# Physics

Suppose that the resistance between the walls of a biological cell is 4.9 × 10^9 ohms (a) What is the current when the potential difference between the walls is 68 mV? (b) If the current is composed of Na+ ions (q = +e), how many such ions flow in 0.45 s?

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1. R=4.9•10⁹ Ω
V=68•10⁻³V
I=V/R=68•10⁻³/4.9•10⁹=1.39•10⁻¹¹ A
I=q/t =Ne/t
N=It/e=1.39•10⁻¹¹•0.45/1.6•10⁻¹⁹=3.9•10⁷

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