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Topic 7.1 Extended D –Nuclear Structure and Nuclear Force Almost everything we observe in the world around us is sensed by the electromagnetic and the gravitational forces. Your seeing an object is because of the interaction between photons at the electrons orbiting atoms. Your feeling an object is because of the interaction between the electrons in your atoms responding to the electrons in the object you are feeling. Of course, weight is the other factor you can sense in an object. There are four fundamental forces in the universe. You have studied two of them in detail - namely the gravitational force (last year) and the electromagnetic force (this year). The next slide shows the relative magnitudes of the four forces of nature: FYI: In this chapter and the next, we will be studying the strong and the Topic 7.1 ofExtended weak forces, completing our overview all the fundamental forces of nature. D –Nuclear Structure and Nuclear Force ELECTRO-WEAK STRONG ELECTROMAGNETIC WEAK GRAVITY + + nuclear force STRONGEST Range: Extremely Short Force Carrier: Gluon light, heat and charge radioactivity freefall WEAKEST Range: Range: Short Force Carrier: Photon Range: Force Carrier: Graviton FYI: An alpha particle () is a double-positively charged particle emitted Topic Extended by radioactive materials such as 7.1 uranium. D –Nuclear Structure and Nuclear Force observing the spectra. But how do we study the nucleus? In 1897 British physicist J.J. Thomson discovered the electron, and went on to propose a "plum pudding" model of the atom in which all of the electrons were embedded in a spherical positive charge the size of the atom: In 1911 British physicist Ernest Rutherford conducted experiments on the structure of the atom by sending alpha particles through gold leaf. If the atomic structure was as Thomson said, a beam of particles should barely be deflected as it passed through the atom. Rutherford's experimental results were quite different, as the next slide will show: scintillation screen We studied the electrons surrounding the nucleus by Topic 7.1 Extended D –Nuclear Structure and Nuclear Force Here we see that the deflections are much more scattered... The nucleus Rutherford proposed that the positive charge of the atom was located in the center, and he The atom coined the term nucleus. "On consideration, I realized that this scattering backward must be the result of a single collision, and when I made the calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus." FYI: By using known values for both mass and speed of the particle, Topic 7.1 and the atomic number of the atom usedExtended in the experiment, it was found thatDthe–Nuclear radius of the nucleus was of the order 10-12 m! Structure andof Nuclear Force FYI: This, by the way, is an upper limit on the size. Why? In fact, if we look at a head-on collision between an alpha particle and a nucleus, we can obtain a rough value for the diameter of a nucleus. The charge of the alpha particle is 2e, and the charge of the nucleus is Ze, where Z is the atomic number of the element. The alpha particle feels a coulomb potential caused by the nucleus given by kQ kZe = V = r r If the particle approaches from infinity, the work required to stop it so that it reverses is given by 2 W = qV = 2eV = 2ekZe = 2kZe rmin rmin where rmin is the closest approach of the particle to 0 the nucleus. From the work-kinetic energy theorem, W = K = K - K0 2 2 mv 2kZe so that = or 4kZe2 Minimum Radius rmin = 2 rmin of a Nucleus mv2 Topic 7.1 Extended D –Nuclear Structure and Nuclear Force All right. Suppose we have two protons in a nucleus. Then we have a huge problem. What is it? Estimate the force between two protons in a helium atom. The two positive charges feel the coulomb force: 9109(1.6 10-19)2 kQq = 0.00023 N F = 2 = (110-12)2 r The two positive charges will want to accelerate apart with an acceleration of F 0.00023 = 1.381023 m s-2 = a = m 1.6710-27 Since most atoms do not spontaneously disintegrate we can postulate that there must be a very strong attractive nuclear force which is able to counter the repulsive electric force. Since even in crystals (in which nuclei are very close together) the nuclei of the nearby atoms are not attracted to each other, the nuclear force must be very short ranged. FYI: Thus there are isotopes of hydrogen, all having the same (+1) Topic 7.1 Extended nucleus, but different numbers of neutrons. A neutron is slightly more D –Nuclear and Nuclear massive then a proton.Structure In fact, we can compare the masses ofForce the electron, proton, and neutron: So what, exactly, a = 1840 electron masses Mass ofis proton nucleus comprised Mass ofof? neutron = 1841 electron masses Perhaps you recall the FYI: Hydrogen has three common isotopes: instrument called the Hydrogen has one proton and no neutrons in the nucleus. mass spectrometer. Deuterium has one proton and one neutron. An element is ionized, Tritium has one proton and two neutrons. and accelerated by an All forms have a single applied voltage in electron. the chamber S. It is then projected into a known perpenThrough use of such an dicular magnetic field. instrument, scientists The radius is discovered different detected, and the mass masses for hydrogen calculated: nuclei, and postulated r = mv qB rqB m = v the existence of the neutrally-charged neutron. FYI: A = Z + N, so that nuclear reactions do not generally include the N Topic 7.1 Extended values (since it can be calculated). D – Nuclear Structure and Nuclear Force FYI: Since periodic tables are readily available, we can even get away with just showing the A number. An element's characteristics are determined by the FYI: Tritium is unstable. But is stable enough to form heavy number of electrons it deuterium has (chemistry). D2O. Note use of the D the special isotope symbol. water The number of the electrons isfordetermined by the number of (opposites attract). FYI:protons A particular isotope of an element is called a SPECIES or a NUCLIDE. ThereforeThus it we follows that isotopes of anof element have three species, or nuclides, hydrogen. behave chemically the same. There are 6 nuclides of carbon: FYI: In chemistry, you need only a chemical symbol, such 11 12 13C 14C 15C 16C C Chydrogen. as H, to represent For In the nuclear physics,weyou distinguish the higher elements, don'tneed have to special names like we do for isotopes. hydrogen. We name these isotopes like this: carbon-12, carbon-14, etc. Only carbon-12 and carbon-13 are stable. NUCLEAR PHYSICS CHEMISTRY H Mass Number = A H H Protons = Z 1 1 H 0 hydrogen 2 1 N = Neutrons 1 deuterium 3 1 H 2 tritium