Physics I
Class 25
Review Activity
Exam 3
Rev. 15-Apr-07 GB
25-1
Exam Time and Rooms
Exam 3 – Apr. 24, 6-7:25 PM
Sections
01,15
02,07
14
04,05
09
10
11,12
Days/Times
MR 8-10,2-4
MR 10-12,4-6
MR 12-2
MR 12-2,2-4
TF 10-12
TF 10-12
TF 12-2,2-4
Professor
Bedrosian
Wilke
Zhang
Yamaguchi
Wetzel
Washington
Eah
Exam Room
DCC 308
DCC 308
DCC 308
DCC 318
DCC 324
DCC 324
Sage 3303
Please come to the assigned room for your section.
Conflict exam or for students who get extra time:
J-R SC 2C06 Apr. 24 – Extra time, please start 5-6 PM. Conflict, start any time 5-8
PM.
25-2
Cover Page of the Exam
Name ___________________________________________
Exam #3
Physics I
Spring 2007
If you would like to get credit for having taken this exam, we need your
name (printed clearly) at the top and section number below.
Your name should be at the top of every page.
Print your name first thing!
Check mark by section #.
Section #
_____ 1
_____ 2
_____ 4
_____ 5
_____ 7
_____ 9
_____ 10
_____ 11
_____ 12
_____ 14
_____ 15
M/R 8-10 (Bedrosian)
M/R 10-12 (Wilke)
M/R 12-2 (Yamaguchi)
M/R 2-4 (Yamaguchi)
M/R 4-6 (Wilke)
T/F 10-12 (Wetzel)
T/F 10-12 (Washington)
T/F 12-2 (Eah)
T/F 2-4 (Eah)
M/R 12-2 (Zhang)
M/R 2-4 (Bedrosian)
Questions
Part A
Value
24
Part B
28
Part C-1
24
Part C-2
24
Total
100
Score
You may not unstaple this exam.
Only work written on the same page as the question will be graded.
Part A = 6 at 4 points each.
Part B = 1 graphing problem.
Part C = 2 problems.
Show all work.
Use only the formulas on the
sheet and/or standard math.
Cheating on this exam will result in an F in the course.
25-3
Relationship of Energy and
Electric Potential Changes
Memorize these tables or know how to derive them!
K = –U
U=qV
or U = q V
Potential Energy Change
+ charge
– charge
V > 0 (+)
+ (increase)
– (decrease)
V < 0 (–)
– (decrease)
+ (increase)
Kinetic Energy Change
+ charge
– charge
V > 0 (+)
– (decrease)
+ (increase)
V < 0 (–)
+ (increase)
– (decrease)
25-4
Fixing Common Errors #1 & #2

48a. | E i | 
1 | qi |
4   0 ri 2

1 qi
48b. E  
(r̂i )
2
4   0 ri
The ri2 in the denominator is never xi2 or yi2 separately!
You must take into account that E is a vector!
25-5
Fixing Common Errors #3 & #4
50.
1 qi
V
4   0 ri
The ri in the denominator is never xi or yi separately!
V is a scalar – just a number with no X or Y components!
25-6
Fixing Common Errors #5

48a. | E i | 
1 | qi |
4   0 ri 2

1 qi
(r̂i )
48b. E  
2
4   0 ri
1 qi
50. V  
4   0 ri
These formulas are for calculating the electric field and
electric potential at a location in space due to point charges
at other locations in space – not at the same location!
25-7
Fixing Common Errors #6
V
x
V
53y. E y  
y
V
53z. E z  
z
53x. E x  
These formulas are for calculating the electric field at a location in
space when you know the electric potential already and it is very
easy to take its derivatives (parallel plates).
It is better to use the point charge formulas for situations where
point charges create the electric field and electric potential.
25-8
Fixing Common Errors #7
49.
51.


F  qE
U  qV
The electric field (E) in 49 and the electric potential (V) in 51 are
not created by q – q is the charge at the point where E and V are
calculated, that experiences force F and has potential energy U.
E and V can exist without q's being there at all.
F and U cannot exist without q.
25-9
Fixing Common Errors #8
55.
mv
r
qB
The "r" in 55 is not the same r as in previous formulas for electric
field and electric potential.
25-10
Review Activity
The purpose of this activity is to help you prepare for Exam 3. It will be
graded like a test with a total of 10 points as your activity grade.
Quick Summary:
1. Answer the questions on your own paper.
2. You must turn in your individual work.
3. You may discuss the questions with anyone in the room. However,
your professor and/or TAs will not tell you if an answer is correct.
4. You may not discuss the questions with students who have not yet
done the activity.
5. The questions and answers will be posted on the website after the last
section has completed the activity.
6. This is a great time to get help from your professor and/or TAs.
25-11