Download Creating Effective Physics Graphs: Titles, Scales, Fitting Data, and Linearizing Equations and more Study Guides, Projects, Research Physics in PDF only on Docsity! Department of Physics & Astronomy Lab Manual Undergraduate Labs Graphical Presentation of Data Guidelines for Making Graphs Titles should tell the reader exactly what is graphed Remove stray lines, legends, points, and any other unintended additions by the computer that does not add to your graph. Axes should be labeled clearly and include the units and scale markings The scales should be chosen such that the data covers most of the area of the graph. The origin 0,0 is oftentimes included, but not always. Include error bars when appropriate, especially when fitting curves to the data. Bad Graph Label without units Way too many tick markings Tick markings too small to read No title Data only fills up small part of graph Choose a better vertical scale Department of Physics & Astronomy Lab Manual Undergraduate Labs Good Graph Scales appropriately chosen so that data can be seen clearly All labels are easy to read A reasonable amount of tick marks displayed Department of Physics & Astronomy Lab Manual Undergraduate Labs Real Data Real life data sets often display several types of behavior that may make your data not look “perfect”. For example, if you measure the period of a simple pendulum over a long period of time, you will not observe a perfect fit to 2 / . For small oscillations over a short period of time you can recover the equation. Over long periods of time the pendulum’s oscillations will decrease in amplitude due to friction at the pivot and air resistance, and you may observe an exponential decay envelope on top of the sinusoidal behavior. Finding the “best line” and the uncertainty in the slope The figure to the right shows the "best" (dark line) or most representative straight line that fits the data points as well as two other (red) lines. Approximately the same number of points lie above and below the best line. The best line is used to find the slope and the intercept. The two red lines might represent the data nearly as well as the best line. One red line has the largest plausible slope and one has the smallest. The largest slope line can be constructed by drawing a line which goes between a point below the best fit line on the left side of the graph and a point above the best fit line on the right side of the graph. The smallest slope line can be constructed by drawing a line which goes between a point above the best fit line on the left side of the graph and between a point below the best fit line on the right side of the graph. The differences between the slopes and intercepts of these lines yields the uncertainties in the slope and intercept. Finding the Uncertainty in the Slope The figure to the right illustrates the method used for finding the uncertainty in the slope of the best line. The dashed lines define the slope triangle of the best fit line. The vertical height of the slope triangle is called the “rise” and the horizontal width is called the “run”. The slope is given by “rise/run.” Please note that the slope triangle is for the best fit line and not for individual data points. Finding the “best fit” Finding uncertainty in slope Department of Physics & Astronomy Lab Manual Undergraduate Labs The uncertainty in the slope, expressed as a fraction of the slope, is slope slope and is found as follows: 2run run2 rise rise 2 slope slope In this example, rise should be approximately the same as the uncertainty in the measurements and similarly run should be approximately the same as the uncertainty in the measurements. Since the uncertainty in the rise does not affect the uncertainty in the run, the uncertainties are added in quadrature. (See the Propagation of Errors section for a complete explanation.) Data that does not fit a straight line When the distances between the data points and the "best" line are much larger than the error bars, the data does not fit a straight line. An experimenter faced with data of this type should conclude one or more of the following: The phenomenon is not described by a linear relationship between variables The uncertainties have been grossly underestimated The data is as precise as indicated but is inaccurate due to mistakes made reading a measuring instrument (e.g., interpreting 1.23 cm as 2.23 cm.) The data is plotted incorrectly Very precise data and a good fit to a straight line The graph on the right shows very precise data (error bars too small to plot). If the errors are too small to draw minimum and maximum slope lines, you might want to consider using the method of least squares. Very good fit to a straight line Department of Physics & Astronomy Lab Manual Undergraduate Labs Linearizing Data A linear relation has the form , which is useful for showing direct relationships such as and . A graph of the force of gravity on the ‐axis and mass on the ‐axis would yield a line with a slope equal to the acceleration due to gravity. A graph of voltage vs. current would give a value for resistance. This is very good stuff! What about equations which are non‐linear? How could a least squares fit help with that? The trick is to linearize your data and then apply a least squares fit. Consider the two graphs below, which both deal with the equation . The left graph vs. is plotted directly, which yields a parabola. In the graph on the right we have linearized the function and plotted vs. , which yields a line with slope . The difference between the two graphs is that in graph A is treated as the independent variable and in graph B is treated as the independent variable. does not always have to be the independent variable; think of plotting vs. a new variable , where . Examples of linearizing equations We would like to perform a measurement that would enable us to graph the charge‐to‐mass ratio of the electron / , just like Sir J.J. Thompson did in 1897. Consider electrons being accelerated by a uniform electric field to a speed . Let be the voltage difference between the starting and endpoints. The electrons then pass through a uniform magnetic field perpendicular to their motion. This causes the electrons to undergo uniform circular motion. Combining the equations and we can obtain the relation 2 (A) (B) (A) vs. of the quadratic function (B) vs. yields a linear relation with slope .