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# Voltage and Length Question! watch

1. As this is getting complicated, let me try to explain. Here is a graph showing two cases. One where the thickness is the same (left) and one where the thickness and resistance changes at some point (A) on the right. In both cases the applied pd goes from -6V to +6V (blue, 12V) and then is increased to -9V to +9V (red 18V)
In the first case (left) the zero point doesn't change. This will only happen if the increase in voltage is done in this way, where the ends are kept equally plus and minus. It would not be the case if the ends were to be, for example, -6V and +12V when increased to 18V pd.
In the second graph (right) the thickness changes at A and the gradient changes.
The line goes through zero at B. When the pd is increased, so long as the same conditions apply (equal negative to positive at either end) then the place where they go through zero will be the same.
But, as I said before, if the voltage doesn't change exactly in this way, the zero points will not be the same.
Without being sure of the setup, and how the voltages at the ends in your experiment are determined, I can't be sure what the result will be.
This is why I say the zero point could change. Hope that has helped to clear it up a bit.
2. (Original post by Stonebridge)
Again, it depends what you want to find out.
If you want to get accurate results for gradient, you take more measurements on the linear parts.
For example, if you want to find out where the transition takes place between thinner and thicker paper, it would be indicated by the place the two lines intercept. This is determined more accurately by making sure your two straight lines are as accurate as possible.

The place where the change occurs could be abrupt or could be gradual.
If you wanted to look at that, you would need to take a lot of measurements close together near the change. This is done assuming that there is a change.

If you believe that there should not be a change in gradient, then you need to measure the gradient more accurately by getting more accurate data points all along the line.

As I say, what you do is determined by what you want to know.
Hi, I know a similar question has been asked but when reducing the uncertainty at the value of x (where x is the length at where the width of the conductive paper changes) would you a) make more readings around that point or b) make sure that there are enough readings so that both straight lines can be accurately plotted.

I think the answer is a) but can you explain why?

Thanks
3. (Original post by georgiemuc)
Hi, I know a similar question has been asked but when reducing the uncertainty at the value of x (where x is the length at where the width of the conductive paper changes) would you a) make more readings around that point or b) make sure that there are enough readings so that both straight lines can be accurately plotted.

I think the answer is a) but can you explain why?

Thanks
The point x is at the intersection of the lines.
In order to find this point accurately you need two good, accurate straight lines.
You can do this effectively by making sure you have a good range of values for both lines so they can be plotted accurately.
Making a lot of measurements at the point x would only be possible once you know where x is. You don't know where it is untill you have the two straight lines.
You could make some extra measurements near that point later, but it isn't actually essential that you do this.
I also need to point out that I don't have the setup information for this experiment as explained in earlier posts, so there is an element of guesswork as to precisely what the examiner is looking for.

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