Millikan's Oil-Drop Experiment Millikan's Qil-Drop Experiment In this lab you will discover the nature of charge in a similar way that Millikan did with his o1l drop experiment. Problem Does there exist a smallest unit of electric charge of which other units are multiples? Equipment charged plates oil droplets potentiometer (to adjust voltage) and power supply s X-ray source New Drop X-ray zap 330V d (between plales) = 0.10m M (of oil droplet) = 1.57x1015 kg g = 9.8 m/s? Procedure 1. Open the Millikan lab. 2. Click the New Drop button to release a droplet. When it gets inside the chamber, click the x-ray button to ionize the chamber and thereby put some charge on the droplet. Millikan's Oil-Drop Experiment 3. Adjust the potentiometer (slider) until the oil drop completely stops moving up or down. 4. Record the voltage in the following table. Voltage (V) Charge (C) ACharge (C) 2 3 4 5 6 7 8 9 - (=] 5. Hit the chamber with another blast of x-rays to change the charge on the oil droplet. Measure the voltage required to stop it again. Repeat this step 8 more times until you have 10 individual values for voltage. If the droplet gets out of the chamber, add a new drop again and continue. Analysis 6. Calculate the amount of charge on the droplet for each voltage. How will you know this? When the droplet completely stops, the electric force up must be exactly equal to the gravitational force down. Fo F, =F, so E,q=mg F, o mg E V _ ) but E = Recall that E, is the Electric d field intensity and q 1s the charge. 56 q= dmg Millikan's Qil-Drop Experiment 8. Finally, calculate the difference between successive charges. 9. Plot a bar graph of the differences against the trial. Conclusion What can you conclude about the existence of a smallest unit of electric charge, of which other charges are multiples? 10. Do some research on the Internet and find an image of the original Millikan apparatus and include it in your write up. 11. Write up your lab and submit it to your teacher. How Did He Do It? How did Millikan find the mass of the droplet? He knew that the oil droplet would accelerate down under the force of gravity until it reached its terminal velocity, the point where the air resistance would stop it from accelerating any more. This of course happens almost instantaneously because the droplet is s0 tiny. The other fact Millikan knew is that the droplet is essentially a sphere shape once it reaches terminal velocity. This is the equation for the terminal velocity of a sphere. 2 . 02222g(D,-D,)r r= n vt is the terminal velocity of the droplet Dy, is the density of the oil or 920 kg/m' DA is the density of the air in the chamber or 1.292 kg/m' n is the dynamic viscosity of the air or 1.80 x 10 * kg/m /s r is the radius of the droplet Using the chamber without charging the droplet, Millikan found the terminal velocity. From that he calculated the radius of the droplet. v, n 0.2222 g (D, -D,) Millikan's Oil-Drop Experiment Using the radius, he calculated the volume. 4 V=nxr 3 Using the volume and the density, he calculated the mass. D=" ; DV m If you are interested, you can use the Pre-Millikan experiment to find the terminal velocity and calculate the mass of each droplet. Does your value match the one you used in the actual Millikan experiment