# Practical: Graph need to pass through orgin?

1. Oct 16, 2004

### dove

Hi there, I need immediate help as my pratical exam is on next Monday [18/10/04] and I would greatly appricate if anyone can help me with my doubts.

We all know that for physics practical, mostly likely we have to draw a graph to conclue our results. However, after ploting the correct points, I always wonder if you have to past through the orgin, O. My teacher says that if we are finding the resistence, then if passes through O beacause it obeys the Ohm's law. However, what are about others? like current? Do we also have to pass through O.

I would just like to confrim with you if am I correct to say that in Physics experiment, we always draw a best fit line in all experiments expect cooling curve. If I'm wrong, please tell me.

2. Oct 16, 2004

### arildno

That depends completely on the function f you're talking about!!
If f(0)=0, then the graph DOES go through the origin, if f(0) is not equal to 0, then your graph does NOT go through the origin.

3. Oct 16, 2004

### ylem

Well, when we did the electricity and magnetism bit of the AS syllabus my teacher said that if the graph didn't go through zero it was called a zero error, and it just meant that the ammeter/voltmeter wasn't set properly or something. Not sure if that helps you though! Sorry!

Ooo, also, isn't current proportional to voltage? So, the graph should also pass through zero? Again, not sure if i'm right!

Last edited: Oct 16, 2004
4. Oct 16, 2004

### MiGUi

That kind of error is verry common and its very easy to solve. If you see Ohm's law V = IR and you plot it, you get a line which passes through origin. If not, then the voltage is not correctly measured. Think that voltage is a potential, so we measure variations but not absolute values. So, if you can get the line pass through the origin, you have to calibrate the voltimeter measuring the voltage of ground and that value will be your zero, so the next values you take have an error by excess, and to get the exact value you have to rest your zero to all of your experimental points of voltage. Then, if you plot it you will get a line which passes trough the origin.

MiGUi

5. Oct 16, 2004

### dove

Okay, what if the graph is plotted without a formula connected together? For example, in the picture below, they ask we to plot (h1-h3) agasint (h2-h3)? I did that experiment before, it didn't get to past through the orgin. So, does that mean that only if the y-axis and x axis are related by a straight line equation (y=mx+c), for exmaple, V=RI, Q=IT & V=W/Q, then it is a best fit graph which passes through the orgin?

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6. Oct 18, 2004

### dove

Thanks for all your help. I had my physics pratical today. I made a huge, unforgiveable mistake. They didn't ask us to draw a striaght line graph, but a curve. I missed this piece of information a drew a straight line graph without passing through the orgin. The total marks for the graph is 4. I did label the axes, give the title of the graph as well as the scale. So, how much do you people think the cambridge markers will deduct because I never follow the instruction? Will they take away all the 4 full marks?

7. Oct 18, 2004