Question

A proton beam in an accelerator carries a current of 134 µa. if the beam is incident on a target, how many protons strike the target in a period of 16.0 s?

Answer 1

The intensity of current is defined as the quantity of charge Q that passes through a certain point in a time $\Delta t$:
$I= \frac{Q}{\Delta t}$

In our problem, the current is 
$I=134 \mu A= 134 \cdot 10^{-6}A$ while the time interval is 16.0 s, so we can use the previous equation to find the total charge that strikes the target in this time:
$Q=I \Delta t=(134 \cdot 10^{-6}A)(16.0 s)=2.1 \cdot 10^{-3}C$

We know that each proton carries a charge of $q=1.6 \cdot 10^{-19}C$, so we can find the number of protons that strike the target by dividing the total charge by the charge of a single proton:
$N= \frac{Q}{q}= \frac{2.1 \cdot 10^{-3} C}{1.6 \cdot 10^{-19}C}=1.3 \cdot 10^{16}$