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An insulating pipe of cross-sectional area A contains an electrolyte with two kinds of ions: one carrying charge -e and the other carrying charge +2e. When a potential difference is applied across the pipe, the negative ions drift with speed v and the positive ions drift with speed v/4. Both ion types have the same number density n. Find the current flowing through the pipe.

1. 3*n*e*v*A / 2
2. n*e*v*A / 2
3. n*e*v*A / 4
4. 5*n*e*v*A / 2

Correct answer: 3*n*e*v*A / 2

Solution

Negative ions give n*e*v*A and positive ions give n*(2e)*(v/4)*A = n*e*v*A/2; under a field the two currents are in the same direction and add to 3*n*e*v*A/2.

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