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The charge of an electron is $1.6 \times 10^{-19}\,\text{C}$. How many electrons strike the screen of a cathode ray tube each second when the beam current is $16\,\text{mA}$?

1. 10 17
2. 10 19
3. 10 – 19
4. 10 – 17

Correct answer: 10 17

Solution

Current is the rate of flow of charge, so the number of electrons per second is $n=I/e$. With $I=16\times10^{-3}\,\text{A}$ and $e=1.6\times10^{-19}\,\text{C}$, we get $n=10^{17}$ electrons per second.

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