Properties of Objects. Quantization of Charge. Conservation of Charge. Conductors, Insulators and Semiconductors. How to Put Charge on an Object

Transcription

1 Properties of Objects Fundamental Particles Atoms and Ions Quantization of Conservation of Conductors, Insulators and Semiconductors Polarization How to Put on an Object 1 Physical Properties That which is proper to anything; a peculiar quality of a thing; that t which his inherent in a subject, or naturally essential to it; an attribute; as, sweetness is a property of sugar. Physical properties, or those which result from the relations of bodies to the physical agents, light, heat, electricity, gravitation, cohesion, adhesion, etc., and which are exhibited without a change in the composition or kind of matter acted on. Examples of physical properties of objects include 2 Color Mass Hardness

2 Intrinsic Physical Properties An intrinsic physical property is a property of a set of objects that is constant throughout the entire set over space and time. Examples of intrinsic physical properties of an NFL football include material shape weight color 3 texture size Intrinsic Physical Properties of Fundamental Particles Everything in the universe is made up of a finite number of fundamental particles. Each of these particles has the following intrinsic physical properties. size mass charge 4 spin

3 There are only two types of charge called positive and negative Unlike charges attract and like charges repel. 5 Why positive and negative? Imagine what it would be like for the types of charges to be named in a different way. Which of these would best lead to an understanding of the difference between these types of charge and why? A and B Good and Evil Left and Right Jack and Jill 1 Good + 2 Evil =? 6

4 The universe is made of 28 fundamental particles 6 quarks, 6 leptons, their antiparticles and some force particles. All figures shown here are from Lawrence Berkley National Laboratory The Particle Adventure 7 Most charge that we experience is found in the quarks and the electron. The fundamental unit of charge is e = Coulombs (C) All figures shown here are from Lawrence Berkley National Laboratory The Particle Adventure Each quark carries a fraction of this charge. Electrons have a charge of -e 8

5 Fundamental Quarks are only typically in combinations of three called protons and neutrons All figures shown here are from Lawrence Berkley National Laboratory The Particle Adventure 9 A proton contains 1 down and 2 up quarks (+e charge) A neutron contains 1 up and 2 down quarks (0 charge) An electron is a fundamental particle (-e charge) found on most objects comes typically from one place The object has either too many or too few electrons. 10 Positive charge is created when an atom loses an electron. This is called a positive ion. Negative charge is created when an atom gains an electron. This is called a negative ion.

6 All ordinary matter consists of Protons Mass = 1.67x10-27 kg = +e Neutrons Mass = 1.67x10-27 kg = 0 Electrons Mass = 9.11x10-31 kg = -ee All ordinary matter must therefore have a total charge which is an integer multiple of e. is designated by either Q or q. 11 Q = N protons ( + e) + N ( e) electrons Example 1 Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6.3 µc of charge. How many excess electrons did the balloon receive from Dr. Mike? Answer The number is given by N electrons in the equation from the previous slide. N electrons 6 Q C 13 = = = e C 12

7 Example 2 Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains µc of charge. By how much did Dr. Mike s weight change? Answer In Example 1, we calculated that the balloon received 3.93 x electrons. The weight of these additional electrons is then found as follows. M = N m electrons electron ( electrons ) ( kg) = = kg 13 ( ) ( ) w= Mg = = kg 9.81 m/s N Example 3 Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. His hair loses 3.5 x electrons. How much charge does Dr. Mike have when he is finished? Answer The charge is given by Q in the equation from the previous slide. 14 protons ( ) ( ) Q = N e + N e electrons ( ) -19 (-. ) = protons C + 0 electrons C = = C 5.6 mc

8 Conservation The amount of charge in the universe never changes. Example Dr. Mike puts electrons on a balloon by rubbing it in his hair. The balloon gains -6.3 µc of charge. How much charge does Dr. Mike s hair have after he has done this? Answer 15 The electrons moved from Dr. Mike to the balloon. Thus, the charge that move to the balloon was taken away from Dr. Mike and he has +6.3 µc of charge more than he had when he began. The total charge in the universe is still the same as it was when he began. You can easily determine the number of electron and protons in a mole of some element. Example How much negative electron charge is in 3 moles of oxygen? Answer 16 There are Avogadro s number of oxygen atoms in one mole. Using the periodic table (look it up on the web or in your chemistry textbook), we see that there are 8 electrons in on oxygen atom. And there is e charge in each electron. So we have atoms electrons C Q = mole oxygen atom electron 6 Q = C Note: There are just as many protons. So 3 moles of oxygen is actually neutral ( 3 moles)

9 In an insulator, all of the electrons are bound to a nucleus. 17 In a conductor, some of the electrons are free to move between atoms. 18 These electrons stay on the surface of a conductor.

10 In a semiconductor, the electrons cannot move unless given a high enough energy. They are insulators when the energy is low. 19 and conductors when the energy is high A photoconductor is a semiconductor that uses light as its input energy. They are insulators when the light energy is low. 20 and conductors when the light energy is high

11 Dipoles Dipoles are pairs of equal positive and negative charge that can rotate but not translate. -q +q The intrinsic physical properties of the dipole are its charge and its length. (It also has mass and spin but we will almost never speak of the mass or spin of a dipole.) The dipole is sometimes described by its dipole moment. The magnitude of the dipole moment is given by the equation p = ql L p r 21 and it points from positive to negative in direction. Polarization Dipoles always attract charged objects. 1. A negatively charged object causes the dipole to rotate so that the positive charge is closer to the conductor. 2. A positively charged object causes the dipole to rotate so that the negative charge is closer to the conductor. 22

12 Inductors and conductors sometimes exchange charge by rubbing them together. Use the Tribolectric Table to know which way the charges move. 23 In conductors, electrons are free to move There are two ways to add charge to a conductor. 1. Touching a charged rod to a conducting sphere transfers electrons to the sphere. A negative total charge remains What would happen if the rod were positive to begin with? 24 This is called charging by contact.

13 In conductors, electrons are free to move There are two ways to add charge to a conductor. 2. If a charged rod is simply brought near a conducting sphere, the electrons will begin to move. And, some will go into the ground. What would happen if the rod were positive to begin with? A positive total charge remains 25 This is called charging by induction. In insulators, electrons are not free to move. They stick to the atom Insulators can shift there charges and will therefore attract any charged conductor. 1. A negative conductor repels 2. A positive conductor attracts the electrons and is then attracted by the closer protons. the electrons and is then attracted by these closer electrons. 26

14 Dipoles are pairs of positive and negative charge that can rotate but not translate. Like insulators they always attract the conductor. 1. A negative conductor causes the dipole to rotate so that the positive charge is closer to the conductor. 2. A positive conductor causes the dipole to rotate so that the negative charge is closer to the conductor. 27 Properties of Objects Fundamental Particles Atoms and Ions Quantization of Conservation of Conductors, Insulators and Semiconductors Polarization How to Put on an Object 28