At a high point in the Earth\'s atmosphere, He 2+ ions in a concentration of 3.2

Question

At a high point in the Earth's atmosphere, He2+ ions in a concentration of 3.2x1012 m-3 are moving due north at a speed of 2.0x106 m/s. Also, a 6.8x10^11 m-3 concetration of O2- ions are moving due south at a speed of 6.6x106 m/s. Determine the magnitude of the current density, j at this point.

So far, I've got:

j=nevd

jHe = (3.2x1012)(1.6x10-19)(2.0x106) = 1.024 A/m2

jO2 = (6.8x1011)(1.6x1019)(6.6x106) = 0.71808 A/m2

I tried subtracting the current densities (jHe - jO2) but this is wrong. I don't know if my work is correct or not - it seems a little too simple - but I do understand that the direction of j at this point will be due North. Please help!

Thanks!

Explanation / Answer

Helium has a +2 charge, not a +1. This means that e should be 2*e, or 2 * (1.6*10^-19) for the Helium density. For Oxygen density, continue using (1.6*10^-19). Simply add up the two values when you're done.

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