Question 26.76: A current is established in a gas discharge tube when a suff......
A current is established in a gas discharge tube when a sufficiently high potential difference is applied across the two electrodes in the tube. The gas ionizes; electrons move toward the positive terminal and singly charged positive ions toward the negative terminal. (a) What is the current in a hydrogen discharge tube in which 3.1 × 10^{18} electrons and 1.1 × 10^{18} protons move past a cross-sectional area of the tube each second? (b) Is the direction of the current density \vec{J} toward or away from the negative terminal?
(a) The current is 4.2 × 10^{18} e divided by 1 second. Using e=1.60 \times 10^{-19} \,C we obtain 0.67 A for the current.
(b) Since the electric field points away from the positive terminal (high potential) and toward the negative terminal (low potential), then the current density vector (by Eq. 26- 11) must also point toward the negative terminal.
\vec{E}=\rho \vec{J} (26-11)