Suppose that the resistance between the walls of a biological cell is 6.8 × 109 ω. (a) what is the current when the potential difference between the walls is 69 mv? (b) if the current is composed of na+ ions (q = +e), how many such ions flow in 0.86 s?

(a) We can find the current flowing between the walls by using Ohm's law:

where  is the potential difference and  is the resistance. Substituting these values, we get


(b) The total charge flowing between the walls is the product between the current and the time interval:

The problem says , so the total charge is


The current consists of Na+ ions, each of them having a charge of . To find the number of ions flowing, we can simply divide the total charge by the charge of a single ion: