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# UT PHYS 2080 - Chapter 15

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Chapter 15 Quiz may be Tuesday or Wednesday of this week!! June 23, 201415.1 – 15.615.1 Properties of Electric Charges- There are two kinds of electrical charge: positive and negative- Like charges repel one another and unlike charges attract one anothero Objects usually contain equal amounts of positive and negative charge (they are neutral)  Electrical forces arise when those objects have net negative or positive charge Objects become charged by gaining or losing electrons because electrons are lighter than protons and are more easily accelerated by forces- Electric charge is always conserved negative charge is transferred from one object to the other - The SI unit of electric charge is the coulomb “C”15.2 Insulators and Conductors- Conductors electrics charges move freely in response to an electric force good conductors: silver, copper, aluminumo When these materials are charged in some small region, the charge readily distributes itself over the entire surface of the material- Semiconductors  somewhere in-between  silicon and germanium- Insulators all other materials  glass and rubbero When these materials are charged by rubbing, only the rubbed areas becomes chargedo Hold copper rod in hand, rub with rod with fur or wool, it will not attract a piece of paper, Do the same but hold the rod with an insulator and the rod will remain charged and attract the paper  in the first case, the electric charge produced by rubbing readily moved from the copper through your body to the ground. In the second case the insulator stopped the flow to the ground- Charging by conductionthe object being charged is always left with a charge having the same sign as the object doing the chargingo Negatively charged rubber rod brought into contact with an insulated neutral conductive sphereo Before contact, the negative rod repels the sphere’s electrons, creating a local positive charge on the neutral sphereo On contact, electrons move from the rod, flow onto the sphere, neutralizing the local positive chargeso When the rod is removed, negative charge remains on the sphere the sphere hasbeen left with a net negative charge- Charging by inductionthe object is left with a charge opposite of the charging object and charging on object by induction requires no contact with the object inducing the chargeo Grounded  when an conducting wire or copper pipe is buried in the Earth (can accept an infinite number of electronso Negatively charged rubber rod brought near a neutral conducting sphere that is insulated o When negatively charged rod is brought close to sphere, causes some electrons in sphere to move to the side of the sphere furthest from the rod, which results in an excess positive charge nearest the negatively charged rodo Conducting grounded wire is added to sphere, some electrons leave the sphere and enter the ground, which leaves the sphere with excess positive chargeo When rubber rod removed from vicinity, the positive charge remains on the ungrounded sphere excess positive charge becomes uniformly distributed over the surface of ungrounded sphere because of repulsive forces of the like charged and high mobility of electrons in a metalo *** remove finger first, then later the rod  if remove rod before finger, the negative charge will go back onto it- Charging insulators by induction polarization  doesn’t change net chargeo In the presence of a charged object, these centers may separate slightly, resulting in more positive charge on one side of the molecule than on the other  polarizationo The realignment of charge within individual molecules produces an induced charge on the surface of the insulator A balloon charged through rubbing will stick to an electrically neutral wall15.3 Coulomb’s Law - The force exerted by one charged particle on another is given by: o K= 8.988*109~ 9.00*109 N*m2/C2- Electric forces between unmoving charges is called electrostaticforces while moving charges create magnetic charges- Charges:o 1/e = 6.25*1018 protons to create a total charge of +1.0 C Electrons to create a total charge of -1.0 C- Forces are vectors so Coulomb’s force law incorporates vectors and obey Newton’s third law of equal and opposite  always equal regardless of whether q1 and q2 have the samemagnitude- Comparing and contrasting electric and gravitational forceso Comparisons: both act at a distance and not in contact, both are inversely proportional to the distance squared with the force directed along a line connecting the two bodies, mathematical form is the sameo Two important differences: electric forces can be either attractive or repulsive but gravitational forces are only attractive and the electric force between charged elementary particles is far stronger than the gravitational force between the same particles - Superposition principle: when a number of separate charges act on the charge of interest, each exerts an electric force. these electric forces can all be computed separately, one at atime, then added as vectors15.4 The Electric Field said to exist in the region of space around a charged object-- SI unit of Newton per coulomb (N/C)- When a positive test charge is used, the electric field always has the same direction as theelectric force on the test charge- Test charge is required to be very small so it doesn’t cause any significant rearrangement of the charge creating the electric field-- Electric field of a point charge: o Charges create electric fields, and these fields in turn exert electric forces on otherchargeso Use vectors for multiple charges, the total electric field is found using the superposition principle15. 5 Electric Field Lines- 1. The electric field vector E is tangent to the electric field lines at each point.- 2. The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region (E is large when the field lines are close together and small when the lines are far apart the lines are closer together as they get near the charge, indicating that the strength of the field is increasing) - Rules for drawing electric field lines:o 1. Closer lines mean a stronger fieldo 2. The field is tangent to the lines at every pointo 3. Field lines start on positive charges and end on negative chargeso 4. The number of lines entering or leaving a charge is proportional to the

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