Introduction to Electromagnetism - 2

Recap Coulomb s Law 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 2 January 12, 2007 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 Charge is quantized As we touch a charged sphere with an uncharged one, the charges will redistribute themselves We can keep doing it and the charge in A will continue to decrease (we can use an electroscope to check) In a famous experiment Millikan determined that no matter the amount of charge you have, it is always a multiple of a particular number We call this number e 1.6022 x 10-19 C which is in fact the charge of an electron (with a negative sign) and of a proton (these are also called elementary charges) All this means that the charge is quantized. Charge is conserved Remember the experiment with metallic spheres. When we measure the value of the charge in the spheres at any time, the sum will always be the same. In all electric phenomena, the total electric charge remains constant. This can be said more formally The sum of all electric charges in a closed system is constant, regardless of whatever electric phenomenon occurs within the system. Consider the wool cloth and the plastic rod. After rubbing, the total charge in the system wool plastic is the same, even though there has been redistribution of charge. L d Quick Quiz 1 Answer (b). The amount of charge present in the isolated system after rubbing is the same as that before because charge is conserved it is just distributed differently. Quick Quiz 1 Quick Quiz 2  Quick Quiz 2 Quick Quiz 3 Quick Quiz 3