# Physics

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A charged disk of radius R that carries a surface charge density σ produces an electric field at a point a perpendicular distance z from the center of the disk, given by:

Edisk = (sigma/2e0) x(1-(z/(square root(z^2 + R^2))

Consider a disk of radius 10 cm and positive surface charge density +3.7 mC/m2. A particle of charge -4.5 mC and mass 75. mg accelerates under the effects of the electric field caused by the disk, from a point at a perpendicular distance from the center of the disk.

The final speed of the particle is 1.0 m/s and the work done on the particle by the electric field is -3.0 mJ.

How fast and in what direction was the particle originally moving?

a) 0 m/s
b) 9.0 m/s towards the disk
c) 9.0 m/s away from the disk
d) 50 km/s towards the disk
e) 50 km/s away from the disk

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