A beam contains 8.7 X 1010 doubly charged positive ions per cubic centimeter. all of which are moving north with a speed of 6.8 K 10S _) 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? (3) Number n Units V (b) v (C) v