Millikan’s Oil Drop Experiment
PHYSICS CHAPTER 22 C
Atoms
 By the early 1900s, we knew:
 Atoms had electrons in them (J. J. Thomson)
 Atoms had positive charges in them (protons)
 Structure???
Millikan’s Experiment
 Milikan Oil Drop
 Millikan
Millikan’s Experiment
 Experiment balanced gravity and charges
 Eq = mg
 If E is known due to voltage (V = Ed)
 And weight is determined
 We can find q
 Q was found to be a multiple of 1.6 x 10-19 C
Example p375
 An oil drop weighs 1.9 x 10-14 N. It is suspended in
an electric field of intensity 4.0 x 104 N/C. What is
the charge on the oil drop?
 What we know:

W = 1.9 x 10-14 N
E = 4.0 x 104 N/C
 Equation:
 Q = W/E
 Substitute:
 Q = (1.9 x 10-14 N)/(4.0 x 104 N/C)
 Math:
 Q= 4.75 x 10-19 C
Example p 375 continued
 If the drop is attracted toward the positive plate, how
many excess electrons does it have?
 What we know:

Q = 4.75 x 10-19 C
e- charge: 1.6 x 10-19 C
 Equation:
 # e- = q/charge of e-
 Substitute:
 #e- = (4.75 x 10-19 C)/(1.6 x 10-19 C)
 Math:
 #e- = 2.9688 = 3