Millken's Oil Drop Experiment

MILLIKAN OIL DROP EXPERIMENT DATA GIVEN LAB Introduction In the traditional Millikan Oil Drop experiment, oil drops are injected into a small region between two charged plates separated by a distance d. The oil drops acquire a charge (Q = nq) and experience a force due to the electric field between the plates. At a particular stopping voltage V, an oil drop can be held stationary by causing the upward electrical force to balance its weight (g = 9.8 m/s2). Hence QV  mg d (1) To be completely correct, the equation for the downward force on the oil drop (Fg) should use the apparent mass of the oil drop. This is the mass corrected for the buoyant (upward) force of the air. The buoyant force is equal to the weight of the air displaced by the oil drop—a very minor correction. Assuming the drop is shaped like a sphere then the downward force is given by 4 r 3 mg  ( o  a ) g 3 (2) where o is the density of the oil drop (o = 851 kg/m3) and a is the density of air (a = 1.29 kg/m3). Thus when determining the electric charge on an oil drop you need only measure the voltage V required to stop the oil drop and the radius r of the oil drop. However when the oil drop is allowed to fall freely under gravity (no applied voltage) it will experience a retarding force due to air resistance. This viscous force Fv caused by air resistance on a drop of radius r, moving with terminal velocity vt, through air of viscosity µ = 1.81 x 10 -5, is given by Stoke's law as Fv  6  vt (3) In this case the upward viscous force balances the downward force of gravity. Thus equating the two equations (F v = Fg) yields the following equation for the radius of an oil drop r 9 vt 2 g ( o  a ) (4) By measuring the terminal velocity of the oil drop you can determine its radius. Using given values for the density of air, density of oil, and viscosity of air show that 3/ 2 8 t Q  3.3  10 dv V (5) MILLIKAN OIL DROP EXPERIMENT DATA GIVEN LAB DATA COLLECTION: Screen Shot of Drop 10, Date: June 21st, 2005, note plate separation is given in millimetres Table 1: Oil Drop Data Table Drop # 1 2 3 4 5 6 7 Stopping Voltage (Volts) 154 180 539 216 270 539 1091 261 149 132 267 728 884 221 # of Divisions travelled in 4 seconds 9 9 9 9 9 9.1 9.1 8.8 8.8 7.3 7.3 11 7.9 7.9 Drop # 8 9 10 Stopping Voltage (Volts) 315 252 210 252 210 405 202 152 243 817 204 163 136 102 # of Divisions travelled in 4 seconds 10 10 10 10 10 9.8 9.8 9.8 9.8 7.5 7.5 7.5 7.5 7.5 For this lab you have been given a set of data, however you may choose to check out the following simulations in order to understand the experiment better: 1.​ https://mpv-au.vlabs.ac.in/modern-physics/Millikans_Oil_Drop_Experiment/experiment.ht ml 2.​ https://www.thephysicsaviary.com/Physics/Programs/Labs/MillikanOilDropLab/ Formal Lab Report Sections Title: Make up an informative title for what the lab is about; make sure your rewrite the title in your table of contents. Date: When did you do the lab? (Year, Day, Month, Time) Abstract: The abstract is a miniature version of the whole report. Your abstract should emphasize the objective, procedure, results and significance. Use this section to demonstrate how all the information holds together. The Abstract should be no more than a paragraph in length. Since the abstract is a summary of the report, you cannot write it until after you have completed the report. Data Processing and Presentation: This will usually involve constructing a results table and/or plotting a graph. Make sure your final result(s) is to the correct number of significant figures. In this lab, you will determine the number n of electron charges (n=Q/1.6e-19) on your oil drops. You will complete the calculations by constructing a results table using a spreadsheet program (e.g., Google Sheets or Excel). Then, plot a histogram of your values. (Both should go in the Data Processing section) Conclusion (Note: this conclusion has one section) i) Interpretation: This is the most important part of the report because it shows how well you have understood the experiment. Answer the question, “What is the relationship between my results and theory?” You need to decide if you have done what you set out to do in the Object, and to state clearly what you have concluded from your results. Analysis Questions: Put your answers to the questions below in this section. 1.​ Why do electrical fields not affect some of the drops? 2.​ Why do some of the drops require a negative rather than a positive balancing voltage? 3.​ Each time a new drop is injected, the radius can vary over a range of 20%. How much possible change can this cause the mass of the drop? 4.​ What is your best estimate of the minimum electrical charge? 5.​ Does your experiment support the idea of a quantized electrical charge? 6.​ If we had neglected the buoyancy of the air, how big an error would have been introduced into the calculation of the charge? 75 Data Given Lab Rubric Section Date Title Abstract Analysis (Data Processing Presentation) Interpretation Post-Lab Questions Details Year, Day, Month, Time (Communication) Clear and Descriptive (Communication) What you did, and your results. Should be brief!!!! (Communication) Processing raw data (Inquiry) INQ 4 3 COM 2 Presenting processed data (Communication) Drawing conclusions (Making Connections) Answered all Questions (Making Connections) ONTARIO MARK INQ ___/4 MC 0.5 0.25 0 0 0.5 0.25 0 0 2 1.5 1 0.5 2 1.5 1 0.5 1 COM___/5 4 3 2 1 2 1.5 1 0.5 AP___/4(or 6)