Calculating Speed of Block with Dielectric in Capacitor

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The discussion revolves around calculating the speed of a block connected to a dielectric in a capacitor setup. The initial energy stored in the capacitor without the dielectric is calculated as 0.25 J, while the energy with the dielectric is reduced to 0.05 J due to the dielectric constant of 5. The energy change of 0.2 J is then equated to the kinetic energy of the block to find its speed. However, the calculated speed of 0.4 m/s is incorrect; the actual speed at the moment the dielectric leaves the capacitor is 4.4091 m/s. The importance of considering the potential energy change of the block is emphasized in the calculations.
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Homework Statement


Consider a horizontal square plate capacitor of area 1*1 m2, capacitance in vacuum 2 uF, which contains a dielectric material with dielectric constant K=5. The dielectric slides frictionlessly and is attached via a massless string and a massless pulley to a block of mass 2.5 kg. The block pulls the dielectric from the capacitor as it falls. Compute the speed of the block at the instant the dielectric leaves the capacitor assuming it starts at rest and that the voltage across the capacitor after the dielectric is removed is measured to be 500 V.


Homework Equations


U = 1/2 CV^2 = q^2 / 2C


The Attempt at a Solution


Energy without dielectric = 1/2 * 2 * 10^-6 * 500^2 = 0.25
Energy with dielectric = 0.25 / k = 0.05

Energy change = 0.25 - 0.05 = 0.2

0.2 = 1/2 mv^2
v^2 = 0.4 / 2.5
v = 0.4 m/s

Actual answer is 4.4091 m/s .
 
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123yt said:
Energy change = 0.25 - 0.05 = 0.2

Do not forget the change of the potential energy of the box.

ehild
 
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