A battery of emf ? is inserted in series with the resistor in Fig. 27.39, with its positive terminal toward the top rail. The bar is initially at rest, and now nothing?s pulling it.(a) Describe the bar?s subsequent motion.(b) The bar eventually reaches a constant speed. Why? (c) What is that constant speed, in terms of the magnetic field, the battery emf, and the rail spacing l? Does the resistance R affect the final speed? If not, what role does it play?

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В х хх х_х x x x X x x X .R FIGURE 27.39 Problems 46-49 and 75