PHYS2424 – SPRING 2000
EXAM #1 – Part 2 (Version A)
Electric Force, Fields, Potential, Capacitance, and Circuits
NAME: __________________________________________________________
Problem 1: __________________________________
(40 marks)
Problem 2: __________________________________
(30 marks)
Problem 3: __________________________________
(20 marks)
Problem 4: __________________________________
(50 marks)
Problem 5: __________________________________
(20 marks)
Problem 6: __________________________________
(20 marks)
Problem 7: __________________________________
(30 marks)
Problem 8: __________________________________
(40 marks)
Problem 9: __________________________________
(30 marks)
Problem 10:__________________________________
(30 marks)
Total Part 2 __________________________________
(310 marks)
1.
ACME Corporation’s Chief designer, Bugs Bunny, has designed a capacitor with
the specifications:
Capacitor Specification
Type: Parallel-Plate
Conductor Material: Aluminum
Conductor Length: 25 cm
Conductor Width:
10 cm
Dielectric:
Pyrex Glass
Gap Width:
7.5 mm
While setting up, ACME Technician Wiley E. Coyotte incorrectly chooses pyrex
glass with a thickness of 5 mm leaving a 2.5 mm air gap between the pyrex glass
and the top conductor plate.
Air
Pyrex
Glass
A.
Al plates
What is the capacitance of the capacitor designed by Bugs Bunny?
1B.
What is the capacitance of the capacitor produced by ACME corporation?
2.
The electrostatic potential in a certain region of space is given by
V  4x 3  2xy  3y 2 volts, where x and y are in meters.
A.

Calculate the electric field E within this region in terms of x and y.
B.

Assuming that point (1.50m, 3.00m) is within this region of space, calculate E at
point (1.50m, 3.00m).
3.
In constructing an electrical circuit, your circuit diagram calls for a 35- resistor.
You have several 10- resistors. How would you construct an equivalent 35-
resistor? (To receive full credit including a drawing of how you would connect
these resistors.)
4.
Three isolated point charges q1, q2 and q3 are arranged as shown below:
y
q1 = 6.00 C
x
x
q1
q2 = 6.00 C
q2
q3 = - 3.00 C
x = 3.00 cm
y
y = 4.00 cm
q3
x
A.
Calculate the magnitude and direction of the electric field (due to q1, q2 and q3) at
the origin.
4B.
Calculate the magnitude of the acceleration imparted to an electron placed at rest
at the origin.
C.
What is the electric potential at the origin? (Use r = , as your zero potential
reference point)
D.
What is the electrical potential energy stored in this charge configuration?
5.
What is the electric potential difference between point B and point A on the xaxis, if the x-component of the electric field varies along the x-axis as shown
below:
Ex (N/C)
A
12
6
-6
3
-3
6
9
x (m)
B
-3
6.
Calculate the total capacitance for the circuit below:
C
2C
A
2C
2C
B
2C
7.
You are given the unique MARBLE electric dipole that can exist in one of four
states as shown below. If the Marble dipole has a dipole strength of 2.00x10-29
coulombmeter and is interacting with an external electric field of 1.00x104 V/m,
what is the energy in electron volts for each of its four states.

P
60
120
State 1
Zero degrees
State 2
State 3
State 4
8.
Two charged metal plates in vacuum are 15cm apart as shown below. The electric
field between the plates is uniform and has a magnitude of 100 N/C. An electron
(mass = 9.1x10-31 kg) is released from rest just inside the negative plate.
E = 100 N/C
A.
+
+
-
+
-
+
-
+
-
How long will it take to reach the positive plate?
8B.
How fast will the electron be going just before it hits the positive plate?
8C.
If the positive plate of the capacitor has a net charge of 5.00 C, what is the
capacitance of the capacitor.
9.
As chief physicist for ACME products, a subsidiary of Roadrunner industries, you
are given the following device: a long conducting cylinder (length L) carrying a
total charge +q surrounded by a conducting cylindrical shell of total charge –2q,
as shown in the figure below.
c
b
Find the following:
A.
What is the charge on the inside surface of the outside conductor ( r = b )?
B.
What is the charge on the outside surface of the outside conductor ( r = c )?
9C.
What is the electric field in the region b<r<c ?
9D.
What is the electric field in the region c<r ?
10.
For the circuit shown below:
20 v
6 k
20 k
30 k
15 k
5 k
4 k
Find the
A.
current through the 5 k resistor.
10B.
Find the voltage across the 5 k resistor.
10C.
Find the power dissipated by the 5 k resistor.