Electricity Test Review
Please answer the questions on a separate sheet of paper.
Concepts:
1. What is your rule for determining how adding bulbs to a circuit affects resistance? Look at section 3 of the lab.
2. What is your role for determining how adding bulbs to a circuit affects flow? Look at section 3 of the lab.
3. What is the difference in series and parallel? Look at section 2 of the lab.
4. How do you find the equivalent resistance of resistors in parallel? Look at your Ohm’s Laws notes.
5. How do you find the equivalent resistance of resistors in series? Look at your Ohm’s Laws notes.
6. What is Ohm’s law and why doesn’t it always apply? Look at your Ohm’s Laws notes.
7. What is a short and how do you create it? Look at any section of the lab.
8. Draw the symbols for a battery, bulb, switch, and resistor. Look at section 1 of the lab.
9. Why does the symbol for a bulb have two identical terminals? Look at section 1 homework.
10. What is Coulomb’s law? Look at Coulomb’s law notes.
11. What is superposition? Look at Coulomb’s law notes.
12. How are Amperes related to Coulomb’s? Look at Ohm’s law notes.
13. What is the difference in an insulator and a conductor? Look at Coulomb’s law notes.
14. Is charge conserved? Look at Coulomb’s law notes.
15. What is the difference in parallel branches that are independent and dependent across the battery? Look at section 3 of the lab.
16. What does the prefix micro, µ, mean? Seriously, you have a phone, a computer, and you’re asking me. ;P
17. The potential difference across the ends of a wire is doubled in magnitude. If Ohm’s law is obeyed, which one of the following
statements concerning the resistance of the wire is true? C
(a)
The resistance is one half of its original value.
(b)
The resistance is twice its original value.
(c)
The resistance is not changed.
(d)
The resistance increases by a factor of four.
(e)
The resistance decreases by a factor of four.
Problems:
1.
Two isolated charges +q and -2q, are 2 centimeters apart. If F is the magnitude of the force acting on the charge -2q, what are the
magnitude and direction of the force acting on charge +q? F in the opposite direction.
2.
Find the equivalent resistance between X and Y in the two circuits shown below.
Left Req 
3.
(4)(2) 4
 
(4  2) 3
right Req  2 
(2)(2)
 3
(2  2)
In the circuit on the above left, 12 A enter the branch at X and exit at Y. What is the current through each resistor?
Assume x is the current in the 1 Ω and 3 Ω resistor and 6 is the current in the 2 Ω resistor.
x + y = 12 A. and 2x = y
so x +2 x = 12 A
thus, x = 4 A and y = 8 A
4.
A point charge q1 = 4.0 µC is at the origin and a point charge q2 = 6.0 µC is on the x axis at x = 3.0 m. Find the electric force on
charge q2?
4.0 µC
x=0
F
6.0 µC
x=3m
k (4 x106 C )(6 x106 C )
 0.024 N ,180
(3m) 2
5.
For problem 4 above, what is the electric force on q1? 0. 024 N, 0º
6.
How would your answers to 4 and 5 change if q2 = -6.0 µC? Both forces would be in the opposite direction.
7.
A -2.0 µC point charge and a 4.0 µC point charge are a distance L apart. Where should a third point charge be placed so that the
electric force on the third charge is zero?
x
+4.0µC
L
-2.0µC
The point where there is zero net force has to be closer to the smaller charge. It can’t be between the two forces because the net
force would be to the left. Thus, the point where the force is zero is on the left side of the charges.
k (2  C )q k (4  C )q

2
x2
 L  x
1
2

2
2
x
 L  x
 L  x
2
 2x2
L2  2 Lx  x 2  2 x 2
x 2  2 Lx  L2  0
x
8.
2 L  4 L2  4 L2
2 L2  L2
 L
 L(1  2)
2
2
Three point charges, each of magnitude 3.00 µC, are at separate corners of a square of edge length 5.00 cm. The two point charges
at opposite corners are positive, and the third point charge is negative. Find the force exerted by these point charges on a fourth
point charge q4 = 3.00 µC.
3µC
3µC
F3
F1
3µC
F2
3µC
k (3x106 C ) 2
F1 
 32.4 N ,180
(.05m) 2
F2  32.4, 270
k (3x106 C ) 2
F3 
 16.2 N , 45
( 2(.05m)) 2
F
Theta
x
y
32.364
180
-32.36
0.00
32.364
270
0.00
-32.36
16.18
45
11.44
11.44
-20.92
-20.92
F = -(20.9i+20.9j) N = 29.6 N, 225º
9.
A wire carries 2.00 A of current for 3.00 seconds. What is the charge that passes through that wire during that time?
q=it = (2C/s)(3s) = 6.0 C
10. What is the number of electrons that move through the wire in #9 during that time period?
6.0Cx
electron
 3.75 x1019 electrons
19
1.6 x10 C
11. How many electrons flow through a battery that delivers a current of 2.0 A for 15 s?
q=it = (2C/s)(15s) = 30 C
30Cx
electron
 1.875 x1020 electrons
19
1.6 x10 C
12. A 10-A current is maintained in a simple circuit with a total resistance of 200 Ω. What net charge passes through any point in
the circuit during a 1-minute interval?
q=it = (10C/s)(60s) = 600 C
600Cx
electron
 3.75 x1021 electrons
19
1.6 x10 C
13. When a light bulb is connected to a 4.5 V battery, a current of 0.16 A passes through the bulb filament. What is the resistance of
the filament?
R
V
4.5V

 28.125
i 0.16 A
14. Three resistors, 50- Ω, 100- Ω, 200- Ω, are connected in series in a circuit. What is the equivalent resistance of this
combination of resistors?
n
Req   Ri  350
i 1
15. A 4.5-V battery is connected to two resistors connected in series as shown in the drawing. What is the current in the circuit?
4.5 V
68 
68 
n
Req   Ri  68  68  136
i 1
i
V 4.5V

 0.033 A
R 136
16. Two 20- Ω and three 30- Ω light bulbs and a 15 V battery are connected in a series circuit. What is the current that passes through
each bulb?
n
Req   Ri  2(20)  3(30)  13
i 1
i
V
15V

 0.1154 A
R 130
17. Five resistors are connected as shown. What is the equivalent resistance between points A and B?
I
A
4.0 
4.0  3.0 
2.0 
Req  4 
3.0 
(4)(2) (3)(3)

 6.83
(4  2) (3  3)
18. Jason’s circuit has a 24- Ω resistor that is connected in series to two 12- Ω resistors that are connected in parallel. JoAnna’s
circuit has three identical resistors wired in parallel. If the equivalent resistance of Jason’s circuit is the same as that of
JoAnna’s circuit, determine the value of JoAnna’s resistors.
for Jason: Req  24 
(12)(12)
 30
(12  12)
Req  30 
for Joanna:
1
1 r
 
1 1 1 3 3
 
r r r r
r  90
Questions 19 through 20 pertain to the statement and diagram below:
Four resistors and a 6-V battery are arranged as
shown in the circuit diagram.
6V
10 
20 
30 
60 
B
19. Determine the equivalent resistance for this circuit.
for the branch on the right: Req  10 
(30)(60)
 30
(30  60)
combining the left and right branch: Req 
(20)(30)
 12
(20  30)
20. Which resistor has the smallest current passing through it? 60Ω