Objective:
This experiment is a simplified version of the Millikan oil drop experiment. You are to use the computer simulation to determine as much as possible about the electrical charges on the drops. You will do this by balancing the force of gravity with the force on the drop due to the electric field.
Theory:
When an electrically charged object is placed in an electric field, an electrical force is exerted on it. This force is given by:
Fe = E q
In this simulation, oil drops are injected into an electrical field between two plates. You can control the strength of the E-field between the plates.
The oil drop is also in the earth's gravitational field, so it also has the following force acting on it:
Fg = m g
One way of studying the charge on the oil drops is to adjust the E-field between the plates until the electrical forces acting up on the drop exactly balance the pull of gravity down. In this case:
Fg = Fe
m g = E q
You can, therefore, calculate the actual charge on a single drop.
Procedure:
Go to the website http://www68.pair.com/willisb/millikan/experiment.html
Read through the information on the page and then go to the simulation.
Determine the E-field necessary to balance the oil drops. Make sure to leave on "replace new drops automatically." You need to find FOUR different voltages that work. To help decide if a E-field "works" watch one drop for a while, if it doesn't move leave the E-field alone. If the E-field "works" more drops on the screen will begin to stop. Once a few seem stationary, unchek "replace new drops automatically" and some drops should remain on the screen
Calculate the charge based on each E-field. (use one 5-step method)
Fill in the following table...make sure to go in order from low to high for the E-field.
| E- field number | E-field (N/C) | charge (C) |
| 1 | . | . |
| 2 | . | . |
| 3 | . | . |
| 4 | . | . |
Plot a histogram of the balancingE-fields and then another of charges (make sure to go lowest to highest for both) (This is a bar graph)
Questions:
1. Why do different drops require different E-fields to balance?
2. Are the steps of the E-field histogram the same? Explain why.
4. Are the steps on the charge histogram the same? Explain why.
5. What is the smallest charge that you measured?
6. What is the average step size on the charge histogram?
7. What is your best estimate of the minimum electrical charge?
8. Does your experiment support the idea of a quantized electrical charge?
Conclusion: What does this experiment show you about the charge of an electron? How did you determine this charge? Why does it work?