Millikan Oil Drop Virtual Lab Activity
https://www.thephysicsaviary.com/Physics/Programs/Labs/MillikanOilDropLab/index.html
Note: Students will follow instructions from the simulation.
Objectives: Students calculate for the charge of an electron using the virtual lab and suggest ways to improve the experiment.
Part I.
1
Derive an equation for q in terms of density of oil oil, radius of oil roil , acceleration due to gravity g, distance between
capacitor plates d, and voltage V.
2
Complete the table and answer the questions that follow.
Constants: g = 9.8 ms-2
oil = 900 kgm-3
d = 3.0 x 10-3m
4
Vsphere = 𝜋𝑟 3
3
E = V/d
Trials
1
2
3
r/x10-9m
Voil/×10-19m3
moil/×10-16kg
Fg /×10-15N
V/V
E/Vm-1
q/ × 10-19C
4
5
(a) What can you say about the charge q (8th column)?
……………………………………………………………………………………………………………………..
(b) How can you determine the charge for a single electron?
…………………………………………………………………………………………………………………….
(c) Is the charge of a single electron accurate in this experiment? Why?
………………………………………………………………………………………………………………………
(d) Suggest another way to determine the charge of a single electron more accurately.
…………………………………………………………………………………………………………………….
(e) Suggest what is meant by quantization of electric charge
…………………………………………………………………………………………………………………….
(f) What can you conclude about this experiment?
…………………………………………………………………………………………………………………….
Part II. Post Lab
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The data below is from an experiment similar to Millikan’s experiment.
· Density of oil = 900 kg m-3
· Pd across the plates = 613 V
· Plate separation = 0.01 m
· Viscosity of air = 1.8 × 10-5 N s m-2
When the voltage between the plates is turned off, the droplet falls steadily a distance of 2.50 10-3 m in a time of 22
s. What is the charge? Take g as 9.8 m s-2. Neglect any effects of buoyancy force.
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In an experiment to determine the charge on a charged oil droplet, the droplet was held stationary in a vertical electric
field of strength 57 kVm-1. After the field was switched off, the droplet fell at a steady speed, taking 18.3 s to fall
through a vertical distance of 2.0 mm. Neglect any effects of buoyancy force.
Viscosity of air = 1.8 × 10-5 Ns m-2
Density of oil = 970 kg m-3
g = 9.8 m/s-2
(a) Calculate the speed of the droplet as it falls.
(b) Show that the droplet’s radius is 9.7 × 10-7 m
(c) Calculate the charge of the droplet.
(d) Compare this to the electronic charge. What does it suggest?
Mark Scheme
1
Fe = Fg
qE = mg ; E = V/d
q=mgd/V
m = V and V = 4/3r3
q = (4/3r3)gd/V
2 Sample table.
S/N
1
2
3
r/x10-9m
480
520
330
Voil/×10-19m3
4.63
5.89
1.51
moil/×10-16kg
4.17
5.30
1.35
Fg /×10-15N
4.09
5.19
1.33
V/V
19
16
12.3
E/Vm-1
6333.3
5333.3
4100
q/ × 10-19C
6.45
9.74
3.24
(a) Charges are different. Bigger radius/volume have bigger charges.
(b) Finding the least common multiple from the data. However, in the experiment, only 3 data were gathered. In Millikan’s
experiment, there were lots of data gathered, and the least common multiple was determined to be approximately 1.6 × 10-19 C.
(c) No. drag force caused by air is not considered. Big error in reading the radius since it is too small. Consider calculating
the radius using Stoke’s law.
(d) Consider drag force. Use Stoke’s law to find radius.
(e) The important finding from this experiment was that the charge was always a whole number multiple of 1.6 x 10-19 C.
Therefore the electron charge came in definite amounts (quanta); therefore it is said to be quantised.
(f) Electrons are indivisible, i.e. fundamental particles of matter.
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