Scenario #1: Light Emitting Diodes
In the circuit at right, a power supply sends current through a light-emitting diode
(LED). As electrons pass through the LED, they drop to a lower energy level,
emitting a photon in the process. Very little energy is lost; to create 2.0 eV photons
requires a voltage of approximately 2.0 V. The power supply is adjusted to provide
the exact voltage needed by the LED.
Multiple Choice Questions (3 points):
1) One circuit runs a red LED; another runs a blue LED. What can you say about voltage in the two circuits?
A. The red LED requires higher voltage.
B. The two LEDs require the same voltage.
C. The blue LED requires higher voltage.
D. Two LEDs are better than one.
2) One circuit runs a red LED; another circuit runs a blue LED. Each circuit carries exactly 20.0 mA. What can
you say about the number of photons emitted each second by the two LEDs?
A. The red LED emits more photons.
B. The two LEDs emit the same number of photons.
C. The blue LED emits more photons.
D. Led Zeppelin emits more photons than either.
Short Answer Question (6 points): A coil of copper wire has 1200 turns, and an area of 0.0040 m2. The coil of wire is connected to a light emitting
diode that emits 620 nm photons. A cow magnet with field strength 0.050 T is quickly thrust into the coil. What is
the approximate minimum time for the magnet to go from being outside the coil to being inside the coil in order to
light the LED?
!
!
© 2014 Brian Jones, all rights reserved. No part of this document may be reproduced without the express written consent of the author.
Scenario #2: Light Emitting Filaments
Halogen bulbs have filaments like regular incandescent bulbs, but the filaments are much hotter, the bulbs are
physically smaller, and they work at lower voltages. The small bulbs can be used at the focal point of a mirror to
create a tight beam of light; they are used for spotlights, projectors and desk lamps.
Multiple Choice Questions (3 points):
3) Increasing the temperature of a bulb’s filament ______________ the energy radiated from the filament,
_____________ the peak wavelength of the emitted radiation.
A. increases, increases
B. increases, decreases
C. decreases, increases
D. decreases, decreases
4) Light from a halogen desk lamp is focused into a circle 40 cm across. A student using the lamp needs brighter
light; she moves the lamp closer to the desktop to focus the light into a circle 20 cm across. This increases the
intensity by a factor of
A. 2
B. 4
C. 6
D. 8
Short Answer Question (6 points)
A halogen bulb runs at 24 V provided by a transformer connected to the standard 120 V electricity supply. The
transformer draws 20 W from the outlet and has 500 turns of wire on the primary.
Find:
• The number of turns on the secondary of the transformer
• The current drawn from the outlet
• The current in the bulb
!
!
!
© 2014 Brian Jones, all rights reserved. No part of this document may be reproduced without the express written consent of the author.
The test has 4 scenarios, each with 2 multiple choice questions (3 points each) and 1 short answer question (6 points). That’s 12
points possible per scenario. 4 scenarios at 12 points each plus 2 points for putting your name on your paper makes 50 points.
The scenarios below are those from last year’s exam. Solutions will be posted on Tuesday, April 13.
Scenario #3: The Right Hand Rules
Scenario #1: The Right Hand Rules
Multiple Choice Questions (3 points):
Multiple Choice Questions (3 points)
5) Two wires carry equal and opposite currents, as in the cross section diagram
1) right.
Two wires
equalfield
and opposite
currents,
in the cross
at
The carry
magnetic
at a point
exactlyasbetween
thesection
two wires is
diagram at
wires is
right. In this diagram, the magnetic field at a point exactly between the two
A. Directed
down,
toward
thethe
bottom
A. directed
down,
toward
bottomofofthe
thepage.
page
B. Directed
up,
toward
the
top
of
the
page.
B. directed up, toward the top of the page
C. Directed
to the
right.
C. directed
to the
right
D. Zero.
D. zero
6) A
isismoving
current-carryingwire
wire
diagram
2) proton
A proton
movingabove
above aa current-carrying
as as
in in
thethe
diagram
at at
right.
The
wire
and
the
proton
are
in
the
plane
of
the
paper.
In
this
The wire and the proton are in the plane of the paper. In this diagram, the
diagram,
the magnetic
forceis on the proton is
magnetic force
on the proton
!
right.
A. directed
down,
toward
bottomofofthe
thepage.
page
A. Directed
down,
toward
thethe
bottom
B.
directed
up,
toward
the
top
of
the
page
B. Directed up, toward the top of the page.
C. directed
to the
C. Directed
out of
theright
paper.
D.
zero
D. Zero.
Short Answer Question (6 points):
Short Answer Question (6 points):
A proton moves to the right through a region of space with electric and magnetic !
A proton
the right
through
a region
of space
fields moves
at right to
angles
to each
other, as
in the diagram
at with
right. aFor just the right E
electric
field
the traverse
top of the
field
!
speed,
thetoward
proton will
thispage
fieldand
withthe
no magnetic
net deflection.
B
directed out of the page, as in the diagram at right. The electric
• What is the direction of the electric force on the proton?
field magnitude
1500 V/m
; the
magnetic
field
• Whatisis! Ethe= direction
of the
magnetic
force
on the proton?
magnitude
is
!
B
=
50
mT.
• For what speed will the net force be zero?
• isDescribe
the path
proton force
if it moves
at proton?
twice this speed.
• What
the direction
of of
thetheelectric
on the
• What is the direction of the magnetic force on the proton?
Scenario #2: What’s the Flux?
• For what speed will the net force be zero, so the proton
Multiple
Choice
Questionsas(3shown?
points)
moves with
no deflection
transformer
the this
following
• AIfparticular
the proton
moves athas
twice
speed,specifications:
will the proton’s
path curve
upward
or curve
Vp =downward?
120 V
Primary
voltage:
Secondary voltage:
Vs = 6.0 V
!
v
Power output:
P = 10 W
3) 1) If this transformer has 800 turns of wire on the primary, how many turns of wire are on the secondary?
A.
B.
C.
D.
40
400
1600
16,000
4) When the transformer is running at its full rated power, how much current is in the primary?
A.
B.
C.
D.
0.083 A
0.60 A
1.7 A
10 A
© 2009 Brian Jones, all rights reserved
No part of this document may be reproduced without the express written consent of the author.
© 2014 Brian Jones, all rights reserved. No part of this document may be reproduced without the express written consent of the author.
Scenario #4: Rad Medicine
Lung ventilation can be measured with ! 133 Xe , a radioactive isotope of xenon with a 5.2 day half-life. A patient
breathes using a sealed system containing the radioactive gas. Xenon atoms undergo beta-minus decay, emitting a
350 keV electron; the daughter nucleus then emits a gamma ray that can be detected with a gamma camera.
Multiple Choice Questions (3 points):
7) What is the daughter nucleus of the beta-minus decay?
A. ! 131 Te
B.! 133 I
C.! 133 Xe
D.! 133 Cs
8) The ! 133 Xe is supplied to hospitals in sealed vials with a rated activity. After 3 weeks, the activity has dropped
to _______ of the initial value.
A. 1/2
B. 1/4
C. 1/8
D. 1/16
Short Answer Question (6 points)
A patient has xenon gas with total activity 740 MBq in his lungs for 2 minutes. The radiation dose to the lungs is
primarily due to the beta radiation. If 50% of the energy of the emitted electrons is absorbed in 1.2 kg of lung
tissue, what is the dose equivalent in Sv?
© 2014 Brian Jones, all rights reserved. No part of this document may be reproduced without the express written consent of the author.