Some long molecules have charge distributions corresponding to alternating positive and negative source charges, as shown in this figure. The distance units are nanometers. 0 + + + . -4 -3 -2 -1 1 2 31 0 X A professor measured the potential along a line of constant y-position for this molecule and obtained this graph of the electrostatic potential: V -6 12345 x (nm)
Part (d)
Which of the following statements could be true? Select all that apply.
This potential was measured along the horizontal line 1 nm below the charges (y = 0 nm).
The magnitude of the potential is largest at approximately x = -4.5 nm, y = 2.5 nm.
This potential was measured along the line 1 nm above the charges (y = +2 nm).
The professor was working too late and switched the plus and minus x-coordinates when he recorded the results (left and right are reversed).
The potential measurement shown in the second figure indicates that the electric field is pointing up and to the right in the neighborhood of x = 5 nm, y = 0 nm.
The potential measurement shown in the second figure indicates that the electric field is pointing down and to the left in the neighborhood of x = 5 nm, y = 0 nm.
This potential was measured along the horizontal line that passes through the charges (y = +1 nm).
Part (e)
Suppose a Na+ ion approached this potential from the right along the x-axis and was constrained to move along the x-axis (no y-motion allowed). If it had very little KE but was moving slowly to its left when it was at the point x = 15 nm, and if there it essentially felt V = 0 from the molecule, how far along the axis would it come?
It would come in to about 2 nm before bouncing back.
It would be attracted and come in to about 3.5 nm (the bottom of the well). (hint this is not right)
It would stay where it was.;
It would not be attracted, it would be repelled and travel to larger values of x.
It would get to about 0.5 nm and bounce back.
Something else not mentioned in the other choices would occur.
Part (g)
For the set of 8 source charges in the figure at the top, there is an equipotential vertical line (really part of a surface in three dimensions) that corresponds to V = 0. Give a value for the x or y position of the line in nm.
Part (h)
It looks from the graph like the position x = 0 is not the local maximum, which is evidently a little to the left. If the diagram matches the graph would this be correct?
It's impossible to tell
Yes
No
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