Problem 73-7  Source:   Problem 29-4 - page 249 -    OMAN, Robert & OMAN, Daniel -   Book: How to Solve Physics Problems.

What is the current and the current density in a gas discharge pipe 1.2 cm in diameter, where they circulate in a cross section of the tube, every second, 3.0 x 10¹⁷ electrons and 2.0 x 10¹⁶ positive ions?

Solution of the Problem 73-7   

If electrons move in one direction and positive ions move in the opposite direction, then we can consider the total number of particles moving within the tube, as the sum of the quantities of all particles. So, using the eq. 73-4, we have that the value of electric current is:

In possession of the value of the electric current, using eq. 73-06, we can find the value of current density. Remembering that the tube area is worth A = π r² = π x (0.006)² = 1.13 x 10^-4 m².