Question

Judy and Elroy Jetson take a field trip to Mars for science class (recall that the acceleration due to gravity on Mars is smaller than on Earth, gMars = 3.7 m/s2 ). They repeat a version of Millikan’s oil-drop experiment.

(A) Compared to the same experiment on Earth (same drop mass and charge) would the required electric field to hold the oil drop in place be smaller, larger, or the same (why)?

(B) Elroy forgets to write down the charge for one of his droplets. He thinks it might have been 2.30 × 10−19 C. Judy says that Elroy remembered incorrectly, why did she say that?

Answer 1

Part A

In Millikan's Oil Drop Experiment, the electrostatic force on a charged droplet counters the weight(gravitational force) to hold it in place.

As such, the forces are equal, and the net force on the drop is zero.

As we can see, the electric field required is directly proportional to the local gravity. Hence, since Mars has less gravity, the field required will be less as well.

Part B

The charge on an oil drop, or for that metter any object, is always an integral multiple of the charge of an electron.

This is because the charge on a body exists due to an excess or deficit of electrons, and electrons only exist as a whole.

To find out whether Elroy is right, we divide the charge he remembers by the charge of an electron. If the answer is not an integer, he remembers incorrectly.

Since this is not an integer, Judy is correct about Elroy not remembering it correctly.

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