Name ________________________________________________________________________
Exam #3
Physics I
Spring 2003
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.
Section #
_____ 1
_____ 2
_____ 3
_____ 4
_____ 14
_____ 5
_____ 7
_____ 10
_____ 11
_____ 15
_____ 12
M/R 8-10 (Washington)
M/R 10-12 (Schroeder)
M/R 10-12 (Zhang)
M/R 12-2 (Bedrosian)
M/R 12-2 (Adams)
M/R 2-4 (Hayes)
M/R 4-6 (Bedrosian)
T/F 10-12 (Wilke)
T/F 12-2 (Sperber)
T/F 12-2 (Adams)
T/F 2-4 (Wilke)
Questions
Part A
Value
16
B-1,2
18
B-3,4
16
C-1,2
26
C-3
24
Total
100
Score
If we catch you cheating on this exam,
you will be given an F in the course.
Sharing information about this exam with people who have not yet
taken it is cheating on the exam for both parties involved.
The Formula Sheets are the last two pages. Detach carefully for easier reference if you wish.
1
Name ________________________________________________________________________
On this exam, please neglect any relativistic and/or quantum mechanical effects. If you don’t
know what those are, don’t worry, we are neglecting them! On all multiple choice questions,
choose the best answer in the context of what we have learned in Physics I.
On numerical questions, show all work to receive credit.
Part A – Warm-Ups – 16 Points Total (4 at 4 Points Each)
Write your choice on the line to the left of the question number.
_______1.
A)
B)
C)
D)
E)
The velocity of the proton is at a right angle to the magnetic field.
The velocity of the proton is in the same direction as the magnetic field.
The velocity of the proton is in the opposite direction as the magnetic field.
Any of the above (A-C) would cause the proton to slow down.
None of the above would cause the proton to slow down.
_______2.
A)
B)
C)
D)
E)
F)
V=0
Y = 1 cm
V = -50
Y = 0 cm
V = -100
Y = -1 cm
Y
X
An alpha particle (charge = +2e) begins with a kinetic energy of 6.0 x 10–17 J at
point P where the electric potential is +50 V. It moves to point Q where the electric
potential is –50 V. Considering only the force on the alpha particle due to the
electric field, what is its kinetic energy at point Q (to two significant digits)?
2.8 x 10–17 J
4.4 x 10–17 J
6.0 x 10–17 J
7.6 x 10–17 J
9.2 x 10–17 J
_______4.
A)
B)
C)
D)
E)
F)
The figure below shows electric equipotential lines in a certain region of space.
What is the direction of the electric field in this region?
–X
+X
–Y
+Y
+Z = Out of the page.
–Z = Into the page.
_______3.
A)
B)
C)
D)
E)
A proton is moving in a uniform magnetic field. Which of the following conditions
(if any) would cause the proton to slow down?
An electron is moving East in a uniform magnetic field directed North. What is the
direction of the Lorentz force on the electron?
N
North.
South.
East.
West.
Up (out of the page).
Down (into the page).
B
W
v
electron
2
E
S
Name ________________________________________________________________________
Part B – Short Questions and Problems – 34 Points Total
B-1 (8 Points)
A neutron star has mass = 2.0 10 30 kg and a radius of 10.0 km. What is the gravitational
acceleration at the surface of the neutron star?
Answer: _____________________ units ________
B-2 (10 Points)
Match the following quantities with the short description on the right. Put the corresponding
letter in front of each description. (Some quantities in the list are not used; none are used twice.)
A.
B.
C.
D.
E.
F.
G.
Electric Force
Electric Field
Electric Potential
Electric Potential Energy
Electric Charge
Magnetic Field
Universal Gravitation Constant
______ Scalar, SI unit = Joule
______ Scalar, SI unit = Coulomb
______ Scalar, SI unit = Volt
______ Vector, SI unit = Newton
______ Vector, SI unit = Volt / meter
3
Name ________________________________________________________________________
B-3 (8 Points)
An electron is traveling in space at 1.4 10 7 m/s in a circle. The only significant force on the
electron is due to a magnetic field of 5.7 10 5 T at a right angle to the motion of the electron.
What is the radius of the circle?
Answer: _____________________ units ________
B-4 (8 Points)
An electric field of 300 N/C is created by adjusting an electric potential difference between two
parallel plates that are 5.0 cm apart (d). (The plates are equipotential surfaces.) What is the
magnitude of the potential difference needed to get this electric field?
Answer: _____________________ units ________
4
Name ________________________________________________________________________
Part C – Extended Problems – 50 Points Total
C-1 (14 points)
Two point charges are located as shown below. The variable “q” is a non-zero charge. Find the
X coordinate of a location on the X axis (not infinity) where the electric field created by the two
charges is zero, in terms of the variable d.
Y
X
+9q
-4q
d

E  0:
X Coordinate: _________________________ d
C-2 (12 points)
For the same configuration as C-1 above, find the X coordinate of a location on the X axis (not
infinity) where the electric potential created by the two charges is zero, in terms of the variable d.
Assume that the electric potential is zero at infinity.
V  0:
X Coordinate: _________________________ d
5
Name ________________________________________________________________________
C-3 (24 points)
Let d = 1.0 cm and q = 1.0 x 10–9 C. Calculate the electric field and electric potential at point P
(“X marks the spot”). Assume that the electric potential is zero at infinity.
Y
Point P
at (d,d)
d
X
+9q
Point P:
d
-4q
E field X Component: ___________________ units ________
E field Y Component: ___________________ units ________
Electric Potential:
___________________ units ________
6
Name ________________________________________________________________________
Formula Sheet for Exam 3 and Final – Page 1 of 2
1.
v  v 0  a t  t 0 
21.
2.
x  x 0  v 0 ( t  t 0 )  12 a ( t  t 0 ) 2
K  12 m v 2  12 m (v x  v y )
22.
3.
x  x 0  12 ( v0  v)( t  t 0 )
23.
K f  K i  Wnet


U    Fcons  dx
4.
x  x 0  v( t  t 0 )  12 a ( t  t 0 ) 2
24.
U g  m g (y  y 0 )
25.
U s  12 k ( x  x 0 ) 2
26.
27.
28.
 K   U  Wnoncons
s  r
v tangential   r
a tangential   r
2
2
6.
v 2  v 02  2a x  x 0 
 

 F  Fnet  m a
7.
T
8.
a centripetal 
29.
30.
  0  t  t 0 
Fcentripetal


p  mv

 
dp
 F  Fnet  d t



J   Fnet dt   p


P   pi


dP
  Fext
dt
31.
   0  0 ( t  t 0 )  12 ( t  t 0 ) 2
32.
   0  12 (0  )( t  t 0 )
33.
   0  ( t  t 0 )  12 ( t  t 0 ) 2
M   mi
38.
5.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
2r
v
v2
 2 r
r
v2
m
 m 2 r
r
35.
 2  02  2   0 
   
a  b  a b sin( )
36.
I   m i ri
34.
37.
39.
1
1
x cm   m i x i y cm   m i y i
M
M


P  M v cm
   
a  b  a b cos()  a x b x  a y b y
 
W  Fd
 
W   F  dx
40.
41.
42.
43.
44x. m1 v1, x ,before  m 2 v 2, x ,before  m1 v1, x ,after  m 2 v 2, x ,after
44y. m1 v1, y ,before  m 2 v 2, y ,before  m1 v1, y ,after  m 2 v 2, y,after
m1  m 2
2 m2
v1,i 
v 2 ,i
m1  m 2
m1  m 2
2 m1
m  m1

v1,i  2
v 2 ,i
m1  m 2
m1  m 2
45a. v1,f 
45b. v 2,f
7
2
K rot  12 I  2
 
W     d
  
  r F

 dL

  I  d t
  
l  r p


L  l i


L  I
Name ________________________________________________________________________
Formula Sheet for Exam 3 and Final – Page 2 of 2

m m
46a. | F |  G 1 2 2
r

m m
46b. F  G 1 2 2 r̂
r

1 | q1 || q 2 |
47a. | F | 
4  0
r2

1 q1 q 2
47b. F 
(r̂ )
4  0 r 2

1 | qi |
48a. | E i | 
4   0 ri 2

1 qi
(r̂i )
48b. E  
4   0 ri 2


49. F  q E
50.
51.
52.
1 qi
4   0 ri
U  qV
 
V    E  dx
V
V
x
V
53y. E y  
y

 
54. F  q v  B
mv
55. r 
qB
53x. E x  
Useful Constants
(You can use the approximate values on tests.)
Universal Gravitation Constant
G  6.67310 11 N m 2 kg 2  6.67 10 11
Electrostatic Force Constant
1
 8.987551788 10 9 N m 2 C  2  9.0 10 9
4  0
Magnetic Constant
 0  4  10 7 H m 1  1.26 10 6
Speed of Light in Vacuum
c  2.99792458 10 8 m s 1  3.010 8
Charge of a Proton
e  1.602176462 10 19 C  1.6 10 19
Electron-Volt Conversion Constant
1eV  1.602176462 10 19 J  1.6 10 19
Mass of a Proton
m p  1.6726215810 27 kg  1.67 10 27
Mass of an Electron
m e  9.10938188 10 31 kg  9.110 31
8