00:01
We're given a plot in problem 74 that has some data, and it is a plot where you have the potential difference on the vertical axis and the current on the horizontal axis, and it has a straight line slope for the resistance.
00:24
And i've actually reproduced a portion of the plot with two points that i thought were very clear.
00:31
Marked so those are the blue points here and the green line is the slope and we'll make use of this information in part a of the problem where it has to find the emf and the internal resistance and the way to do this is to write down this particular equation one which has of course the emf and the internal resistance which is e is equal to vab plus i are.
01:06
So we want to find two things, but we only have one equation.
01:10
But the good thing about this plot is that all the, any point along here will supply us with a value of i and a value of vab.
01:23
So we can have several equations.
01:26
But since we have two unknowns here, let's go ahead and write down two equations.
01:32
So the emf is not changing.
01:35
And let's call some v on this craft v1 and a corresponding i1, and of course the internal resistance doesn't change.
01:47
And then again, write down a second equation v2 plus i2r.
01:56
So now we have two equations and two unknowns.
01:59
So we can solve for e and we can solve for r.
02:03
And we want to pick points v1, i1, that are particularly clear.
02:10
So it's easy if you inspect the graph that the point for a current of three ampires, you have a potential difference of 30 volts.
02:22
So that's this one point here.
02:24
So that can be our v1 and i1.
02:28
So i1, v1, we pick this.
02:35
Set of points.
02:36
And we can do a similar thing here, where we have i2 v2 will correspond to those set of points.
02:46
And so we can eliminate, so let's solve for our two, for the internal resistance first.
02:55
So we can eliminate the emf.
03:00
So we just set these two equations equal to each other...