A beam contains 2.7 X 106 doubly charged positive ions per cubic centimeter, all of which are moving north with a speed of 9.9 X 105 _, m/s. What is (a) the magnitude of the current density and (b) direction of the current density J .(c) What additional quantity do you need to calculate the total current i in this ion beam? CD (a) Number l Units lb) (C) 45 1b