Physics Question & Answer Key

A proton with an initial horizontal velocity of 2.3X10^6 m/s enters a region of uniform electric field strength of 1.1X10^5 N/C between two oppositely charge parallel plates. If the plates are 1.3cm apart and 6.7cm long, how far below the top plate must the particle be shot so as to just miss hitting the right edge of the top plate? Ignore gravitational forces.

t = d / v

   = 0.067 / (2.3e6)

   = 2.9e-8

E = F / q

F = Eq

   = (1.1e5)(1.6e-19)

   = 1.76e-14

F = ma

a = F / m

    = (1.76e-14) / (1.67e-27)

    = 1.05e13

s = ut + .5at²

   = 0 + .5(1.05e13)(2.9e-8)²

   = 0.0045 m (from top)

Physics Question & Answer Key

A 5.00 x 10^ -2 kg mass with a charge of .75μC is hung by a thin insulated thread. A charge of -0.9μC is held .15m directly to the right so that the thread makes an angle Θ with the vertical. What is the angle Θ and the tension in the string?

Force between charges:

F = [(9e9)(0.75e-6)(0.9e-6)] / (0.15²) 

   = 0.27 N

Gravitational force:

mg = 5e-2 x 9.8 

      = 0.49 N

From a force vector diagram:

0.49N vertically down & 0.27N horizontal - -  combining to form θ from vertical) 

tan θ = 0.27 / 0.49 

θ = 28.9º

sin θ = 0.27 / T 

T =0.27 / (sin28.9) 

   = 0.56 N