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Ions are charged atoms or molecules. The plasma membrane is analogous to a capacitor, storing and separating charge. The voltage across the membrane varies from cell to cell, but a typical value is that the inside of the cell is -65 mV (millivolts) relative to the outside.
This may suggest that positive ions will flow into the cell when ion channels open, but, as mentioned in the question, the movement of ions is determined by both concentration and voltage.
This so called "electrochemical potential" can be calculated using the Nernst equation.
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