Atomic Models and Millikan’s Apparatus Atomic Physics Lesson 2

John Dalton 1803 Solid Sphere model of the atom He believed that the atom could not be broken down any further

William Crooke 1885 First discovered cathode rays but could not identify what they were except that they were negatively charged A CRT is also called a Crooke’s tube

J.J. Thompson 1856 - 1940 Proved that the cathode rays were actually streams of electrons Determined the charge to mass ratio This led to the next model of the atom – Plum Pudding Model or Raisin Bun Model negative electrons floating in a sea of positive charge

Raisin Bun Model

Robert Millikan 1909 determined the charge on an electron.

Concept Parallel, oppositely charged plates Small charged mass

Apparatus

If the mass is stationary (or moving at a constant velocity): q is a multiple of the charge on one electron

Millikan’s Apparatus

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.

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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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?

Solution:

Solution b. (no way to tell if e¯ added or e¯ removed)

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?

Formula Manipulation

Solution:

Finally: not notes 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 sometimes called the quantization of charge.

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).

Homework: Read text. P. 761-762 Do questions p. 763 #1,2 and p. 764 #1,2 Workbook p. 120 Do #4 - 9