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# electron drift velocity watch

1. A copper wire has a cross sectional area of 2mm^2 and carries a current of 6A calculate the mean drift velocity of electrons in the wire. The charge on an electron is -1.6 x 10^-19 C. The number density of copper atoms (the number of atoms per unit volume) is 8 x 10^28 m-3. Each copper atom provides one conduction electron.

I did this:

v = I/nAe

v = 6/ (8 x 10^28) (2 x 10^-6) (-1.6 x 10^-19) = -3.75 x 10^19

The actual answer is 3.75 x 10^19

What did I do wrong?
2. (Original post by tazmaniac97)
A copper wire has a cross sectional area of 2mm^2 and carries a current of 6A calculate the mean drift velocity of electrons in the wire. The charge on an electron is -1.6 x 10^-19 C. The number density of copper atoms (the number of atoms per unit volume) is 8 x 10^28 m-3. Each copper atom provides one conduction electron.

I did this:

v = I/nAe

v = 6/ (8 x 10^28) (2 x 10^-6) (-1.6 x 10^-19) = -3.75 x 10^19

The actual answer is 3.75 x 10^-19

What did I do wrong?
Nothing. (I assume you meant 10-19 in your answer. That's what it is. If you did get 1019 check your calculation on the calculator))

You don't need the minus sign for the charge on the electron, you are only interested in the amount of charge, not whether it's positive or negative in this question.
If you really want the minus sign, then the minus in the answer simply means that the electron is moving in the opposite direction to that which a positive charge (conventional current) would move.

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