일반물리학 2 — Current and Resistance: Problem 53

53 A beam contains \(4.5 \times 10^8\) doubly charged negative ions per cubic centimeter, all of which are moving north with a speed of \(300 \text{ m/s}\). What are the (a) magnitude and (b) direction of the current density \(\vec{J}\)? (c) If the particle distribution is uniform across a cross-sectional area of \(2.5 \text{ mm}^2\), what is the current?