Millikan Oil Drop: Pre-lab Assignment

Your name: _____________________

Please print this page, fill it in, and show it to your TF at the start of your lab session.

The Millikan Oil-Drop experiment is an important experiment in the history of physics. The American physicist Robert A. Millikan used it to produce an accurate measurement of the charge on the electron.

In Millikan's apparatus a low-level radioactive source, an alpha emitter, was used to change the amount of charge on the oil droplets. Our apparatus has a similar source, a thorium-232 source with an initial activity level of 0.008 mC (millicuries). The manual for our apparatus states: "Thorium-232 is a naturally occuring, low-level alpha particle emitter. It is not regulated in its use and poses no hazard to the user of the apparatus." Do a little research about thorium-232 (what is the half-life, for instance?), alpha sources in general, and safe levels of radioactivity, and answer this question: Do you feel comfortable using an apparatus containing a 0.008 mC thorium-232 source? Why or why not?

The lab manual for this experiment mentions the electrostatic unit (e.s.u.). What is this a measure of, and how do you convert from e.s.u. to the corresponding standard MKS unit?

The simulation above gives you some idea of what to expect in the experiment. Five oil droplets are shown. Which of the droplets are positively charged?

Which of the droplets are negatively charged?