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Next Week Experiment 1: Equipotential Lines Week of January 18 Tutorial and Test 1 on chapter 18 Chemistry 1310 Midterm Is NOT at the same time as the PHYS 1030 midterm! Aurora: CHEM1310, Thursday, March 4, 7-9 pm Friday, January 8, 2010 43 WileyPLUS If you bought a NEW copy of Cutnell & Johnson (not second-hand) without a WileyPLUS code for edition 8, you can claim a WileyPLUS code... ...see me. Friday, January 8, 2010 44 The story so far... Electric charge is carried by electrons, protons. Because they each have a fixed amount of charge, charge is conserved. The electrostatic (Coulomb) force is exerted between charges. Like charges repel, unlike charges attract. The force between charges is given by Coulomb’s law: F = kqq0/r2, Electric field due to q is E = F/q0 E q, q0 in Coulombs (C), k = Coulomb constant = 9x109 N.m2/C2. Friday, January 8, 2010 45 The story so far... Electric charge is carried by electrons, protons. Because they each have a fixed amount of charge, charge is conserved. The electrostatic (Coulomb) force is exerted between charges. Like charges repel, unlike charges attract. The force between charges is given by Coulomb’s law: F = kqq0/r2, Electric field due to q is E = F/q0 E q, q0 in Coulombs (C), k = Coulomb constant = 9x109 N.m2/C2. Friday, January 8, 2010 46 Prob. 18.66/27: A ball (mass = 0.012 kg) carries a charge of –18 !C. What electric field is needed to cause the ball to float above the ground? �F = q�E q = –18 !C m = 0.012 kg mg What is the weight of the ball? What magnitude and direction of electric field supplies an equal upward force? Friday, January 8, 2010 47 iClicker Question A positive charge +q is fixed at the centre of a square. A second charge is fixed to either corner B, corner C or corner D. The net electric field at corner A is zero. At which corner is the second charge located? At B, C or D ? Answer: C Friday, January 8, 2010 Q 48 iClicker Question A positive charge +q is fixed at the centre of a square. A second charge is fixed to either corner B, corner C or corner D. The net electric field at corner A is zero. Is the second charge (A) positive, or (B) negative ? Answer: B Q Friday, January 8, 2010 49 iClicker Question A positive charge +q is fixed at the centre of a square. A second charge is fixed to either corner B, corner C or corner D. The net electric field at corner A is zero. Does the second charge have (A) greater magnitude, (B) smaller magnitude, or (C) the same magnitude as the charge at the centre? Answer: A Q Friday, January 8, 2010 50 On a thin, nonconducting rod, positive charges are spread evenly, so that there is the same amount of charge per unit length at every point. Deduce the direction of the electric field at point P that is located directly above the midpoint of the rod. E C P B A D +++++++++++++++++++++++++++++++++++++ Pair off charges on equal sections of the rod equidistant from the midpoint. Work out the direction of the field from the paired-off charges... Friday, January 8, 2010 51 iClicker Question On another identical rod, positive charges are spread evenly over only the left half, and the same amount of negative charge is spread evenly over the right half. Deduce the direction of the electric field at point P that is located directly above the midpoint of the rod. E C P Answer A) B A D +++++++++++++++++++––––––––––––––––––– Hint: pair off charges on equal sections of the rod equidistant from the midpoint. Work out the direction of the field from the pairedoff charges... Friday, January 8, 2010 52 Where should q1 to placed so that the net force on it is zero? Assume all charges are positive. q1 q2 q3 L Draw in the forces on q1 due to q2 and q3 Find where the forces are equal in magnitude and opposite in direction When F = 0, the electric field due to q2 and q3 is also zero Friday, January 8, 2010 53 Prob. 18.35/29: Where is the electric field equal to zero? Let point P be a guess for where E = 0, a distance x from the charge at the left. E=0? P x q1 = –16 !C Friday, January 8, 2010 L=3m 3–x q2 = +4 !C 54 18.-/65: A proton is moving parallel to a uniform electric field. The electric field accelerates the proton and thereby increases its linear momentum to 5x10-23 kg.m/s from 1.5x10-23 kg.m/s in a time of 6.3 !s. What is the magnitude of the electric field? Friday, January 8, 2010 55 Prob. 18.28/68: Two small, identical objects are 0.2 m apart. Each carries a charge and they attract each other with a force of 1.2 N. The objects are brought into contact so that the net charge is shared equally, and then they are returned to their initial positions. There is now a repulsive force of 1.2 N between the objects. What is the initial charge on each object? Write down Coulomb’s law for charges q1 and q2 Apply conservation of charge to find the charge q on each object after the charges have equalized Write down Coulomb’s law again Friday, January 8, 2010 56 18.51/45: Two point charges of the same magnitude but opposite signs are fixed to either end of the base of an isosceles triangle. The electric field at the midpoint M between the charges has a magnitude EM. The field at point P has magnitude EP. The ratio of these two field magnitudes is EM/EP = 9.0. Find the angle αin the diagram. L L cos α = d Friday, January 8, 2010 d L d 57