12Apr/10Off
Floats Corks
A Floating Cork Ball and Charge Question :S?
A cork ball of mass 3.85 g is placed between two very large horizontal plates. The bottom plate has a uniform charge density of +0.52 μC/m2, whereas the upper plate has a uniform charge density of -0.90 μC/m2. The cork ball, which carries an unknown charge, is placed between the plates and is observed to float motionlessly. What is the charge on the ball?
any help on this question would be very much appreciated
The electric field due to each plate is E = σ/2εo where εo = 8.854x10^-12
Between the plates the fields add so E = σ/2εo)top + σ/2εo)bottom = (σt + σb)/2εo
=> E = (0.90x10^-6 + 0.52x10^-6)/(2*8.854x10^-12) = 80190N/C directed upward
So a positive charge would experience an upward force.
For the cork to float the weight (m*g) must be the force from the field Eq
So Eq = m*g => q = m*g/E = 0.00385kg*9.8m/s^2/80190N/C = 4.71x10^-7C = 0.471μC
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