Transcript Lecture 2
PHY 2049: Physics II There is a proposal to move the final to Monday (8/4) from Wednesday (8/6). Any objections? To purchase Wiley Plus, the URL is: http://he-cda.wiley.com/WileyCDA/Section/id-320351.html PHY 2049: Physics II About bicycle riding: If some one shows you a trick, it means that, It is possible to do that trick The person who showed you, can do the trick It does not mean that you can do the trick. Try it a lot of times and you can too. PHY 2049: Physics II Coulomb’s law: Two kinds of charges, positive and negative, unlike attract, like repel. Electrons and Protons: q = ± 1.602 x 10-19 C Current I = dq/dt 1 Amp. = 1 C/s Add forces as vectors F k q1 q 2 r12 2 rˆ PHY 2049: Physics II A rule Try to calculate the force magnitude by ignoring the signs of charges. Then assign the direction by looking at the signs. Never do both (using the charge signs in the calculation of the force and then assign the direction). PHY 2049: Physics II Charges are introduced by rubbing (depends) Touching (same) By induction (opposite) PHY 2049: Physics II Balls are touched and then separated. Rank them by Charge left Magnitude of Force before After Charge transferred 3,1,2 PHY 2049: Physics II d Force on the center charge = 6kq2/d2 PHY 2049: Physics II q1 = q4 = Q; q2 = q3 = q what is Q/q so that the force on 1 is zero All charges same sign. There is no solution. Q and q different sign. There is a possible solution. Is the answer the same if the force on 3 is to be zero PHY 2049: Physics II What is the force on the central chlorine ion? Suppose that we put in an electron that sits on a cesium site and neutralizes it, which way does the Chlorine move? PHY 2049: Physics II Electric Field Force F = qE E = k ∑ rˆ qi/ri2 •Electric field lines denote direction •Their density represents magnitude •They do not cross PHY 2049: Physics II q1 = 2Q q2 = - 2Q q3 = -4Q E at origin PHY 2049: Physics II Ek z qz 2 R 2 3/ 2 zˆ