2 In a particular cathode-ray tube, the measured beam current is
4 How many electrons strike the tube screen every $\Delta t = 40.0\U{s}$
5 \end{problem*} % problem 21.1
8 Current is defined as \emph{charge passing through a given surface per
9 unit time} or in SI units:
11 1\U{A} = \frac{1\U{C}}{1\U{s}} \;.
15 \Delta Q &= I \Delta t = 1.20\U{mC} \\
16 N_e &= \frac{\Delta Q}{e} = \ans{7.50\E{15}} \;,
18 where $e = 1.60\E{-19}\U{C}$ is the charge on one electron.