Physics 212 Electric Charge TOC Properties of Objects

Physics 212 Electric Charge TOC Properties of Objects Fundamental Particles Atoms and Ions Quantization of Charge Conservation of Charge Conductors, Insulators and Semiconductors Polarization How to Put Charge on an Object 1

Physics 212 Electric Charge Physical Properties TOC That which is proper to anything; a peculiar quality of a thing; that which is 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. http: //www. wordiq. com/dictionary/Properties Examples of physical properties of objects include… 2 Color Mass Hardness

Physics 212 Electric Charge Intrinsic Physical Properties TOC 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… shape material weight 3 texture color size

Physics 212 Electric Charge Intrinsic Physical Properties of Fundamental Particles TOC 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 4 charge mass spin

Physics 212 Electric Charge TOC There are only two types of charge called positive and negative Unlike charges attract and like charges repel. 5

Physics 212 Electric Charge Why positive and negative? TOC 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 6

TOC All figures shown here are from Lawrence Berkley National Laboratory “The Particle Adventure” 7 Physics 212 Electric Charge The universe is made of 28 fundamental particles 6 quarks, 6 leptons, their antiparticles and some force particles.

Physics 212 Electric Charge TOC Most charge that we experience is found in the quarks and the electron. The fundamental unit of charge is 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

Physics 212 Electric Charge Fundamental Charge TOC 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)

Physics 212 Electric Charge found on most objects comes typically from one place… TOC 10 The object has either too many or too few electrons. Positive charge is created when an atom loses an electron. Negative charge is created when an atom gains an electron. This is called a positive ion. This is called a negative ion.

Physics 212 Electric Charge All ordinary matter consists of … TOC Protons Neutrons Electrons Mass = 1. 67 x 10 -27 kg Mass = 9. 11 x 10 -31 kg All ordinary matter must therefore have a total charge which is an integer multiple of e. Charge is designated by either Q or q. 11 Charge = +e Charge = 0 Charge = -e

Physics 212 Electric Charge Example 1 TOC 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 Nelectrons in the equation from the previous slide. 12

Physics 212 Electric Charge Example 2 TOC Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. The balloon gains -6. 30 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 1013 electrons. The weight of these additional electrons is then found as follows. 13

Physics 212 Electric Charge Example 3 TOC Dr. Mike puts electrons on a neutral balloon by rubbing it in his hair. His hair loses 3. 5 x 1016 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

Physics 212 Electric Charge Conservation TOC 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 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. 15

Physics 212 Electric Charge TOC 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 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… 16 Note: There are just as many protons. So 3 moles of oxygen is actually neutral.

Physics 212 Electric Charge TOC 17 In an insulator, all of the electrons are bound to a nucleus… even the excess charges which stay right where they were forced to be by friction.

Physics 212 Electric Charge In a conductor, some of the electrons are free to move between atoms. TOC 18 Excess electrons will move freely as well, but they will stay on the surface of the conductor.

Physics 212 Electric Charge TOC 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

Physics 212 Electric Charge TOC 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

Physics 212 Electric Charge Dipoles TOC Dipoles are pairs of equal positive and negative charge that can rotate but not translate. -q L +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 21 and it points from positive to negative in direction.

Physics 212 Electric Charge Polarization TOC Dipoles always attract charged objects. 1. 22 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.

Physics 212 Electric Charge Inductors and conductors sometimes exchange charge by rubbing them together. TOC Use the Tribolectric Table to know which way the charges move. 23

Physics 212 Electric Charge In conductors, electrons are free to move… TOC 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 24 This is called charging by contact. What would happen if the rod were positive to begin with?

Physics 212 Electric Charge In conductors, electrons are free to move… TOC 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. A positive total charge remains 25 What would happen if the rod were positive to begin with? This is called charging by induction.

Physics 212 Electric Charge In insulators, electrons are not free to move. They stick to the atom… TOC Insulators can shift their charges and will therefore attract any charged conductor. 1. 26 A negative conductor repels 2. the electrons and is then attracted by the closer protons. A positive conductor attracts the electrons and is then attracted by these closer electrons.

Physics 212 Electric Charge Dipoles are pairs of positive and negative charge that can rotate but not translate. TOC Like insulators they always attract the conductor. 1. 27 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.

Physics 212 Electric Charge TOC Properties of Objects Fundamental Particles Atoms and Ions Quantization of Charge Conservation of Charge Conductors, Insulators and Semiconductors Polarization How to Put Charge on an Object 28