# Chapter 15: Electric Charge, Forces, and Fields

Chapter 15: Electric Charge, Forces, and Fields Static Electricity Electrical charge that stays in one place Electric Charge: a fundamental property of matter associated with the particles that make up the atom. Electricity is the study of the interaction between electrically charged objects. Structure of matter

Neutral vs Charged Objects Atoms with equal numbers of protons and electrons are described as being electrically neutral. Atoms with an unequal number of protons and electrons are electrically charged (and in fact, is then referred to as an ion rather than an atom). Any particle which contains less electrons than protons is said to be positively-charged. Any particle which contains more electrons than

protons is said to be negatively charged. Quantifying Charge, Q The Coulomb The magnitude of charge on an electron is the fundamental unit of charge, or the smallest observable charge in nature, abbreviated e To determine the total charge of an object with an excess of protons, subtract the total number of electrons from the total number of protons. Example Problem 1: Identify the following particles as being charged or uncharged. If charged, indicate whether

they are charged positively or negatively. Measurement of Charge SI Unit for charge is the coulomb, C The charge on a single electron is qe= -1.602 x 10 -19 C. The charge on a single proton is qp=+1.602 x 10 -19 C. In calculations, always convert charges to coulombs Example Problem 2 (rewrite false answers to make them true) A. TRUE or FALSE: An object which is positively charged contains all protons and no electrons.

B. TRUE or FALSE: An object which is negatively charged could contain only electrons with no protons. C. TRUE or FALSE: An object which is electrically neutral contains only neutrons. Example Problem 3: Determine the quantity and type of charge on an object which has 3.62 x 1012 more protons than electrons. Example Problem 4: Complete the following statements After some rather exhausting counting a physics student determines that a very small

sample of an object contains ... A. 8.25749 x 1017 protons and 5.26 x 1014 electrons; the charge on this object is ____ Coulombs. B. 3.12 x 1014 protons and 4.5488 x 1016 electrons; the charge on this object is ____ Coulombs. C. 2.40277 x 1019 protons and 9.88 x 1016 electrons; the charge on this object is ____ Coulombs. D. 2.6325 x 1015 protons and 2.6325 x 1015 electrons; the charge on this object is ____ Coulombs. Example Problem 5: The amount of charge carried

by a lightning bolt is estimated at -10.0 Coulombs. What quantity of excess electrons is carried by the lightning bolt? Interactions of Charges Electrical force keeps electrons in orbit around the nucleus and holds matter together. Law of Charges (aka The Charge-Force Law) : Like charges repel, and unlike charges attract. Charge on an electron and proton are equal in

magnitude, but opposite in sign, thus acting on separate objects, the forces follow Newtons Third Law Interaction Between Charged and Neutral Objects What type of interaction is observed between a charged object and a neutral object? Any charged object - whether positively charged or negatively charged - will have an attractive interaction with a neutral object.

Positively charged objects and neutral objects attract each other; and negatively charged objects and neutral objects attract each other. Example Problem 6: On two occasions, the following charge interactions between balloons A, B and C are observed. In each case, it is known that balloon B is charged negatively. Based on these observations, what can you conclusively confirm about the charge on balloon A and C for each situation.

Example Problems 7 and 8 7. Two objects are charged as shown at the right. Object X will ____ object Y. A. Attract B. Repel C. Not affect 8. Two objects are shown at the right. One is neutral and the other is negative. Object X will ____ object Y. D. Attract

E. Repel F. Not affect Conductors and Insulators Conductors are materials which permit electrons to flow freely from atom to atom and molecule to molecule. Conductors allow for charge transfer through the free movement of electrons. A conductor will permit charge to be transferred across the entire surface of the object.

Insulators are materials which impede the free flow of electrons from atom to atom and molecule to molecule. If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging and charge is seldom

distributed evenly across the surface of an insulator. Semiconductors have an intermediate ability to conduct charge. Movement of electrons is more complex than the valence electron theory can describe and is only understood by the aid of quantum mechanics Conductivity of semiconductors can be adjusted by adding atomic impurities to the

substance, making them useful in technology Superconductors are elements, inter-metallic alloys, or compounds that will conduct electricity without resistance below a certain temperature. Distribution of Charge via Electron Movement To reduce the overall repulsive affects within the object, there is a mass migration of excess electrons throughout the entire surface of the object. Excess electrons migrate to distance themselves from their repulsive neighbors so excess negative charge distributes itself

throughout the surface of the conductor. What if electrons are removed from a conductor at a given location, giving the object an overall positive charge? If protons cannot move, then how can the excess of positive charge distribute itself across the surface of the material? Electrons are loosely bound within atoms and are free to move within a conductor. Electron migration happens across the entire surface of the object, until the overall sum of repulsive affects between electrons across the whole surface of the object are minimized.

Example Problem 9: One of these isolated charged spheres is copper and the other is rubber. The diagram below depicts the distribution of excess negative charge over the surface of two spheres. Label which is which and support your answer with an explanation. Electrostatic Charging Law of Conservation of Charge: The net charge of an isolated system remains constant

When one material becomes negatively charged, another must become positively charged. Charging by Friction: transfer of charge due to contact between materials Amount of charge depends on the nature of the materials Charging by Conduction: transfer of charge due to the

flow of electrons by the contact of a charged object to a neutral object for electrons to move from the atoms of one material to the atoms of another material, there must be an energy source (a motive) and a lowresistance pathway. Electrostatic Charging, cont. Charging by Induction: separation of charges in a

two-body system due to repulsion between like charges. Induction charging is a method used to charge an object without actually touching the object to any other charged object. Electrostatic Charging, cont. Polarization: a separation of charge within an object where the net charge is zero Electric dipoles can be induced or permanent The density of the electron cloud can be distorted by

nearby atoms Electrons shared in covalent bonds may establish a dipole when the atoms involved have different electronegativity Example Problem 10: You shuffle across a carpeted floor on a dry day and the carpet acquires a net positive charge. Will you have a deficiency or an excess of electrons? If the charge the carpet acquired has a magnitude of 2.15 nC, how many electrons were transferred?

Since the carpet has a net positive charge, it must have lost electrons and you must have gained them. Given: qc = +2.15 nC = 2.15 x 10-9 C qe = -1.602 x 10-19 C net charge on you, q = -qc = -2.15x10-9 C Grounding Grounding is the process of removing the excess charge on an object by means of the transfer of electrons between it and another

object of substantial size. A ground is simply an object which serves as a seemingly infinite reservoir of electrons, often called an electron sink **Why would grounding a charge be important?** Homework p. 530 531; 1, 6, 9, 10, 12, 13, 18 Egg Car Plan Sheet Due 02/10/11

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