Question

A negatively charged oil drop whose mass is 10-15 kg and whose charge is -6.4 10-19 Coulombs remains stationary under the influence of the Earth’s gravitational field and a uniform external electric field made by two oppositely-charged parallel plates (see diagram below). Ignore the influence of Earth’s electric field on the oil drop.

1. Determine the magnitude and direction of the net electric field created by the plates (not the electric field of the oil drop !). Explain completely how you determined the direction of the field - you’ll need to explain how you apply the positive test charge rule as part of your explanation. Yes this is a net force problem.

2. On the diagram above, draw the electric field lines created by the plates – make your field lines clearly show that the magnitude and direction of the electric field are constant. Make sure your diagram is consistent with the information given in the problem statement.

3. How many excess electrons does the drop have ?

4. If the charge of the oil drop were doubled, but everything else in the problem remained the same, what would happen to the oil drop ? Determine the magnitude and direction of the oil drop’s acceleration (if any) for this situation.

Answer 1

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