Exemplo 20.2 de College Physics, 20.1 Current

Calculating the Number of Electrons that Move through a Calculator

Problema e solução em inglês, como no livro original.

If the 0.300-mA current through the calculator mentioned in the Example 20.1 example is carried by electrons, how many electrons per second pass through it?

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Estratégia
The current calculated in the previous example was defined for the flow of positive charge. For electrons, the magnitude is the same, but the sign is opposite, $I_{electrons} = - 0.300 \times 10^{-3} C/s$ .Since each electron $(e^{-})$ has a charge of $–1 . 60 \times 10^{- 19} C$ , we can convert the current in coulombs per second to electrons per second.
Solução
Starting with the definition of current, we have

$I_{electrons} = \frac{Δ Q_{electrons}}{Δ t} = \frac{–0 . 300 \times 10^{- 3} C}{s} .$
20.5

We divide this by the charge per electron, so that
$\begin{array}{l} \frac{e^{–}}{s} & = & \frac{–0 . 300 \times 10^{- 3} C}{s} \times \frac{1 e^{–}}{–1 .60 \times 10^{- 19} C} \\ = & 1.88 \times 10^{15} \frac{e^{–}}{s} . \end{array}$
20.6
Discussão
There are so many charged particles moving, even in small currents, that individual charges are not noticed, just as individual water molecules are not noticed in water flow. Even more amazing is that they do not always keep moving forward like soldiers in a parade. Rather they are like a crowd of people with movement in different directions but a general trend to move forward. There are lots of collisions with atoms in the metal wire and, of course, with other electrons.

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