With an open switch, capacitor C in the figure is given a charge Q = 240 mu C an

Question

With an open switch, capacitor C in the figure is given a charge Q = 240 mu C and capacitor 3C is left uncharged. The switch is then closed at t = 0. If C = 60 mu F and R = 20 Ohm, what is the current through the resistor R at the initial instant t = 0? A. 2.7 A B. 240 A C. 0.2 D. 40 mu A E. 1.2 mA A metal crossbar of mass m = 0.5 kg rides on a parallel pair of long horizontal conducting rails separated by a distance L = 1.25 m and connected to a device that supplies constant current I = 1.4 A to the circuit, as shown below (viewed from the top). The circuit is in a region with a uniform magnetic field B of magnitude 0.8 T directed in to the screen as shown. There is no friction and the bar starts from rest at t = 0. The velocity of the bar t seconds later is: A. (2.8 t) m/s to the right. B. (1.4 t) m/s to the left. C. (0.2 t) m/s in the direction of the magnetic field B. D. (11.2 t) m/s to the right. E. (16.0 t) m/s to the left.

Explanation / Answer

Force = ILB =1.4*1.25*0.8 =1.4

Accelaration = Force/m =2.8 m/s2

Velocity = 0.8*t m/s

option A is correct

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