Integrated Science I ELECTRICITY

Transcription

1 Integrated Science I ELECTRICITY Natural Electricity Electricity occurs naturally around us form the lightning in the sky to the small shock you give someone from rubbing you shoes across the carpet. Static Electricity Electrostatics Electricity at rest A positively charged nucleus is surrounded by electrons. Early 1900 s Niels Bohr & Ernest Rutherford The protons in the nucleus: attract the electrons hold them in orbit like our sun holding the planets in orbit around it. Electrons are: attracted to the protons repelled by other electrons Page 1

2 Charge Attracting and repelling is a property called CHARGE. Electrons: negatively charged Protons: positively charged Neutrons: no charge Important facts about atoms: Basic Atomic Rules 1) Every atom has a positively charged nucleus surrounded by negatively charged electrons. 2) All electrons are identical; each has the same mass and the same quantity of negative charge. 3) The nucleus is composed of protons and neutrons. A proton has about 1830 times the mass of an electron, but its positive charge is equal to the negative charge of the electron. A neutron has slightly greater mass than a proton and has no charge. Zero Net Charge 4) Atoms usually have a as many electrons as protons, so the atom has a zero net charge. Page 2

3 Fundamental Law Like charges repel; opposite charges attract. Ion A charged atom is said to be a ion. A positive ion has a positive charge or is said to have lost one or more electrons or gained one or more protons. A negative ion is said to be negatively charged if it has lost one or more protons or gained one or more extra electrons. The gain or loss of electrons comes from the outer most shell of the atom. Carpets & Electrical Shock If you walk across the carpet or scrape your shoes across the carpet while standing, are you positively or negatively charged? You probably will have fewer electrons on your feet so you will be positively charged while the carpet may become negatively charged. Page 3

4 Conductor Materials in which the electrons can move freely about the atoms make good conductors. The electrons are very loosely bound in the outer orbits and are free to roam around in the material. Metals are good conductors of electricity and heat. Insulator Materials such as glass and rubber have electrons which are very tightly bound. The electrons are not free to roam about the material, thus they are very poor conductors of electricity and heat. Semiconductor Materials such as germanium and silicon, are good insulators in their pure crystalline form but increase greatly in conductivity when even one atom in ten million is replaced with an impurity that adds or removes an electron from the crystalline structure. Thus, these materials are called Semiconductors. At temperatures near absolute zero, there are Page 4

5 Superconductor certain metals, which acquire infinite conductivity (zero resistance to the flow of a charge). These are called superconductors. Charging By Conduction Electrons can be transferred from one material to another by simply touching. When a charged rod is placed in contact with a neutral object, some of the charge will transfer to the neutral object. This method of charging is simply called "Charging By Conduction." Charging By Induction If a charged object is brought close to a conducting surface, even with out physical contact, electrons will move in the conducting surface; similar to a set of magnets. Thus, a charge may be exchanged with out any physical contact between the objects. This is called; charging by induction. Electric charge is measured in units called, coulombs. A charge of one coulomb (1 C ) Page 5

6 Coulomb is equal to the combined charge of 6.25 billion billion (6.24 X ) electrons. This represents the amount of charge that passes through a common 100-W light bulb in about 1 second. Charge Electric Potential Energy Every single electron has a charge of 1.6 X C. Every proton has a charge of +1.6 X C, this is why the charges can cancel each other out. When an object is charged, the electrons in the object have the ability to do work. If you put the North Pole of two magnets together you would have to force them close to each other. Thus, you do work. When you release one of the magnets it will move and work is done. The energy in moving electrons is called electric potential energy. Voltage is the amount of electrical potential energy per coulomb of charge. Heat flows from higher temperature to the side of the container with a lower Page 6

7 temperature until both are at equilibrium (same temperature). When equilibrium is reached (both containers have the same temperature) the flow of heat stops. The difference in electrical potential difference causes a flow of energy (electrons) or charge from higher potential to lower potential. This is the process in a common battery. The charge flows when there is a potential difference, or voltage difference. Potential Difference The flow of charge will continue until both objects reach a common potential. When there is no potential difference, no flow of charge will occur. Electric current is simply the flow of electric charge. In solid conductors, it is the electrons that carry the charge through the circuit. Electrons are free to move throughout the atomic network. These electrons are called Page 7

8 conduction electrons. Protons, on the other hand, are bound inside atomic nuclei that more or less locked into place. Electric current is measured in Amperes, symbol A. The ampere is named for the French Scientist Andre Marie Ampere ( ). Conduction Electrons An ampere is the measurement of the rate of flow ( I ) which is one coulomb of charge per second. The amount of current that flows in a circuit depends on the electrical potential difference (voltage) and the resistance that the conductor offers to the flow of charge, or the electrical resistance. This is similar to the rate of water flow in a pipe, which depends not only on the pressure behind the water but on the Page 8

9 resistance offered by the pipe itself. Electrical Resistance The resistance of a wire or object depends upon the conductivity (how well it conducts electricity or how well electrons flow through it) of the material in which the wire is made of and also the thickness and length of the wire. Electrical resistance is measured in units called, Ohm s (_) in honor of George Simon Ohm, a German Physicist who tested different wires in circuits to see what effect the resistance of the wire had on the current. The OHM Ohm discovered a relationship between voltage, current and resistance. Current = voltage resistance This relationship between voltage, current, and resistance is called Ohm s Law. The relationship between units of measurement for these three quantities is: Page 9

10 or more commonly as 1 ampere = 1 volt ohm I = V R As an example, consider a normal vacuum cleaner, which uses 15.0 amps of current with 120 V. Using the above equation calculate the resistance to the flow of electrons that the vacuum cleaner does. I = V R R = V I R = 120 V / 15.0 A = 8.0 _ Page 10

11 21:3 Conductors, Insulators, and Semiconductors 21:6 Coulomb's Law What equation have we used in the past that allowed us to calculate the attractive force between two objects? Newton's Law of Gravitational Force between two objects of mass m 1 and mass m 2, where G is the universal gravitational constant. F = G m 1 m 2 d 2 Page 11

12 The electrical force between any two objects obeys a very similar inverse-square relationship with distance. This relationship was discovered by the French physicist Charles Coulomb ( ) in the eighteenth century. This relationship is called Coulomb's Law in honor of its discoverer. The law states that for charged particles or objects that are small compared to the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. This law is expressed as: F = Kq 1 q 2 d 2 where d is the distance between the charged particles; q 1 represents the quantity of charge of one particle and q 2 the quantity of charge on the other particle; and K is the proportionality constant ( 9.0 X 10 9 N m 2 /C 2 ), similar to the G in Newton's Gravitational constant, but the electrical proportionality constant K is a very large number. Rounded off, it equals ( k = N m 2 /C 2 ). The units N m 2 /C 2 convert the right hand side of the equation to the unit force, the newton (N), when the charges are in coulombs (C) and the distance in meters (m).the SI unit of charge is the coulomb, abbreviated C. 21:7 The Unit Charge: The Coulomb Common sense may say that it is the charge of a single electron, but it is not! For historical reasons, it turns out that a charge of 1 C is the charge of 6.25 billion billion (6.24 X ) electrons. This might seem like a huge number of Page 12

13 electrons, but it represents only the amount of charge that passes through a common 100-W light bulb in about 1 second. Unlike Newton's law gravitation for masses which is an attractive force, electrical force are much stronger and may attract or repel. To aid in understanding the strength of electrical attraction consider the following: EXAMPLE: The hydrogen atom has the simplest structure of all atoms.its nucleus is a proton (mass 1.7 X kg), outside of which there is a single electron (mass 9.1 X kg) at an average separation distance of 5.3 X m. Compare the electrical and gravitational forces between the proton and the electron in the hydrogen atom. FOR THE ELECTRICAL FORCE distance proton charge electron charge d = 5.3 X m qp = +1.6 X C qc = -1.6 X C The electric force F c is: F c = K q c q p d2 = (9.0 X 109 N m2/c2) (((1.6 X C)2)) / ((5.3 X m )2) Page 13

14 = 8.2 X 10-8 N The gravitational force F g between them is: F g = G m c m p d2 = (6.7 X N m2/kg) ((9.1 X kg)(1.7 X kg)) (5.3 X m)2 = 3.7 X N A comparison of the two forces is best shown by their ratio: F c = 8.2 X 10-8 N = 2.2 X 1039 F g = 3.7 X 10-47N Thus, the electrical force is more than times greater than the gravitational force. In fact, the electrical forces are so strong that the gravitational forces become almost non-existent. 21:8 Forces On Neutral Bodies Read, has to do with how photocopiers work Page 14

15 Electricity in one form or another is involved with almost everything around us. From the lightning in the sky, to the spark beneath your feet as you drag your foot across the carpet. The basis of these natural electrical charges found in the forces which bind atoms together to form molecules. Technology has taken these natural phenomenon and harness their power through understanding and knowledge to bring us to today, The Age of Technology. This New Age has brought the human race to experience the most prosperous of times in human existence, never known before recent times. Today, we will be beginning our understanding ELECTROSTATICS, or electricity at rest. 21:1 The Electrical Atom By now, everyone is familiar with the attractive force of gravity by the earth, which effects us in terms of our weight. But now consider a force acting on you which is billions upon billions of times stronger. A force this strong which has the capabilities of squeezing your entire body to a thickness of less a sheet of paper. What keeps this force from doing that is the addition of another force, of the same strength, but acting opposite in the direction of the first. This second force would like explode every atom in your body as far away from every other atom as possible. Yet, we exist because these two forces acting on our bodies are balanced, these forces are electrical forces. Electrical forces come about from the existence of particles in atoms. In a very simple model of an atom proposed by Ernest Rutherford and Niels Bohr in the early 1900's, a positively charged nucleus is Page 15

16 surrounded by electrons. The protons in the nucleus attract the electrons and hold them in orbit, just as the sun holds the planets in orbit around it. Electrons are attracted to the protons, but are repelled by other electrons. This attracting and repelling is attributed to a property called CHARGE. Electrons are negatively charged and protons are positively charged. Neutrons have no charge, and are neither attracted or repelled by charged particles. Some important facts about atoms are: 1) Every atom has a positively charged nucleus surrounded by negatively charged electrons. 2) All electrons are identical; each has the same mass and the same quantity of negative charge. 3) The nucleus is composed of protons and neutrons (The common form of hydrogen is the exception, it has no neutrons). All protons are identical; all neurons are identical. A proton has about 1830 times the mass of an electron, but its positive charge is equal in magnitude to the negative charge of the electron. A neutron has slightly greater mass than a proton and has no charge. 4) Atoms usually have a as many electrons as protons, so the atom has a zero net charge. QUESTION: WHY DON'T PROTONS PULL THE OPPOSITELY CHARGED ELECTRONS INTO THE NUCLEUS? Page 16

17 ANSWER: Electrons are not pulled into the nucleus for the same reason that the earth is not pulled into the sun by gravitational forces. The electrons are in motion and overshoot the nucleus but are held in orbit by the pull of the protons. This is extremely oversimplified but is a starter for a more in depth look into the electrical nature of atoms. An electron is really a wave rather than an orbiting particle. It is not understood why electrons repel electrons and are attracted to protons. We simply say that it is one of the fundamental Law's of Nature. This fundamental Law can also be explained with the following: Like charges repel; opposite charge attract. 21:2 Transferring Electrons Both electrons and protons have electric charges. In a neutral atom the amount of negative charge is balanced by an equal amount of positive charge. Thus, there is no net charge. But, if an electron were to be removed from an atom, the atom would no longer be neutral. It would now have one more positive charge (proton) than negative (electron) charge. It would said to be positively charged. A charged atom is said to be a ion. A positive ion has a positive charge or is said to have lost one or more electrons. A negative ion is said to be negatively charged or has lost Page 17

18 one or more protons. Although the inter-most electrons of an atom are bound extremely tightly around the nucleus, the outer most electrons may be freely dislodged. The amount of force required to strip off electrons from atoms depends upon the type of material be considered. As an example, the electrons in a rubber rod are held in place much more firmly than those in a piece of fur. Thus, if you were to rub the rubber rod with a piece of fur, electrons will transfer from the fur to the rubber rod. the rubber rod now has an excess of electrons and is negatively charged. The fur, now has fewer electrons than protons and is now positively charged. If you rub a glass or plastic rod with silk, you will find that the rod becomes positively charged. the silk has a greater affinity for electrons than the glass or plastic rod. Electrons are rubbed off the rod and onto the silk. QUESTION: IF YOU SCRAPE YOUR SHOES ACROSS THE CARPET WHILE WALKING, ARE YOU POSITIVELY OR NEGATIVELY CHARGED? ANSWER: YOU WILL HAVE FEWER ELECTRONS AFTER YOU SCRAPE YOUR FEET ON THE CARPET WHILE WALKING, THUS YOU WILL BE POSITIVELY CHARGED AND THE CARPET WILL BE NEGATIVELY CHARGED. 21:3 Conductors, Insulators, and Semiconductors Page 18

19 Electrons can move more freely in some materials as opposed to others. Such is the case for most metals. The atoms are very loosely bound in the outer orbits and are free to roam around in the material. Because of this "electron freedom" the material is said to be a good "conductor." Metals are good conductors for the motion of electric charges and for heat. Other materials such as glass and rubber have electrons which are very tightly bound and remain attached to particular atoms. The electrons are not free to roam about the material, thus they are very poor conductors of electricity and our poor conductors of heat. They are what is known as good Insulators. Any material can be classified by the way in which it transfers electric charges. Those that transmit electric charges very well are call conductors and those that do not are called insulators. The conductivity of a metal can be more than a million trillion times greater than the conductivity of an insulator such as glass. It is for this reason than electricity contained in a powerline can flow hundreds of miles along in a bundle of wires more easily than in a few centimeters of a conductor. The classification of conductor or insulator depends upon how tightly the atoms which make up the substance hold onto their electrons. Some materials such as germanium and silicon, are good insulators in their pure crystalline form but increase greatly in conductivity when even one atom in ten million is replaced with an impurity that adds or removes an electron from the crystalline structure. Thus, these materials are called Semiconductors. Thin layers of semiconducting materials sandwiched together make up transistors, which are used in a wide variety of electrical applications. Page 19

20 At temperatures near absolute zero, there are certain metals which acquire infinite conductivity (zero resistance to the flow of a charge). These are called superconductors. Since 1987, superconductivity at high temperatures (above 100 k) has been found in a variety of nonmetallic compounds. Once electric current established in a superconductor, the electrons flow indefinitely. Explanations for this occurrence are being researched. 21:4 Forces and Charged Bodies We are familiar with the electrical effects produced by friction. On very windy days one may actually see sparks fly from their hair as they pass a plastic comb through it and hear the sparks. Or you can scuff your shoes across a carpet and feel the small electrical shock as you touch someone or something else. In all of these examples electrons are being transferred by friction when one material rubs against another. Electrons can be transferred from one material to another by simply touching. When a charged rod is placed in contact with a neutral object, some of the charge will transfer to the neutral object. This method of charging is simply called "Charging By Conduction." OVERHEAD: FIRST DATE WITH CLEO AT AUTOMOVIES AND FIRST REAL PHYSICS EXPERIMENT. 21:5 Charging By Induction If a charged object is brought close to a conducting surface, even with out physical contact, electrons will move Page 20

21 in the conducting surface. Consider, the two insulated metal spheres. 1. In the first drawing, the uncharges spheres touch each other, so in effect they form a single noncharged conductor. 2. In the second drawing a negatively charged rod is near sphere A. Electrons in the metal are repelled by the rod, and an excess negative charge has moved onto sphere B, leaving sphere A with excess positive charge. The charge on the two spheres has been redistributed. A charge is said to have been induced on the spheres. 3. In the third diagram spheres A and B are separated while the rod is still present. 4. In this diagram the rod has been removed. The spheres are charged equally and oppositely. They have been charged by induction. Since the rod never touched the spheres it retains its original charge. 21:6 Coulomb's Law What equation have we used in the past that allowed us to calculate the attractive force between two objects? Newton's Law of Gravitational Force between two objects of mass m 1 and mass m 2, where G is the universal gravitational constant. F = G m 1 m 2 d 2 Page 21

22 The electrical force between any two objects obeys a very similar inverse-square relationship with distance. This relationship was discovered by the French physicist Charles Coulomb ( ) in the eighteenth century. This relationship is called Coulomb's Law in honor of its discoverer. The law states that for charged particles or objects that are small compared to the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. This law is expressed as: F = Kq 1 q 2 d 2 where d is the distance between the charged particles; q 1 represents the quantity of charge of one particle and q 2 the quantity of charge on the other particle; and K is the proportionality constant ( 9.0 X 10 9 N m 2 /C 2 ), similar to the G in Newton's Gravitational constant, but the electrical proportionality constant K is a very large number. Rounded off, it equals ( k = N m 2 /C 2 ). The units N m 2 /C 2 convert the right hand side of the equation to the unit force, the newton (N), when the charges are in coulombs (C) and the distance in meters (m).the SI unit of charge is the coulomb, abbreviated C. 21:7 The Unit Charge: The Coulomb Common sense may say that it is the charge of a single electron, but it is not! For historical reasons, it turns out that a charge of 1 C is the charge of 6.25 billion billion (6.24 X Page 22

23 10 18 ) electrons. This might seem like a huge number of electrons, but it represents only the amount of charge that passes through a common 100-W light bulb in about 1 second. Unlike Newton's law gravitation for masses which is an attractive force, electrical force are much stronger and may attract or repel. To aid in understanding the strength of electrical attraction consider the following: EXAMPLE: The hydrogen atom has the simplest structure of all atoms.its nucleus is a proton (mass 1.7 X kg), outside of which there is a single electron (mass 9.1 X kg) at an average separation distance of 5.3 X m. Compare the electrical and gravitational forces between the proton and the electron in the hydrogen atom. FOR THE ELECTRICAL FORCE distance proton charge electron charge d = 5.3 X m qp = +1.6 X C qc = -1.6 X C The electric force F c is: F c = K q c q p d2 = (9.0 X 109 N m2/c2) (((1.6 X C)2)) / ((5.3 X m )2) Page 23

24 = 8.2 X 10-8 N The gravitational force F g between them is: F g = G m c m p d2 = (6.7 X N m2/kg) ((9.1 X kg)(1.7 X kg)) (5.3 X m)2 = 3.7 X N A comparison of the two forces is best shown by their ratio: F c = 8.2 X 10-8 N = 2.2 X 1039 F g = 3.7 X 10-47N Thus, the electrical force is more than times greater than the gravitational force. In fact, the electrical forces are so strong that the gravitational forces become almost non-existent. 21:8 Forces On Neutral Bodies Read, has to do with how photocopiers work Page 24