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The Structure of Matter. Physics January 29, 2013 Coach Stephens. Examples of Static Electricity . Clothes tumble in the dryer and cling together. You walk across the carpeting to exit a room and receive a door knob shock.
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The Structure of Matter Physics January 29, 2013 Coach Stephens
Examples of Static Electricity • Clothes tumble in the dryer and cling together. • You walk across the carpeting to exit a room and receive a door knob shock. • You pull a wool sweater off at the end of the day and see sparks of electricity. • During the dryness of winter, you step out of your car and receive a car door shock as you try to close the door. • Sparks of electricity are seen as you pull a wool blanket off the sheets of your bed. • Bolts of lightning dash across the evening sky during a spring thunderstorm. • And most tragic of all, you have a bad hair day. • These are all static electricity events - events that can only be explained by an understanding of the physics of electrostatics.
Electrostatic Forces • Without the forces associated with static electricity, life as we know it would be impossible. • Electrostatic forces - both attractive and repulsive in nature - hold the world of atoms and molecules together in perfect balance. • Without this electric force, material things would not exist. • Atoms as the building blocks of matter depend upon these forces. • And material objects, including us Earthlings, are made of atoms and the acts of standing and walking, touching and feeling, smelling and tasting, and even thinking is the result of electrical phenomenon. • Electrostatic forces are foundational to our existence.
Structure of an Atom • One of the primary questions to be asked in this unit of The Physics Classroom is: • How can an object be charged and what affect does that charge have upon other objects in its vicinity? • The answer to this question begins with an understanding of the structure of matter. • Understanding charge as a fundamental quantity demands that we have an understanding of the structure of an atom. • So we begin this unit with what might seem to many students to be a short review of a unit from a Chemistry course.
Early Greek Philosophers • The search for the atom began as a philosophical question. • It was the natural philosophers of ancient Greece that began the search for the atom by asking such questions as: • What is stuff composed of? • What is the structure of material objects? • Is there a basic unit from which all objects are made? • As early as 400 B.C., some Greek philosophers proposed that matter is made of indivisible building blocks known as atomos. (Atomos in Greek means indivisible.) • To these early Greeks, matter could not be continuously broken down and divided indefinitely. • Rather, there was a basic unit or building block that was indivisible and foundational to its structure. • This indivisible building block of which all matter was composed became known as the atom.
Philosophical to Experimental • The early Greeks were simply philosophers. • They did not perform experiments to test their theories. • In fact, science as an experimental discipline did not emerge as a credible and popular practice until sometime during the 1600s. • So the search for the atom remained a philosophical inquiry for a couple of millennia. • From the 1600s to the present century, the search for the atom became an experimental pursuit. • Several scientists are notable; among them are Robert Boyle, John Dalton, J.J. Thomson, Ernest Rutherford, and Neils Bohr.
Robert Boyle & Dalton • Boyle's studies (middle to late 1600s) of gaseous substances promoted the idea that there were different types of atoms known as elements. • Dalton (early 1800s) conducted a variety of experiments to show that different elements can combine in fixed ratios of masses to form compounds. • Dalton subsequently proposed one of the first theories of atomic behavior that was supported by actual experimental evidence.
J.J. Thomson • English scientist J.J. Thomson's cathode ray experiments (end of the 19th century) led to the discovery of the negatively charged electron and the first ideas of the structure of these indivisible atoms. • Thomson proposed the Plum Pudding Model, suggesting that an atom's structure resembles the favorite English dessert - plum pudding. • The raisins dispersed amidst the plum pudding are analogous to negatively charged electrons immersed in a sea of positive charge.
Ernest Rutherford • Nearly a decade after Thomson, Ernest Rutherford's famous gold foil experiments led to the nuclear model of atomic structure. • Rutherford's model suggested that the atom consisted of a densely packed core of positive charge known as the nucleus surrounded by negatively charged electrons. • While the nucleus was unique to the Rutherford atom, even more surprising was the proposal that an atom consisted mostly of empty space. • Most of the mass was packed into the nucleus that was abnormally small compared to the actual size of the atom.
Neils Bohr • Neils Bohr improved upon Rutherford's nuclear model (1913) by explaining that the electrons were present in orbits outside the nucleus. • The electrons were confined to specific orbits of fixed radius, each characterized by their own discrete levels of energy. • While electrons could be forced from one orbit to another orbit, it could never occupy the space between orbits.
Protons & Neutrons • Bohr's view of quantized energy levels was the precursor to modern quantum mechanical views of the atoms. • The mathematical nature of quantum mechanics prohibits a discussion of its details and restricts us to a brief conceptual description of its features. • Quantum mechanics suggests that an atom is composed of a variety of subatomic particles: • The three main subatomic particles are the proton, electron and neutron. • The proton and neutron are the most massive of the three subatomic particles; they are located in the nucleus of the atom, forming the dense core of the atom. • The proton is charged positively. • The neutron does not possess a charge and is said to be neutral. • The protons and neutrons are bound tightly together within the nucleus of the atom.
Electron Shells • Outside the nucleus are concentric spherical regions of space known as electron shells. • The shells are the home of the negatively charged electrons. • Each shell is characterized by a distinct energy level. • Outer shells have higher energy levels and are characterized as being lower in stability. • Electrons in higher energy shells can move down to lower energy shells; this movement is accompanied by the release of energy. • Similarly, electrons in lower energy shells can be induced to move to the higher energy outer shells by the addition of energy to the atom. • If provided sufficient energy, an electron can be removed from an atom and be freed from its attraction to the nucleus.
Atomic Theory Conclusions • This brief excursion into the history of atomic theory leads to some important conclusions about the structure of matter that will be of utmost importance to our study of static electricity. Those conclusions are summarized here:
Continued… • All material objects are composed of atoms. • There are different kinds of atoms known as elements; these elements can combine to form compounds. • Different compounds have distinctly different properties. • Material objects are composed of atoms and molecules of these elements and compounds, thus providing different materials with different electrical properties.
Continued… • An atom consists of a nucleus and a vast region of space outside the nucleus. • Electrons are present in the region of space outside the nucleus. • They are negatively charged and weakly bound to the atom. • Electrons are often removed from and added to an atom by normal everyday occurrences. • These occurrences are the focus of this Static Electricity unit of The Physics Classroom.
Continued… • The nucleus of the atom contains positively charged protons and neutral neutrons. • These protons and neutrons are not removable or perturbable by usual everyday methods. • It would require some form of high-energy nuclear occurrence to disturb the nucleus and subsequently dislodge its positively charged protons. • These high-energy occurrences are fortunately not an everyday event and they are certainly not the subject of this unit of The Physics Classroom. • One sure truth of this unit is that the protons and neutrons will remain within the nucleus of the atom. • Electrostatic phenomenon can never be explained by the movement of protons.
Check Your Understanding • Use your understanding of charge to answer the following questions. • 1. ________ are the charged parts of an atom. a. Only electrons b. Only protons c. Neutrons only d. Electrons and neutrons e. Electrons and protons f. Protons and neutrons
Answer • Answer: E • Electrons are negatively charged and protons are positively charged. • The neutrons do not have a charge. Visit The Physics Classroom's Flickr Galleries and enjoy a photo overview of the topic of static electricity. http://www.flickr.com/photos/physicsclassroom/galleries/72157624896741488/
Neutral vs. Charged Objects Physics January 30, 2013 Coach Stephens
Review • As discussed in a previous section of Lesson 1, atoms are the building blocks of matter. • There are different types of atoms, known as elements. • Atoms of each element are distinguished from each other by the number of protons that are present in their nucleus. • An atom containing one proton is a hydrogen atom (H). • An atom containing 6 protons is a carbon atom. • And an atom containing 8 protons is an oxygen atom.
Continued… • The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral. • The amount of charge on a single proton is equal to the amount of charge possessed by a single electron. • A proton and an electron have an equal amount but an opposite type of charge. • Thus, if an atom contains equal numbers of protons and electrons, the atom is described as being electrically neutral. • On the other hand, if an atom has an unequal number of protons and electrons, then the atom is electrically charged (and in fact, is then referred to as an ion rather than an atom). • Any particle, whether an atom, molecule or ion, that contains less electrons than protons is said to be positively charged. • Conversely, any particle that contains more electrons than protons is said to be negatively charged.
Electrons are Migrants • In the previous section of Lesson 1, an atom was described as being a small and dense core of positively charged protons and neutral neutrons surrounded by shells of negatively charged electrons. • The protons are tightly bound within the nucleus and not removable by ordinary measures. • While the electrons are attracted to the protons of the nucleus, the addition of energy to an atom can persuade the electrons to leave an atom. • Similarly, electrons within atoms of other materials can be persuaded to leave their own electron shells and become members of the electrons shells of other atoms of different materials. • In short, electrons are migrants - constantly on the move and always ready to try out a new atomic environment.
Motive & Pathway • All objects are composed of these atoms. • The electrons contained within the objects are prone to move or migrate to other objects. • The process of an electron leaving one material object to reside (perhaps only temporarily) in another object is a common everyday occurrence. • Even as you read the words of this web page, some electrons are likely moving through the monitor and adhering to your clothing. • If you were to walk across the carpeting towards the door of the room, electrons would likely be scuffed off the atoms of your shoes and moved onto the atoms of the carpet. • And as clothes tumble in the dryer, it is highly likely that electrons on one piece of clothing will move from the atoms of the clothing onto the atoms of another piece of clothing. • In general, for electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source, a motive, and a low-resistance pathway.
Charged Objects • The cause and mechanisms by which this movement of electrons occurs will be the subject of Lesson 2. • For now, it is sufficient to say that objects that are charged contain unequal numbers of protons and electrons. • Charged objects have an imbalance of charge - either more negative electrons than positive protons or vice versa. • And neutral objects have a balance of charge - equal numbers of protons and electrons. • The principle stated earlier for atoms can be applied to objects. • Objects with more electrons than protons are charged negatively; objects with fewer electrons than protons are charged positively.
Neutrons • In this discussion of electrically charged versus electrically neutral objects, the neutron has been neglected. • Neutrons, being electrically neutral play no role in this unit. • Their presence (or absence) will have no direct bearing upon whether an object is charged or uncharged. • Their role in the atom is merely to provide stability to the nucleus, a subject not discussed in The Physics Classroom. • When it comes to the drama of static electricity, electrons and protons become the main characters.
The Coulomb • Like mass, the charge of an object is a measurable quantity. • The charge possessed by an object is often expressed using the scientific unit known as the Coulomb. • Just as mass is measured in grams or kilograms, charge is measured in units of Coulombs (abbreviated C). • Because one Coulomb of charge is an abnormally large quantity of charge, the units of microCoulombs (µC) or nanoCoulombs (nC) are more commonly used as the unit of measurement of charge. • To illustrate the magnitude of 1 Coulomb, an object would need an excess of 6.25 x 1018 electrons to have a total charge of -1 C. • And of course an object with a shortage of 6.25 x 1018 electrons would have a total charge of +1 C.
Quantity of Charge • The charge on a single electron is -1.6 x 10 -19 Coulomb. • The charge on a single proton is +1.6 x 10 -19 Coulomb. • The quantity of charge on an object reflects the amount of imbalance between electrons and protons on that object. • Thus, to determine the total charge of a positively charged object (an object with an excess of protons), one must subtract the total number of electrons from the total number of protons. • This operation yields the number of excess protons. • Since a single proton contributes a charge of +1.6 x 10 -19Coulomb to the overall charge of an atom, the total charge can be computed by multiplying the number of excess protons by +1.6 x 10 -19 Coulomb. • A similar process is used to determine the total charge of a negatively charged object (an object with an excess of electrons), except that the number of protons is first subtracted from the number of electrons.
Total Charge This principle is illustrated in the following table:
Conclusion • In conclusion, an electrically neutral object is an object that has a balance of protons and electrons. • In contrast, a charged object has an imbalance of protons and electrons. • Determining the quantity of charge on such an object involves a counting process; the total number of electrons and protons are compared to determine the difference between the number of protons and electrons. • This difference is multiplied by 1.6 x 10 -19 Coulombs to determine the overall quantity of charge on the object. The type of charge (positive or negative) is determined by whether the protons or the electrons are in excess.
Check Your Understanding Use your understanding of charge to answer the following questions. • 1. TRUE of FALSE: An object that is positively charged contains all protons and no electrons.
Answer Question #1 • Answer: False • Positively charged objects have electrons; they simply possess more protons than electrons.
Question #2 • 2. TRUE of FALSE: An object that is negatively charged could contain only electrons with no accompanying protons.
Answer to Question #2 • Answer: False • Negatively charged objects have protons; it's just their number of electrons is greater than their number of protons.
Question #3 • 3. TRUE of FALSE: An object that is electrically neutral contains only neutrons.
Answer Question #3 • Answer: False • Electrically neutral atoms simply possess the same number of electrons as protons. This gives the objects a balance of both type of charge.
Question #4 • Identify the following particles as being charged or uncharged. If charged, indicate whether they are charged positively or negatively. (n = neutron, p = proton, e = electron)
Answer Question #4 • Answers: A is negative; B is neutral; C is positive • Determining whether a particle is + or - is a matter of comparing the number of electrons to the number of protons. If there are more electrons than protons, then it is negative. If there are more protons than electrons, then it is positively charged.
Question #5 • Consider the graphic at the right of a neutral oxygen atom. • a. Explain what must happen in order for the oxygen atom to become negatively charged. b. Explain what must happen in order for the oxygen atom to become positively charged.
Answer Question #5 • Answers: a. Gain electrons AND b. Lose electrons • Protons are tightly bound in the nucleus and can be neither gained nor loss. So any change in the charge of an atom is due to changes in its electron count. If a neutral atom gains electrons, then it will become negatively charged. If a neutral atom loses electrons, then it become positively charged.
Question #6 • Determine the quantity and type of charge on an object that has 3.62 x 1012 more protons than electrons.
Answer Question #6 • Answer: +5.8 x 10^-7 Coulombs (rounded) • To determine the charge on an object, determine the number of excess protons or excess electrons. Multiply the excess by the charge of an electron or the charge of a proton - 1.6 x 10-19 C. Finally, adjust the sign of the object to + or -.
Homework Go to the following link and complete problems #1-10: http://www.physicsclassroom.com/calcpad/estatics/problems.cfm
