Millikan’s Oil Drop Experiment

P H Y S I C S C H A P T E R 2 2 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 10 4 N/C. What is the charge on the oil drop?

What we know:  W = 1.9 x 10 -14 N E = 4.0 x 10 4 N/C Equation:  Q = W/E Substitute:  Q = (1.9 x 10 -14 N)/(4.0 x 10 4 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