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Atomic Physics Lesson 2

Atomic Models and Millikan’s Apparatus

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POS Checklist­ explain Millikan’s oil-drop experiment and its significance relative to charge quantization.

(from Unit B)

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John Dalton 1803

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William Crooke 1885

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J.J. Thompson 1856 - 1940

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Starring…

(1905)

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Experimental Design:• large steel chamber with microscope and atomizer attached.

• atomizer sprayed a fine mist of oil into the chamber.

• oil becomes charged by friction when passing through the nozzle of the atomizer.

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Millikan’s Apparatus

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• Through the microscope, Millikan could watch the oil drops fall.

• He placed parallel plates in the chamber and activated it when a good looking drop came along.

• The electric force acting on the drop counteracted gravity, suspending the drop.

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• By measuring the diameter of the drop (and knowing the oil density), he could determine the drop’s mass.

• By setting the electric field strength so that the electric force was equal to the force of gravity, he could determine the charge of the drop.

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Concept

Small charged mass

Parallel, oppositely charged plates

If the mass is stationary (or moving at a constant velocity):

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The Complications• The mass used by Millikan was tiny drops of oil

created using an atomizer (think perfume sprayer).

• Mass of each drop was different.

• Mass was calculated by measuring (very small) radius to determine volume, then using density to get the mass.

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More Complications• Many drops appeared between the plates at once,

making it difficult to observe a single drop.

• The number of e¯ added or removed when the drops were formed was not known.

• The drops are very small (about 10-16 kg) so they “float” on the air. This buoyant force must be factored into the calculations.

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http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%25::100%25::/sites/dl/free/0072512644/117354/02_Millikan_Oil_Drop.swf::Milikan%20Oil%20Drop

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Example:• An oil drop with the mass of 9.80 x10-16 kg is

suspended in a 20 kV/m electric field between two parallel, oppositely charged plates.

• Find the charge on the drop.

• How many e¯ have been added or removed from the drop?

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Solution:

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Solution b.

• (no way to tell if e¯ added or e¯ removed)

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Example:• An oil drop (weight = 4.80 x 10-14 N) is

suspended between two parallel, oppositely charged plates that are 3.00 cm apart. The potential between the plates is 1.80 kV.

• How many e¯ does the drop have in excess or in deficit?

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Formula Manipulation

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Solution:

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Finally: • After thousands of trials, Millikan had enough

successful trials to show that all of the charges he calculated were multiples of one number, 1.60x10-19 C.

• Since he never found a smaller common multiple of this charge, he concluded this must be the charge on a single electron.

• The idea that charge could be quantized (broken down to an indivisible piece) had been proven.

• This is called the quantization of charge.

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Example• An oil drop is suspended between two parallel,

oppositely charged plates, 6.00 cm apart. The potential between the plates is 600 V. The oil drop radius is 4.94 x 10-5 cm. The oil density is 0.952 g/mL. Find the number of electrons of imbalance on the drop (the number of e¯ added or removed).

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Homework: • Read text. P. 761-762

• Do questions p. 763 #1,2 and p. 764 #1,2

• Workbook p. 120 Do #4 - 9