GENERAL PHYSICS II         SAMPLE EXAM 1         PHYS 2120

1]     Below is a diagram showing the relative positions of 3 charges. Determine the force on the
- 5 µC charge due to the other 2 charges. [ remember you need magnitude & direction ]

10 cm
+ 8 µC · ------------------------ ·+ 2 µC
|
| 5.0 cm
|
- 5 µC ·

Answer: 57 N @ 62 degrees above -x axis

2]     In the hydrogen atom a single electron orbits around a single proton. The smallest electron
orbit has a radius of 5.29 x 10-11 m. How much work is required to completely remove the
electron from this orbit ( i.e., move the electron to infinity where we take the electric potential to
be zero)

Answer: +4.3 x 10-18 J

3]     {A} Determine the electric field at the point labeled, x, in the figure below.
[ remember magnitude & direction ]

+ 2 µC
· ------------------- x
10 cm
Answer: 1.8 x 10+6 N/C pointing to right

{B} A charge that is now placed at point x above experiences a force of 1.8 N that is directed
toward the + 2 µC charge. What is the magnitude and sign of the charge placed at point x?

Answer: -1.0 x 10-6 C

4]     {A} The cross sectional area of a parallel plate capacitor is 6.25 cm2 . The plates are separated
by a distance of 4.00 mm with only air between them. What is the capacitance of this
capacitor?

Answer: 1.38 x 10-12 F

{B} Explain what a capacitor does and how it does it.

7]     The cross sectional area of a parallel plate capacitor is 6.25 cm2 . The plates are separated
by a distance of 4.00 mm with only air between them. If the work done on a +4.0 m C in
moving it from one plate to the other is 2.0 x 10-4 J , what is the total charge, Q, on the
capacitor?

Answer: 7.0 x 10-11 C

MULTIPLE CHOICE

1 ]   Which of the following is not a vector?
a.  electric force
b. electric field
c. electric charge
d. electric lines of force
e. acceleration

2]   If a conductor is in electrostatic equilibrium near an electric charge
a.  the total charge on the conductor must be zero
b.  any charge on the conductor must be uniformly distributed
c.  the force between the conductor and the charge must be zero
d.  the total electric field of the conductor must be zero
e.  the electric field of the conductor is perpendicular to the surface

3]   For an electron moving in a direction opposite to the electric field
a. its potential energy increases and its electric potential increases
b. its potential energy decreases and its electric potential increases
c. its potential energy increases and its electric potential decreases
d. its potential energy decreases and its electric potential decreases
e. both the potential energy and the electric potential remain constant