Electrical potential difference

1. Feb 24, 2010

coz

1. The problem statement, all variables and given/known data

A grey rectangle is a uniform resistive but conducting coating on a substrate. This means that electrons can flow through it, but only slowly, and hence their paths follow the field lines. Attached, and in electrical contact with this, are two silvered metallic conductors A and B and a silvered conducting ring C.
Conducting strip A was connected to the −ve terminal of a 5 V DC power supply and strip B was connected to the +ve side of the supply.
Two sets of experimental measurements were taken. The first of these was along the line S−S′, where the potential differences between the point S at the outer edge of conducting strip A and various points along this line were recorded. The second set of measurements was made along the line T−T′, where again the potential differences between point T and selected points along the line T−T' were recorded. The results of the student's measurements are given in the tables below.

The questions I am struggling with are:

(c) From your potential difference graphs determine how E x varies along the lines SS′ and TT′. Draw two sketches of Ex versus x to show the variation along each line. Your sketches should show clearly the positions of the silvered regions, but you do not need to calculate any actual values to enable you to draw the sketches. Explain, in a few sentences, the major features of your sketches (such as sign, non-uniformity, or abruptness of change), interpreting them in terms of physical principles and the experimental arrangement.

(d) Sketch again your graph of Ex versus x for the line SS′ only, and superimpose on it a sketch of the x-component of the force F x that would be felt by an electron at all points on the line SS′. Explain briefly the relationship between the two sketches.

2. Relevant equations

Ex = -dV(r)/dx

3. The attempt at a solution

(c) Ex, i.e. the electric field is found by negative potential difference / distance. Is this all the question is asking me to sketch and describe? Without working out the values I am unsure how to determine how Ex and even when I do work out the values, I'm not really sure what it shows me.

Sketch of Ex against x for S: plateau at beginning, then decreases very steeply (negative), then trough, small peak then trough. I'm not sure where the 'silvered' regions are in this sketck.

Sketch of Ex against x for T: plateau at beginning, then decreases very steeply to a trough (negative), then fairly large peak, then another trough. Again I can't pick out the silvered regions in this sketch.

(d) not sure where to begin :S