Error Bars Chemistry
on September 9, 2013 by John Vagabond In your MYP or IG course you should have been told to use a sharp pencil to draw a small cross to represent a data point on a graph, or a dot with a ring round it. The diameter of the ring should represent the error in the reading. In IB, we do things more precisely. Error bars show the actual uncertainty above and below the data point. They can be errors in either the dependent x or the independent y variable or both. In the following example we'll only concern ourselves with an uncertainty in y. Suppose we want to try to plot a graph of the speed of a car, starting from rest for the first few seconds. If we try to read off the numbers on the speedometer and write them down, there'll be a lot of uncertainty in the result. Nevertheless, let's try just to see if we can determine the error. This is what the speedometer might look like. The speed theoretically can be read to the nearest 2km/h, as you can see. However, let's suppose that the best we can do in a moving vehicle where the speed changes all the time is 10 km/h. Here's a table of results that we might obtain. and this is the graph. You should be able to produce something like this in Excel or similar by yourself. If you don't know how to do this kind of thing, you MUST ask. Look at the error bars. They show an uncertainty in the speed of +/- 10 km/h. This is actual, not percentage uncertainty. I have used Excel to draw a line of best fit - called a trendline. It MUST go through all the error bars - this is true whether the line is curved or straight. (exam tip). In this case, the computer has calculated the gradient for us as well - the acceleration in this case. Gradients and areas under the graph have UNITS. It tells us that the gradient is 42.9 km/h per second. You could work out that this represents an acceleration
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