In the famous Millikan oil-drop experiment, tiny sphericaldroplets of oil are sprayed into a uniform vertical electric field.The drops get a very small charge (just a few electrons) due tofriction with the atomizer as they are sprayed. The field isadjusted until the drop (which is viewed through a small telescope)is just balanced against gravity, and therefore remains stationary.Using the measured value of the electric field, we can calculatethe charge on the drop and from this calculate the charge e of theelectron. In one apparatus, the drops are 1.40 mm in diameter andthe oil has a density of 0.850 g/cm3.
a) If the drops are negatively charged, which way should theelectric field point to hold them stationary? Up or down? Pleaseexplain your reasoning. (1)
b) If a certain drop contains four excess electrons, whatmagnitude electric field is needed to hold it stationary?Please express your answer to three significant figures.(2)
c) If you measure a balancing field of 5183 N/C for anotherdrop. How many excess electrons are on this drop? Pleaseexpress your answer as an integer. (2)
