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PHYS 202
Final Exam
5/7/16
Dr. Holmes
NAME
DO ALL THE PROBLEMS. THE WORTH OF EACH PROBLEM IS MARKED NEXT TO THE
PROBLEM. SHOW YOUR WORK FOR PARTIAL CREDIT. (For directions, use the six standard
directions: N, E, S, W, up or out, and down or in.)
1) Consider two charged particles (q1 = +3 C, m1 = 24 grams; q2 = -9 C, m2 = 14 grams). The
first particle is 18 cm to the West of the second one.
a) What is the magnitude and direction of the electric FORCE on the 1 st particle due to the
presence of the 2nd particle? [when answering this direction question, answer with North, East, South, West,
Up or Down.]
__7.5 Nt______,__East________.
b) Is the magnitude of the force on the 2 nd particle due to the 1st particle [bigger, the same as, or
smaller] than the force on the 1st particle due to the 2nd particle?
___same_______.
c) Is the direction of the force on the 1 st particle due to the 2nd particle the same or opposite that
of the force on the 2nd particle due to the 1st particle?
__opposite_.
d) Is the magnitude of the acceleration of the 2 nd particle due to the presence of the 1 st particle
[bigger, the same as, or smaller] than the acceleration of the 1st particle due to the 2nd particle?
(Assume there are no other forces acting on the two particles).
bigger
.

N
1st particle
2nd particle
W E
+
-
S

18 cm
2) A particular light bulb is rated at 7 Watts when a voltage of 12 volts is placed across it.
a) What is the electric current through the light bulb in this circumstance?
__0.583 Amps___.
b) What is the resistance of the light bulb?
____20.6 _______. c) In designing a new wattage
light bulb, should the resistance of the light bulb be lowered or raised from that in part b above if
the power of the light bulb is to be increased when used with the same 12 volts power supply?
__lowered__.
PHYS 202
Final Exam
page 2
3) Consider three resistors: R1 = 3 , R2 = 5 , R3 = 8  .
a) Connect the three resistors in a circuit (make a circuit drawing) such that the effective
resistance is the smallest it can be using all three resistors:
b) What is the effective resistance of this circuit that has the smallest effective resistance using
these three resistors?
__1.52 ______.
c) Connect the three resistors in a circuit (make a
circuit drawing) such that the effective resistance is between 5  and 8 :
4) Consider an electron moving  East at a speed of 8 x 105 m/s in a magnetic field of strength
0.133 T directed North. a) What is the magnitude of the magnetic force on the electron?
1.7 x 10-14 Nt
. b) What is the direction of the magnetic force on the electron?
_____ down__. c) Will this magnetic force cause the electron to speed up, slow down, or
change direction?
___change direction__.
5) A certain mass spectrograph is to be designed so that ions (singly charged) of mass 56 amu
when accelerated to a speed of v = 8x104 m/s, go in a semi-circle of diameter of 12.0 cm when a
magnetic field is applied. a) What is the mass of the 56 amu ions in kg?
____9.3 x 10-26 kg__.
b) What should the strength of the magnetic field, B, be in the spectrograph to make the 56 amu
ions going at the above speed go in a semi-circle of diameter = 12.0 cm?
0.775 T
.
c) Will ions of mass less than 56 amu have diameters of orbit [smaller than, equal to, or bigger
than] the 56 amu ion’s diameter of orbit?
smaller than .
PHYS 202
Final Exam
page 3
6) In each of the four diagrams below, indicate on the A circuit the direction of the induced
current in circuit A due to the situation described (if no current, then write the word NONE on
circuit A):
a) the external magnetic field is increasing
in strength and is directed UP through
circuit A :
b) the South pole of the bar magnet is
moved away from the solenoid (cylinder
wrapped with wire) in circuit A:

N
S
 Bext
Circuit A
Circuit A is the loop of wire
c) The bar is moving to the RIGHT:
 Bexternal

Circuit A
v
d) The switch in circuit B is closed
and remains closed:
B

Switch
A

bar
+
Vbattery
none
7) A person is nearsighted and can see things clearly only if they are 63 cm away or nearer.
a) What focal length should the person's glasses (lenses) have if they are designed to let the
person see something at a far viewing distance of 100 m clearly?
-63.4 cm
.
b) Draw a diagram showing the position of the eye, the lens, the object and the image.
c) Design a lens that has the above focal length from part a), that is, specify nglass, R1 and R2 and
draw a picture of the lens:
nglass
R1
R2
picture:
.
PHYS 202
Final Exam
page 4
FOR PROBLEMS 8 AND 9 USE THE FOLLOWING INFORMATION: A 35 mm camera (film size
is 24 mm x 36 mm) uses a lens with a focal length of 120 mm and the f-stop setting is 4 (which
means the diameter of the opening to the lens is 1/4 the focal length or 30 mm). You take a
picture of a newspaper located 80 meters away. The size of the print on the page is 5 mm. For
purposes of calculation assume that the wavelength of the light is one in the middle of the visible
spectrum. [If you do not know what wavelength this is, you may ask and I will give it to you but
you will be marked down one point.]
8) a) What is the object distance?
80 m
. b) What is the image distance? (if the
distance is close to the focal length, make sure you indicate whether the distance is a little more
or less than the focal length)
120.2 mm . c) Given that the print size on the paper is 5
mm in height, what is the image height on the film?
7.5 m
.
9) a) What is the smallest angle that this camera can resolve with an f-stop setting of 4 based on
the Rayleigh criterion?
1.83 x 10-5 rad = 1.1 x 10-3 o. b) What angle does the print size (5 mm on
the paper) make when viewed from 80 meters away?
6.25 x 10-5 rad = 3.58 x 10-3 o .
c) Assuming the camera has quality lenses and a fine grain film, can the picture the camera
takes be enlarged big enough and clearly enough so that you can read what the newspaper said
[definitely, just barely, not quite, definitely not] ?
Just barely or not quite .
10) a) List two experiments that indicate light behaves as a wave and not as a particle:
(1)
(2)
b) List two experiments that indicate light behaves as a particle and not as a wave:
(1)
(2)
PHYS 202
Final Exam
page 5
11 a) What is the energy of a photon given off by a hydrogen atom in going from the n=5 state to
the n=2 state?
2.86 eV = 4.57 x 10-19 J . b) What is the wavelength of light from this transition?
435 nm
. c) What type (or if visible, what color) is this photon?
Visible: blue-green.
12) a) What is the speed of an electron accelerated through 115 volts?
6.36 x 106 m/s
.
b) What is the momentum of this electron ?
5.79 x 10-24 kg*m/s
c) What is the wavelength of the electron that has the above energy?
.
0.115 nm
.
d) What is the wavelength of a photon that has the above energy?
1.08 x 10-8 m
13) Fill in the missing particle(s):
233
92U
goes to
90X
27Co
58
goes to
26X
27Co
60
goes to
28X
229
+ alpha + energy
58
+
58
(There is one stable isotope of
+
+1
-1
27Co
0
0
+
0
0.
+ anti- 00 .
at a mass of 59.)
.
PHYS 202
Final Exam
page 6
14) a) Given that the half life of 27Co60 is 5.27 years, what is the decay constant,  for this
isotope?
0.132/yr = 4.17 x 10-9 /sec . b) How many atoms of Co60 are there in 1 gram?
1.0 x 1022 .
c) What is the activity of 1 gram of Co60 in dis/sec?
In Curies ?
4.17 x 1013 .
1,130
.
d) What will be the activity of this one gram after 100 years (in dis/sec) ?
8.1 x 107
.
15) C14 has a half life of 5,730 years, and the ratio of C14 to C12 is 1.3 x 10-12. a) What is the
decay constant, , for C14 ?
1.21 x 10-4 /yr = 3.84 x 10-12 /s
. b) How many atoms of C14 are
there in 5 grams of carbon, assuming the ratio given above?
3.25 x 1011 . c) What is the
present activity of 5 grams of carbon taken from a modern "bone" (this assumes the present
ratio of C14 to C12 ) ?
Express your answer in two forms:
in dis/sec:
and in Curies:
1.25
3.37 x 10-11
.
14
12
d) Assuming the ratio of C to C in the atmosphere has remained the same, what should the
age of a bone be if 5 grams of carbon taken from the bone have an activity of 0.06 counts/sec ?
25,100 years
.