Q:    I am wondering if you can clarify Q15 on the bean lab we did this past week. 

This was my answer to that question: 

The cell will become more positive, because it is gaining K+..is this correct?

The resting membrane potential difference will decrease..this is what I do not understand. If the difference decreases how is it going closer to zero, when it is already negative?

A:   Yes, the membrane potential in question 15 does become more positive.

As for the second part of your question, this concept is a bit tricky to understand at first, so I'm glad you asked this question!

Remember that every time we say "membrane potential" we actually mean "resting membrane potential difference" (see page 17 of lab manual for explanation). Don't forget that "difference" because that is the most important part (I always have to remind myself!).  

The membrane potential simply measures the difference in charge between the ICF and the ECF. 

Because we always set the ECF charge to 0, a positive membrane potential means the ICF is more positive than the ECF. A negative membrane potential means that the ICF is more negative than the ECF. 

But, in this case, it doesn't actually matter what sign (+ or -) the membrane potential is, the most important thing is how big the number (i.e. absolute value) is. The larger the number, the larger the difference in charge between the two sides of the membrane. If the membrane potential is 0mV, that means there is no difference in charge between the ICF and the ECF.

So in this case, we initially have a negative membrane potential (lets say -10mV, just as an example). This means the ICF is more negative than the ECF. 

Then suddenly the membrane potential becomes more positive (Let's say from -10mV to -8mV). This means that the the ICF becomes less negative, right? So the absolute value ( which is 8) of the membrane potential becomes smaller, i.e. closer to 0.

You can also look at it another way. If you use the number line on page 17. As you can see, -8mV is closer to 0 than -10mV.

  I---I---I---I---I---I---I---I---I---I---I---I

-10     -8                               0