00:01
We're given a table which i've reproduced to you below of resistance versus length.
00:06
And in particular, there's a spider thread that some researchers had, and they stretched it by one millimeter increment, and then let it relax back to its original length.
00:23
And they measure the resistance as the thread, the spider web, is being used.
00:30
Stretched and so i reproduce the the table that's given here and in question 80 which all of the if the conductivity of the thread results from the aqueous coding only how does the cross -sectional area of the coding compare when the thread is 13 millimeters long versus the starting length of 5 millimeters and assume that the resistivity of the coding remains constant and that the coding is uniform along the thread and then we're asked to find basically how the cross -sectional area of the 13 millimeter length thread compares to the cross -sectional area of the five millimeter length thread so we're told to actually use the starting value because there is an end value associated with the 5 millimeter length.
01:39
So the values that we will consider in this problem are circled in red.
01:50
And the equation that i'll be using is that the resistance is equal to resistivity times length over cross -sectional area.
02:06
But to make things very clear, i will label with subscript 5 and 13, which will correspond to the length 5 and the length of 13.
02:23
So we have resistance 5, which is equal to row, l5 over a5.
02:37
We have r13 is equal to row l13 over a13.
02:52
And what we can do here is pretty much divide these two equations, or you can do this the long way, which is to solve one for row, and then substitute it into the other.
03:09
But it's pretty short if you just divide them.
03:14
So that's the approach i'll take care.
03:17
So i have r5 divided by r13...