Download Lesson 6: Millikan`s Oil Drop Experiment

Lesson 6: Millikan’s Oil Drop Experiment Lesson 6: Millikan’s Oil Drop Experiment Introduction Activity: In 1897, J.J. Thomson discovered the electron. Twelve years later, in 1909, Robert Millikan, one of the most brilliant experimentalists of his time, was the first to measure the mass and charge of an electron in his famous oil drop experiment. The electron is a particle so small that you can’t even know how many of them have in a sample. How do you measure a particle that is essentially invisible? This is the challenge that Millikan faced when he set out to determine the charge of the electron. The goal of this activity is to help you understand the challenge that Millikan faced and the problem solving process that used to measure the charge and mass of the tiny electron. For this activity, imagine that scientists recently discovered the existence of a new subatomic particle they have called Particle X and so far no one has been able to determine its mass. Each of the paper bags in the lab contains an unknown amount of particle X. In this problem solving lab activity you will determine the mass of particle X and calculate how many particles are in each bag. Step 1: Use the electric balances to measure the mass of each paper bag. DO NOT OPEN OR LOOK IN THE BAGS! Bag # Mass (kg) Bag # Mass (kg) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Lesson 6: Millikan’s Oil Drop Experiment Step 2: Meet with the other students in your table group and brainstorm methods that you could use to determine the mass of particle X using the masses of each of the bags containing unknown quantities of the particle. Assignment (this is a group activity): •
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By the end of class, submit a group copy of your solution on a separate sheet of paper with all group members name on it. Solutions should be well organized with clear and detailed communication. The mark for your solution will be assessed for both the Managing Information and the Problem Solving competency. Managing Information: The data that you found measuring the bags is very disorganized. It is difficult to analyze and problem solve with data that is sorted randomly. • Organize and sort the information from your data table on the previous page in two different ways. For example, you can create a new data table, use visual diagraming, or create a graph. Problem Solving: Answer the following two questions: • Question 1: Determine the mass of Particle X using the data you have gathered. • Question 2: How many particles are in each bag? Part 1: Experimental Design First chamber – Charging the oil drops: When oil drops were released into the first chamber, they picked up a small amount of electrons through friction with the air. Each oil drop picked up a different amount of electrons. Second chamber – Balancing the force of gravity: The oil drops fell into the second chamber containing an electric field. Millikan set up a potential difference between the top and bottom of the chamber so that any charged oil drop would experience an upward force, countering the force of gravity in the chamber. Determining the charge on the oil drop: Millikan looked through a small microscope viewer and located a stationary oil drop. For a stationary oil drop, he knew that Fg=Fe because the forces were balanced. Determining the elementary charge: Millikan found the charge of thousands of oil drops. Eventually he started to notice a pattern. The oil drops could be organized into groups of very close amounts of charge. Since you can’t have half of an electron, he assumed that each oil drop in a group had the same number of electrons. Each group was separated by the same amount of charge: -­‐19
1.6x10 C. He deduced that the groups separated by this amount must be one electron different and the difference in charge must be the charge of one electron. Lesson 6: Millikan’s Oil Drop Experiment Activity: Millikan’s oil drop simulation Google the physics aviary Millikan simulation. Collect the voltage and radius for 10 oil drops. Record your data below: Voltage (V) Radius (nm) Once you have filled in your data table, go to the following google sheet: goo.gl/iF6ZBa Fill in the voltage and radius in the next empty yellow shaded cell. The spread sheet will use your data to calculate the charge on each of your oil drops. When we have data for the whole class, we will be able to look at patterns in the charge and deduce the elementary charge. Part 2: Oil Drop Calculations The charge on an oil drop Suspended or constant speed Positive acceleration (up) Negative acceleration (down) Lesson 6: Millikan’s Oil Drop Experiment Example: In the Millikan experiment below, the plates are separated by 10 cm and the battery potential is 4.5 V. The mass of each -­‐16
oil drop is 7.8x10 kg. What is the charge on each oil drop and how many electrons does each contain? Summary Notes: Describe using diagrams and explanations how Millikan used drops of oil to measure the charge of the electron Draw a free body diagram for each of the following oils drops. Derive an equation that you can use to calculate their charge. Suspended or constant speed Positive acceleration (up) Negative acceleration (down) the elementary
Lesson 6: Millikan’s Ocharge!
il Drop Experiment Practice Problems:!
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1. An oil drop, of mass 2.6 x 10-15 kg, is suspended between two parallel plates 0.50 cm apart, and
remains stationary when the potential difference between the plates is 270 V. What is the charge on
the oil drop, and how many excess or deficit electrons does it have? (4.7 x 10-19 C, ± 3 electrons)!
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2. A metallic Ping-Pong ball, of mass 0.10 g, has a charge of 5.0 x 10-6 C. What potential difference,
across a large parallel plate apparatus of separation 25 cm, would be required to keep the ball
stationary? (49 V)!
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3. An oil drop weighs 3.84 x 10-15 N. If it is suspended between two horizontal parallel plates where
the electric field strength is 1.20 x 104 N/C, what is the magnitude of the charge on the oil drop?
(3.2 x 10-19 C, 2e-)!
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4. An oil drop whose mass is 3.50 x 10-15 kg accelerates downward at a rate of 2.50 m/s2 when
placed between two horizontal parallel plates that are 1.00 cm apart. Assuming that the oil drop is
negatively charged and that the top plate is positive, how many excess electrons does the oil drop carry
if the potential difference between the plates is 5.38 x 102 V? (4.76 x 10-19 C)!
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5. An electron is accelerated upward at 5.2 m/s2 towards the positive plate when released. If the plates
are set 15 mm apart, what is the potential difference applied across the plates? (-1.28 x 10-12 V)!
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6. An oil drop with a mass of 7.20 x 10-16 kg is moving upward at a constant speed of 2.50 m/s
between two horizontal parallel plates. If the electric field strength between these plates is 2.20 x 104
V/m, what is the magnitude of the charge on the oil drop? (3.2 x 10-19 C)!
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7. During a Millikan oil drop experiment, a student records the weight of five difference oil drops. A
record is also made of the electric field intensity necessary to hold each drop stationary between the
two horizontal parallel plates.!
a) Using E as the manipulated variable, draw a graph showing the relationship between the weight and
the electric field.!
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b) Using only your graph, determine the elementary charge. [about 1.6 x 10-19 C]!
Lesson 6: Millikan’s Oil Drop Experiment