Charge, voltage and current

Electrons carry negative charge. Losing one leaves the atom with one more proton than electrons, so it becomes a positive ion (a cation), Na+. Ions are the charge carriers in your body.

• A negative ion, Na−:
• Correct: A positive ion, Na+:
• A neutral atom with a smaller mass number:

Opposite charges attract and like charges repel. Because opposite charges attract, pulling them apart takes energy, which is the starting point of this page.

• They repel each other:
• No force acts until they touch:
• Correct: They attract each other:

Dissolved particles move down their concentration gradient with no energy input. On this page, ions feel that same concentration push plus an electrical one.

• Correct: From higher to lower concentration, down its gradient:
• From lower to higher concentration, up its gradient:
• In neither direction without a pump:

Voltage, also called potential difference, is always a difference between two points: how much electrical potential energy each unit of charge has at one point compared with the other. It exists only where charge is separated.

• The amount of electric charge that flows past a given point in a wire or fluid each second: That describes electrical current, the flow of charge. A voltage can exist with no charge flowing at all.
• Correct: The difference in electrical potential energy per unit of charge between two points: Correct. Voltage is the potential difference between two points, measured in volts.
• The opposition a path offers to the flow of charge: That describes electrical resistance, which opposes current.
• The total number of ions dissolved in a fluid: Counting dissolved particles gives osmolarity, not voltage. A fluid full of ions can be perfectly neutral and have no voltage.

Chloride carries negative charge, so bringing it in makes the inside more negative. By convention, current points the way positive charge moves, and bringing negative charge in has the same effect as taking positive charge out, so the current is outward.

• The inside becomes more positive, and the current is inward: Chloride is negative, so it makes the inside more negative, not more positive.
• The inside becomes more negative, and the current is inward: The inside does become more negative, but current direction follows positive charge. Negative ions moving in make an outward current.
• Correct: The inside becomes more negative, and the current counts as outward: Correct. Negative charge entering makes the inside more negative, which counts as an outward current.
• The inside becomes more positive, and the current is outward: The current is indeed outward, but chloride is negative, so it makes the inside more negative, not more positive.

Sodium's concentration gradient pushes it inward, and as a positive ion it is attracted to the negative inside. Both parts of its electrochemical gradient point inward, so it rushes in.

• Sodium moves out: the negative inside pushes it away: A negative inside attracts positive ions such as sodium; it does not push them away.
• Correct: Sodium moves in strongly: its concentration gradient and the voltage both push inward: Correct. The chemical and electrical pushes add together, giving a strong inward electrochemical gradient.
• Sodium does not move, because its two gradients cancel each other: The two gradients would cancel only if they pointed in opposite directions. For sodium, both point inward.
• Sodium moves in slowly, because only its concentration gradient acts on it: Sodium is charged, so the voltage acts on it too. Both pushes act, and they add together.

Potassium is higher inside, so its concentration gradient pushes it out. It is a positive ion, so the negative inside attracts it, pulling it in. The two pushes oppose each other, and the net push is the difference between them.

• Both push potassium outward: The voltage does not push potassium out. A negative inside attracts positive ions.
• Both push potassium inward: The concentration gradient does not push potassium in; there is more potassium inside than outside.
• Correct: The concentration gradient pushes it out, and the voltage pulls it in: Correct. The chemical push is outward and the electrical pull is inward, so they partly cancel.
• The concentration gradient pushes it inward, and the voltage pushes it outward: This reverses both directions. Potassium moves from high to low concentration (outward), and the negative inside attracts it (inward).

I = V / R = 6 V ÷ 2,000 Ω = 0.003 A. Since 1 A = 1,000 mA, that is 3 mA.

• Correct: 3 mA: Correct. 6 ÷ 2,000 = 0.003 A, which is 3 mA.
• 12,000 mA: This multiplies voltage by resistance instead of dividing. Current is voltage divided by resistance.
• 0.3 mA: This misplaces the decimal point. 6 ÷ 2,000 = 0.003 A, and 0.003 A is 3 mA, not 0.3 mA.
• 333 mA: This divides resistance by voltage (2,000 ÷ 6 ≈ 333) instead of voltage by resistance.

A concentration gradient depends on the difference between the two sides. More potassium outside, with the same amount inside, shrinks that difference, so the outward chemical push weakens. Less positive charge leaves, and the voltage across the membrane becomes smaller.

• It becomes stronger, because there is more potassium overall: The push depends on the difference between inside and outside, not the total amount. Raising the outside level shrinks the difference.
• It is unchanged, because the potassium inside the cells has not changed: The push depends on both sides. Changing the outside changes the difference even though the inside is the same.
• It reverses, so potassium now flows into the cells: The question says potassium is still more concentrated inside, so the chemical push still points outward, just more weakly.
• Correct: It becomes weaker, because the inside-outside difference is smaller: Correct. A smaller difference means a gentler gradient and a weaker outward push.

The voltage comes from a very thin layer of separated charge. Moving a tiny number of ions changes that layer a great deal, so the voltage changes a lot while the total amount of each ion inside and outside hardly changes.

• The ions that cross are replaced from the other side at the same moment by other ions: Nothing replaces the ions instantly. The concentrations stay steady because so few ions are needed to change the voltage.
• The voltage changes without any ions moving: A voltage changes only when the charge separation changes, which means charges must move.
• Correct: The separated layer is so small that a tiny number of ions changes the voltage a great deal: Correct. A few ions make a big change in charge separation but a negligible change in concentration.
• Only uncharged particles cross during the change: Uncharged particles carry no charge, so their movement cannot change a voltage.