A beam contains $2 \times 10^8$ doubly charged positive ions per cubic centimeter, all of which are moving with a speed of $10^5 \,m/s$. The current density is ............. $A/m^2$
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$J = nqv = n(ze)v = \frac{{2 \times {{10}^8} \times 2 \times 1.6 \times {{10}^{ - 19}} \times {{10}^5}}}{{{{({{10}^{ - 2}})}^3}}}$ $= 6.4\,A/m^2$
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