# Work and electric potentials confused

1. Jun 21, 2004

### supermenscher

Living cells actively pump sodium ions inside the cell to outside the cell. Given that the electric potential is 0.070V higher outside the cell than inside the cell and that the cell membran is 0.09E-6 meters thick, how much work must be done to move one sodium ion from inside the cell to outside the cell.

So far I am lost, does voltage have to negative, but then negative work has to be done

2. Jun 21, 2004

### kuengb

Signs are a matter of luck

Apart from that this seems not too difficult. Note that the thickness of the wall makes no difference.

3. Jun 21, 2004

### Staff: Mentor

Are you positive?

It requires positive work to move a positive charge from a low to a high potential. (Just like it requires positive work against gravity to lift an object.)

So what's the charge on the sodium ion?

4. Jun 21, 2004

### supermenscher

positive 0.070?

5. Jun 21, 2004

### Staff: Mentor

A sodium ion is formed by removing an electron. So the charge of the ion is +e = 1.6 E-19 Coulombs.

6. Jun 21, 2004

### supermenscher

so is the answer 0.070 * 1.6*10^-19 ?

7. Jun 21, 2004

### Staff: Mentor

Yes. Don't forget to use the proper units: work is measured in Joules.

8. Jun 21, 2004

### supermenscher

thank you so much :)

9. Jun 21, 2004

### turin

I didn't catch that at first ( ). I wonder if it was on purpose.