How can Current be positive when electrons have a negative charge?

I am a little confused. I have been told that electrons carry a charge of $-1.6 \cdot 10^{-19}$ coulombs, and that 1 coulomb is $6.25 \cdot 10^{18}$ electrons, and $1 \,\mathrm{A}$ is the current from when $1$ coulomb of charge flows in $1$ second.

However, when we are asked questions such as 'How many electrons pass a point when a current of $0.4\,\mathrm A$ flows for $900$ seconds?'

I understand $Q = I \cdot t = 0.4 \cdot 900 = 360\,\mathrm C$ and

$$\text{no. of electrons} = \dfrac{\text{total charge}}{\text{charge of one electron}}$$

so

$$\text{no. of electrons} = \frac{360}{-1.6 \cdot 10^{-19}} = -2.25 \cdot 1021$$

So do I just give my answer as a negative number of electrons? Or do I just totally ignore the negative sign?

• It is a sign convention. Note also that the question did not ask which way the electrons went. – Jon Custer Sep 12 '16 at 13:48
• conventionnal sense of the current ... – user46925 Sep 12 '16 at 13:49
• Related: physics.stackexchange.com/q/17109/2451 and links therein. – Qmechanic Sep 12 '16 at 14:35

A more rigorous version would be to recognize that what's actually happening is that you have $2.25\cdot 10^{21}$ electrons traveling in the opposite direction of the current. It's that opposite direction that accounts for the minus sign. 