 100 elements </li></ul><ul><li>By 1936, back to three particles: proton, neutron, electron </li></ul>
J ust as the equation x 2 =4 can have two possible solutions (x=2 OR x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. Dirac interpreted this to mean that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron, for instance, there should be an "antielectron" called the positron identical in every way but with a positive electric charge.
History of Antimatter 1928 Dirac predicted existence of antimatter 1932 antielectrons (positrons) found in conversion of energy into matter 1995 antihydrogen consisting of antiprotons and positrons produced at CERN In principle an antiworld can be built from antimatter Produced only in accelerators and in cosmic rays