II. A charge of 10 -9 Coulombs is placed on a rectangular metal plate 10 cm by 2
ID: 1864993 • Letter: I
Question
II. A charge of 10-9 Coulombs is placed on a rectangular metal plate 10 cm by 20 cm on a side. The plate is parallel to and spaced 1.5cm from a similar plate with an equal and opposite -10-9 Coulombs of charge on it to form a parallel plate capacitor.
6. What is the charge density on the positive plate?
a) 5x10-8 C/m2
b) 5x10-12 C/m2
c) 3.18x10-8 C/m2
d) 7.96x10-9 C/m2
e) 2x10-9 C/m2
7. What is the electric field strength in the space between the plates? You can neglect edge effects.
a) 226 N/C
b) 0.565 N/C
c) 5650 N/C
d) 3590 N/C
e) 1.77x10-4 N/C
8. What is the potential difference between the two plates in Volts?
a) 339 Volts
b) 84.8 Volts
c) 53.9 Volts
d) 71.8 Volts
e) 1.70 Volts
9. The potential in Volts is highest
a) at the surface of the negative plate
b) at the surface of the positive plate
c) at the point half way between the two plates where the forces are in equilibrium
d) nowhere. The potential is constant in the region between the plates so this is misleading
e) at either the positive or negative plate. The sign of the potential depends on the sign of the charge placed between the plates so there is not enough information to answer this question.
10. If a proton with charge 1.6x10-19 C and a mass of 1.67x10-27 kg is released from rest from the surface of the positive plate, what is its velocity when it reaches the negative plate in m/s?
a) 0 m/s since the proton is not attracted to the negative plate and remains near the positive plate
b) 5.4x1011 m/s
c) 1.62x1010 m/s
d) 127,000 m/s
e) 104,000 m/s
11. The proton as it moves freely from the positive to negative plate is
a) gaining potential energy while the kinetic energy stays constant
b) losing kinetic energy while the potential energy stays constant
c) gaining kinetic energy as its potential energy decreases
d) losing kinetic energy as its potential energy increases
e) neither gaining or losing kinetic energy since energy is conserved
Explanation / Answer
6) Charge density, sigma = q/A = (10^-9)/(0.01 x 0.02) = 5 x 10^-8 C/m^2
Option (a)
7) Electric field, E = 5 x 10^-8 / 8.85 x 10^-12 = 5649.72 N/C
Option (c)
8) Potential difference, V = 5650 x 0.015 = 84.75 V
Option (b)
9) Potential difference will be highest at negative plate.
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