Lesson 1 (1) Structure of atoms. An atom consists of a nucleus and electrons. The nucleus is composed of nucleons, of which there are two kinds: protons and neutrons. The mass of proton is about the same as that of a neutron: m p = 1.67 !10 "27 kg Mass of an electron is much smaller: me = 9.1!10 "31 kg The size of an atom is about 10 !10 m , while that of a nucleus is about 10 !15 m . A proton carries an electric charge of +1 unit, and electron carries -­‐1 unit. A neutron carries no charge. In SI unit system, quantity of charge is measured in C (Coulomb). The proton charge is e = 1.6 !10 "19 C The attractive force on the electrons from the nucleus causes the electrons to go around the nucleus like planets surrounding the sun. However, according to quantum mechanics, it is more correct to imagine the electrons like clouds. The atomic number Z of an atom is the number of protons. It determines what element X the atom belongs to. The number of neutrons N determines the different isotopes of the element. A = Z + N is called the mass number. A nucleus (nuclide) can be denoted by Z X A . Examples: 6 C12 , 92 U 238, 92 U 235 An atom has equal number of electrons and protons, and is therefore electrically neutral. When it loses one of more electrons, it becomes a positive ion, and when it gains one or more electrons, it becomes a negative ion. Examples: Na +, Al +++, S !! . Atoms cam combine together to form molecules, such as H 2O . Aggregate solid or liquid matter consists of atoms or molecules bonded together. In a conductor, mostly metals, the atoms are partially ionized. The resulting positive ions form a crystal lattice, and the free electrons can move around the solid. An insulator does not have free electrons. Dielectric is an insulator in which the charge distribution of the electron cloud in the molecule is skewed, so that the molecule is an electric dipole: one end is slightly negative and the other slightly positive. 1 Example: Calculate the number of molecules and the charge carried by all electrons in 1.0 kg of water. Solution: Mass number of hydrogen 1 H 1 = 1 , mass number of oxygen atom 8 O16 =16 Molecular mass of a single H2O molecule= 2 !1+16 = 18 Mass of a single H2O molecule = 18 !1.67 !10 "27 kg 1.0
Number of molecules in 1.0 kg of water = = 3.3!10 25 "27
18 !1.67 !10
Number of electrons in each molecule = 2 !1+ 8 = 10 Total charge carried by the electrons = 10 ! 3.3!10 25 ! ("1.6 !10 "19 ) C = 5.3!10 7 C (2) Charging An object becomes charged when electrons or positive ions are removed from it. This can be accomplished through friction. When two objects are rubbed against each other, charge transferred take place. As a result, one becomes positively charged while the other becomes negatively charged to the same degree. The sum of charge remains zero if the original objects are both neutral. In any process involving charge transferred among different objects, we have conservation of charge: ! Q = ! Q before
after
where Q denotes the charge on the objects. When a glass rod is rubbed against silk, the charge it carries is taken to be positive according to a convention adopted by Benjamin Franklin. In processes where elementary particles can be created or destroyed, the law of charge conservation is more far reaching. Examples are neutron decay and electron-­‐positron annihilation n ! p + e" + ! e+ + e! " 2! 2 (3) Induction When a positively charged object is brought near a piece of metal without touching, the free electrons in the metal are attracted to the object, leaving the positive ions behind. As a result, the end of the metal closer to the object becomes negatively charged and the opposite end becomes positively charged. The metal becomes polarized. The process is called induction. Charges are induced on the two ends. If the metal is light, it can be attracted to the object, because the attractive force on the near end is stronger than the repulsive force on the far end. Polarization occurs also when a negatively charged object is brought near the metal. When the object is moved away from the metal, the free electrons move back and the metal becomes once again neutral. It is possible to charge up the metal after the object is moved away in the following away. While the object is still near the metal, provide a conducting path from the metal to the earth (grounding). The metal and the earth now form a single conductor, with the earth being the far end, which is now charged up positively while the metal itself is negatively charged. Now break the contact between the metal and the earth. The metal will remain negatively charged after the object is moved away. Induction also occurs when a charged object is brought near an insulator. If the latter is a dielectric, the molecular dipoles will tend to be lined up as shown when a positively charged object is brought near. Again, the induced charge is negative on the near end and positive in the far end. If the insulator is light, it will again be attracted to the object. However in this case, the insulator cannot be made to charge up upon removal of the object by first grounding it, because there is no flow of free electrons. If the insulator is not a dielectric, the electron cloud surrounding each molecule can be shifted by the charged object, causing the molecule to become an induced dipole. These dipoles can then be lied up to cause induced charge on the near and far ends. 3