Transcript
Chapter 23 Electric Fields Electric Charges There are two kinds of electric charges Called positive and negative Negative charges are the type possessed by electrons Positive charges are the type possessed by protons Charges of the same sign repel one another and charges with opposite signs attract one another Every substance is made of atoms. Each atom, in turn, consists of a nucleus (protons + neutrons) and electrons which revolve around the nucleus. An atom is neutral when it has as many protons as electrons. In some atoms the outer electrons are not very attached and the atom can loose one or two easily becoming what is called a positive ion. Conversely, some other atoms hold on to their electrons tightly and even have the tendency to acquire one or two electrons more becoming then a negative ion. Hold on to electrons tightly Sulphur Brass Copper Ebonite (rubber, plastic) Silk Lead Fur Wool glass Hold on to electrons loosely When rubbing the glass rod and silk When rubbing the plastic rod and wool glass (+) silk (-) plastic (-) wool (+) Electric Charges, 2 The rubber rod is negatively charged The glass rod is positively charged The two rods will attract Electric Charges, 3 The rubber rod is negatively charged The second rubber rod is also negatively charged The two rods will repel More About Electric Charges Electric charge is always conserved in an isolated system For example, charge is not created in the process of rubbing two objects together The electrification is due to a transfer of charge from one object to another Conservation of Electric Charges A glass rod is rubbed with silk Electrons are transferred from the glass to the silk Each electron adds a negative charge to the silk An equal positive charge is left on the rod Quantization of Electric Charges The electric charge, q, is said to be quantized q is the standard symbol used for charge as a variable Electric charge exists as discrete packets q = ?Ne N is an integer e is the fundamental unit of charge |e| = 1.6 x 10-19 C Electron: q = -e Proton: q = +e Conductors Electrical conductors are materials in which some of the electrons are free electrons Free electrons are not bound to the atoms These electrons can move relatively freely through the material Examples of good conductors include copper, aluminum and silver When a good conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material Insulators Electrical insulators are materials in which all of the electrons are bound to atoms These electrons can not move relatively freely through the material Examples of good insulators include glass, rubber and wood When a good insulator is charged in a small region, the charge is unable to move to other regions of the material Semiconductors The electrical properties of semiconductors are somewhere between those of insulators and conductors Examples of semiconductor materials include silicon and germanium Charging by Induction Charging by induction requires no contact with the object inducing the charge Assume we start with a neutral metallic sphere The sphere has the same number of positive and negative charges Charging by Induction, 2 A charged rubber rod is placed near the sphere It does not touch the sphere The electrons in the neutral sphere are redistributed Charging by Induction, 3 The sphere is grounded Some electrons can leave the sphere through the ground wire Charging by Induction, 4 The ground wire is removed There will now be more positive charges The charges are not uniformly distributed The positive charge has been induced in the sphere Charging by Induction, 5 The rod is removed The electrons remaining on the sphere redistribute themselves There is still a net positive charge on the sphere The charge is now uniformly distributed Charge Rearrangement in Insulators A process similar to induction can take place in insulators The charges within the molecules of the material are rearranged Charles Coulomb 1736 – 1806 French physicist Major contributions were in areas of electrostatics and magnetism Also investigated in areas of Strengths of materials Structural mechanics Ergonomics Coulomb’s Law Charles Coulomb measured the magnitudes of electric forces between two small charged spheres He found the force depended on the charges and the distance between them Point Charge The term point charge refers to a particle of zero size that carries an electric charge The electrical behavior of electrons and protons is well described by modeling them as point charges Coulomb’s Law, 2 The electrical force between two stationary point charges is given by Coulomb’s Law The force is inversely proportional to the square of the separation r between the charges and directed along the line joining them The force is proportional to the product of the charges, q1 and q2, on the two particles Coulomb’s Law, 3 The force is attractive if the charges are of opposite sign The force is repulsive if the charges are of like sign The force is a conservative force Coulomb’s Law, Equation Mathematically, The SI unit of charge is the coulomb (C) ke is called the Coulomb constant ke = 8.9876 x 109 N.m2/C2 = 1/(4?eo) eo is the permittivity of free space eo = 8.8542 x 10-12 C2 / N.m2 Coulomb's Law, Notes Remember the charges need to be in coulombs e is the smallest unit of charge except quarks e = 1.6 x 10-19 C So 1 C needs 6.24 x 1018 electrons or protons Typical charges can be in the µC range Remember that force is a vector quantity Particle Summary Vector Nature of Electric Forces In vector form, is a unit vector directed from q1 to q2 The like charges produce a repulsive force between them Use the active figure to move the charges and observe the force PLAY ACTIVE FIGURE Vector Nature of Electrical Forces, 2 Electrical forces obey Newton’s Third Law The force on q1 is equal in magnitude and opposite in direction to the force on q2 With like signs for the charges, the product q1q2 is positive and the force is repulsive Vector Nature of Electrical Forces, 3 Two point charges are separated by a distance r The unlike charges produce an attractive force between them With unlike signs for the charges, the product q1q2 is negative and the force is attractive Use the active figure to investigate the force for different positions PLAY ACTIVE FIGURE A Final Note about Directions The sign of the product of q1q2 gives the relative direction of the force between q1 and q2 The absolute direction is determined by the actual location of the charges The Superposition Principle The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present Remember to add the forces as vectors The resultant force on q1 is the vector sum of all the forces exerted on it by other charges: Superposition Principle, Example The force exerted by q1 on q3 is The force exerted by q2 on q3 is The resultant force exerted on q3 is the vector sum of and Zero Resultant Force, Example Where is the resultant force equal to zero? The magnitudes of the individual forces will be equal Directions will be opposite Will result in a quadratic Choose the root that gives the forces in opposite directions Electrical Force with Other Forces, Example The spheres are in equilibrium Since they are separated, they exert a repulsive force on each other Charges are like charges Proceed as usual with equilibrium problems, noting one force is an electrical force Electrical Force with Other Forces, Example cont. The free body diagram includes the components of the tension, the electrical force, and the weight Solve for |q| You cannot determine the sign of q, only that they both have same sign Electric Field – Introduction The electric force is a field force Field forces can act through space The effect is produced even with no physical contact between objects Faraday developed the concept of a field in terms of electric fields Electric Field – Definition An electric field is said to exist in the region of space around a charged object This charged object is the source charge When another charged object, the test charge, enters this electric field, an electric force acts on it Electric Field – Definition, cont The electric field is defined as the electric force on the test charge per unit charge The electric field vector, , at a point in space is defined as the electric force acting on a positive test charge, qo placed at that point divided by the test charge: Electric Field, Notes is the field produced by some charge or charge distribution, separate from the test charge The existence of an electric field is a property of the source charge The presence of the test charge is not necessary for the field to exist The test charge serves as a detector of the field Electric Field Notes, Final The direction of is that of the force on a positive test charge The SI units of are N/C We can also say that an electric field exists at a point if a test charge at that point experiences an electric force Relationship Between F and E This is valid for a point charge only One of zero size For larger objects, the field may vary over the size of the object If q is positive, the force and the field are in the same direction If q is negative, the force and the field are in opposite directions Electric Field, Vector Form Remember Coulomb’s law, between the source and test charges, can be expressed as Then, the electric field will be More About Electric Field Direction a) q is positive, the force is directed away from q b) The direction of the field is also away from the positive source charge c) q is negative, the force is directed toward q d) The field is also toward the negative source charge Use the active figure to change the position of point P and observe the electric field PLAY ACTIVE FIGURE Superposition with Electric Fields At any point P, the total electric field due to a group of source charges equals the vector sum of the electric fields of all the charges Superposition Example Find the electric field due to q1, Find the electric field due to q2, Remember, the fields add as vectors The direction of the individual fields is the direction of the force on a positive test charge Electric Field – Continuous Charge Distribution The distances between charges in a group of charges may be much smaller than the distance between the group and a point of interest In this situation, the system of charges can be modeled as continuous The system of closely spaced charges is equivalent to a total charge that is continuously distributed along some line, over some surface, or throughout some volume Electric Field Lines Field lines give us a means of representing the electric field pictorially The electric field vector is tangent to the electric field line at each point The line has a direction that is the same as that of the electric field vector The number of lines per unit area through a surface perpendicular to the lines is proportional to the magnitude of the electric field in that region Electric Field Lines, General The density of lines through surface A is greater than through surface B The magnitude of the electric field is greater on surface A than B The lines at different locations point in different directions This indicates the field is nonuniform Electric Field Lines, Positive Point Charge The field lines radiate outward in all directions In three dimensions, the distribution is spherical The lines are directed away from the source charge A positive test charge would be repelled away from the positive source charge Electric Field Lines, Negative Point Charge The field lines radiate inward in all directions The lines are directed toward the source charge A positive test charge would be attracted toward the negative source charge Electric Field Lines – Dipole The charges are equal and opposite The number of field lines leaving the positive charge equals the number of lines terminating on the negative charge Electric Field Lines – Like Charges The charges are equal and positive The same number of lines leave each charge since they are equal in magnitude At a great distance, the field is approximately equal to that of a single charge of 2q Electric Field Lines, Unequal Charges The positive charge is twice the magnitude of the negative charge Two lines leave the positive charge for each line that terminates on the negative charge At a great distance, the field would be approximately the same as that due to a single charge of +q Use the active figure to vary the charges and positions and observe the resulting electric field PLAY ACTIVE FIGURE Electric Field Lines – Rules for Drawing The lines must begin on a positive charge and terminate on a negative charge In the case of an excess of one type of charge, some lines will begin or end infinitely far away The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge No two field lines can cross Remember field lines are not material objects, they are a pictorial representation used to qualitatively describe the electric field Motion of Charged Particles When a charged particle is placed in an electric field, it experiences an electrical force If this is the only force on the particle, it must be the net force The net force will cause the particle to accelerate according to Newton’s second law Motion of Particles, cont If is uniform, then the acceleration is constant If the particle has a positive charge, its acceleration is in the direction of the field If the particle has a negative charge, its acceleration is in the direction opposite the electric field Since the acceleration is constant, the kinematic equations can be used Electron in a Uniform Field, Example The electron is projected horizontally into a uniform electric field The electron undergoes a downward acceleration It is negative, so the acceleration is opposite the direction of the field Its motion is parabolic while between the plates Use the active figure to vary the field and the characteristics of the particle. PLAY ACTIVE FIGURE
