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Does anyone know what a supercondutor is? Think about it and we’ll talk about it once everyone is settled. Wow What a shock! Electromagnetism 2 PMHS Pearson 2007 Force and E field Force k q1 q2 Fe r2 E field is force per unit charge: Fe k q1 q2 1 k q1 E 2 2 q r q r +3c p+ The q that matters in the Electric field is the +3c Voltage • The electric potential (voltage) at any point is the work done to move a test charge from infinity to the point in question. • The calculation of potential is inherently simpler than the vector sum required to calculate the electric field. Work done on a charge • Work = F*s k q1 q2 • So : Fe s r 2 r k q1 q2 W r W k q1 V q r +3c Because s = r The q that remains is the charge on the object you are approaching p+ The q that matters in the voltage formula is the +3c Current? The drift of electrons in the direction of the electric Current is the net field? Of course! • flow of electrons The small drift velocity on the free electrons in a metal for ordinary currents is on the order of millimeters per second in contrast to the speeds of the electrons themselves which are on the order of a million meters per second So What would slow the drift? Resistance!! • What slows us down? • How does a car stop? . . . So how does Voltage, Current, and Resistance interact? You’ll have to figure that one out for yourself!! What do we do? • Group 1 use one resistor double voltage and see if current is Linear or Quadratic? • Group 2 Keep adding resistors one after the other. Keep Voltage the same. What happens to current? • Group 3 Add resistors in Parallel. Keep voltage constant. What happens to current? Resistance effects • Series (one after another) R R R 1 R 2 3 • Parallel (each end hooked together) 1 1 1 1 R R R R 1 2 3 Ohms Law • For many conductors of electricity, the electric current which will flow through them is directly proportional to the voltage applied to them. Superconductivity • If mercury is cooled below 4.1 K, it loses all electric resistance. This discovery of superconductivity by H. Kammerlingh Onnes in 1911 was followed by the observation of other metals which exhibit zero resistivity below a certain critical temperature. The fact that the resistance is zero has been demonstrated by sustaining currents in superconducting lead rings for many years with no measurable reduction. An induced current in an ordinary metal ring would decay rapidly from the dissipation of ordinary resistance, but superconducting rings had exhibited a decay constant of over a billion years!