Introduction to Electromagnetism - 1

Administrative Marking Scheme Lab 20 Quizzes 10 Assignments 10 Midterms (2) 20 Final 40 Must meet minimum attendance First Assignment Posted January 23, due January 30 MIDTERM I February 9 (1hr) Lecture room Coulomb s Law Superposition Principle, Example The force exerted by q1 on q3 is F13 The force exerted by q2 on q3 is F23 The resultant force exerted on q3 is the vector sum of F13 and F23 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 Click to edit Master title style Click to edit Master text styles Second level Third level Fourth level Fifth level Instructor Franco Gaspari PHY 1040U (Physics for the biosciences) Introduction to Electromagnetism and Optics Lecture 1 January 9, 2007 Electric Charge. Conductors and Insulators. Every day electric phenomena. television telephone The electrical phenomena per se are hidden, but we are able to perceive their effects we do not see what happens inside the wires going to the light bulb, but we see the light. Natural electrical phenomena The most obvious example of a natural electrical occurrence is lightning Some objects become electrically charged when they are rubbed We now know that electrically charged bodies are able to act on other, non-charged, bodies. The natural question is what happens when we put close to each other two electrically charged bodies p n SUMMARY The Electron Where does electric charge come from Does it appear when we rub things with wool, or it is already there Electron negative charge Proton positive charge Neutral body Same number of negative And positive charges An electrically neutral body does have charges, but possesses an equal number of negative and positive charges A charged body has an excess of one type of charge. Plastic acquires electrons when rubbed with wool (and becomes negatively charged), the wool loses the electrons and becomes positive. Conductors and Insulators Metal Plastic Glass Materials in which electric charges are free to move are called CONDUCTORS. Those who trap the electric charges are called INSULATORS. Metal rod Au or Al thin leaves Metal rod Au or Al thin leaves Charging by contact We know that a body can acquire charges when we rub it, however, metals (and conductors in general) can be charged by simple contact with an already charged body. Charging by contact is at the basis of the first electroscope, that is, an instrument that measures whether a body is charged and whether it is a conductor or insulator. After charging the electroscope, if we contact the sphere with an uncharged body, the leaves will close if the body is a conductor, and will stay open if it is an insulator. Charging by induction Remember the experiment with the plastic and the glass, charged by friction, and brought Close to a small ball. In both cases we observed an attraction, even though we know that the plastic (-) and the glass ( ) are charged with opposite charges. Why Plastic Glass Neutral body If the neutral body is a metal, where charges are free to move, we make the charging permanent by cutting during the induction process. Glass (a) A neutral metallic sphere, with equal numbers of positive and negative charges (c) When the sphere is grounded, some of its electrons leave through the ground wire. (b) The electrons on the neutral sphere are redistributed when a charged rubber rod is placed near the sphere. (d) When the ground connection is removed, the sphere has excess positive charge that is nonuniformly distributed. (e) When the rod is removed, the remaining electrons redistribute uniformly and there is a net uniform distribution of positive Any conductor Free charges in the non-charged conductor will move accordingly. If the conductor is broken in 2 pieces, each piece will have a charge of equal magnitude and opposite sign. Putting a charged body (inductor) close to a non charged conductor. Induction It is efficient only with conductors. Insulators give up only the charge in direct contact. Some of the charge on the charged body will pass on the neutral body (redistribution of charge). We touch a neutral body with a charged body. Contact Insulators or conductors held with an insulating handle. Electrons from one body are transferred to the second body. We rub two bodies. Friction Materials Mechanism Description Method HOW DO WE ELECTRIFY BODIES A Brief Summary Electron the elementary particles, of negative charge, that make up the atom with the positive nucleus. The particles which are transferred from a body to another during charging are electrons (the positive nuclei stay behind). o m Coulomb s law describes quantitatively what we observe when two charged objects react to each other (repulsion or attraction) How do the forces acting on the bodies depend on their charges, their separation, and whatever else might be relevant Let two charged particles (or point charges) have charge magnitude q1 and q2 and be at a distance r from each other, then the electrostatic force of attraction (or repulsion) between them is given by q1 q2 The direction of the force is along the straight line joining the particles, and the sign of the charges determines whether it is attractive or repulsive A and B are conducting spheres. B is fixed. A can rotate (with a counterweight). Assume A and B are charged and that a repulsive force acts on them. As the sphere A rotates, it will induce a moment of torsion, M. The elastic momentum of torsion of the wire, Mr, will oppose the movement. In equilibrium, M Mr. Common aspects Both forces are felt by bodies placed at a distance (there is no direct contact between the bodies, unlike mechanical forces) Both forces vary as 1/r2 Differences Fg is always attractive, while Fe can be both attractive and repulsive, depending on the sign of the charges involved Fg is present for any pair of bodies, while Fe requires that both bodies be charged. Comparison of electrostatic forces and gravitational forces Hydrogen Atom e- Electric Forces can be represented by vectors q -q q q q q