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hydey
Joined: 29 Jan 2012 Posts: 2 Location: United Kingdom

Posted: Wed Feb 22, 2012 6:48 pm Post subject: What charge is a spherical drop?? Stuck on question. HELP! 


Hi there,
I am working through a past paper for physics from home and I am very stuck on the following question, is anybody able to shed any light on it for me?
It's worth 9 marks.
Thank you in advance.
Q. Two parallel plates are placed 0.10m apart with one vertically above the other and their edges aligned. The potential difference of the upper plate is 100kV relative to the lower plate. What charge must be a spherical raindrop of diameter 1.0mm carry if it remains suspended between the plates? Assume that the electric field between the plates is uniform and give your answer in units of the charge on an electron. (The density of the water is 1000 kg m3.) 

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hepcj Site Admin
Joined: 23 Jun 2007 Posts: 125

Posted: Wed Feb 22, 2012 11:32 pm Post subject: 


For the raindrop to be suspended between the plates the sum of the gravitational force pulling the drop down must be equal to the attractive force of the electric field pulling the drop up toward the positive plate.
Since like charges repel there must be a negative charge on the drop.
The magnitude of the force pulling the drop down is just mg. Where m is the mass of the drop.
(m = density x volume of the drop)
The magnitude of force on the drop in a uniform electric field is F =q E.
Therefore, q E = m g or q = mg/E.
Oh, and E = V/d where v is the voltage difference between the plates and d is their separation. 

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