Transcript b - University of Colorado Boulder

Grand Total, April 26. Extra credit from exams and weekly
question are not added.
Final grades A above 86%, B above 78%, C above 70 %
here those correspond to 381,
346,
306
but percentages on exams often lower.
7
Max possible = 443.5
6
A
B
5
C
4
3
2
1
415
400
385
370
355
340
325
310
295
280
265
250
235
220
205
0
• Final Exam: Monday 4:30, G125,
• special help sessions Mon. usual hw room 10-12, 3-4
– Can bring 4 3x5 cards
– Similar to previous exams; Covers whole course (slightly
more on material since Exam 3).
– Good prep is to go back through homeworks, exams, and
look at lecture notes.
Look at questions, explain answers to another
person. Listen to another person explain answer, see if it
makes sense, why or why not?
• Two end-of-term surveys posted at homework site (5 pts
each… we use your feedback!).
• Today: content survey- catch what you do not understand
+ Some review.
– Make note of questions you want to review today
– We will scan these today (review most missed Thurs)
– We need surveys back so take notes on different sheet.
Big Bang origin of universe arguments-- review1) Sky not all bright, implies must be edge (in space or time).
not covered so much in class
2) Hubble expansion, see galaxies moving away. Speed
proportional to distance. Implies started at one place
15 billion years ago.
3) That implies universe started super hot and so
hot radiation (short wavelengths) would be left over,
but as univ. expands wavelength gets stretched out.
Long wavelength looks same as thermal radiation from a
colder object (2.7 deg. above abs. zero).
4) This radiation observed. (cosmic background radiation)
Like cold light bulb filament, not very much and long
wavelength, but exactly as predicted by B.B. theory.
What would happen if all matter in universe start exploding out of big bang?
(George Gamow, CU ) Predicted in 1948.
Start out as EM energy, expands and cools. After ~100,000 years.
cold enough (3000 K) to turn into atoms. Some radiation left continues to
cool off. Should look like light (far IR) emitted from object a few degrees
above absolute zero. (“Cosmic Background Radiation”)
In 1964 Penzias and Wilson (telephone company)
accidentally discovered!
Observations of cosmic background radiation
theoretical calc.
of what should
look like.
Thermal radiation
from 2.7 K source.
(really cold light bulb)
Fusion bomb or “hydrogen bomb”
Basic process is fusion of two deuterium nuclei, like in sun.
PN + PN  helium nucleus 2P2N.
energy
released
when fusion
Deuterium (PN)on
Deuterium
Proton-proton big repulsive hill. If coming together with
enough energy to overcome hill and get very close together
than fall into very deep nuclear-force pit, get lots of energy.
Hard part-- sending the deuteriums towards each other with
enough energy to get up the hill (~100 million degrees).
solution: Heat in sun or in a fission bomb.
Correct Answer is E. Remember battery – circuit applet…
electrons bump into stuff but there are never any electron deaths!
2) The diagram below shows the electronic energy
levels in an atom with an electron at energy level A.
A
e-
When this electron moves from energy level A to B,
B
light is emitted. The greater the energy difference between
the electronic energy level A and B …
A) the more photons emitted.
eB) the brighter the light emitted.
C) the longer the wavelength (the more red) of the light emitted.
D) the shorter the wavelength (the more blue) of the light emitted.
E) a and b.
e-
Correct Answer is D. Only one photon is emitted when electron
jumps between levels and that energy of that photon is equal to
the energy difference in the levels.
Since Ephoton = hc/wavelength. The higher the energy the shorter
the wavelength.
3) If no electrons are flowing through a conducting wire, we can conclude
(A) that the resistance of the wire is large.
(B) that the resistance of the wire is small.
(C) that no electrons are present in the wire.
(D) that there is no voltage difference between the two ends of the wire.
(E) that there is a large voltage difference between the two ends of the wire.
Correct Answer is D. If there were a voltage difference than
electrons would flow. Note that the whole wire can be at 10
Volts, but as long as both ends are at 10 volts, no electrons will
flow.
4) What is the main source of energy in an atomic bomb?
A) Release of energy that occurs during a massive chemical reaction
between atoms
B) Release of electrostatic potential energy when protons and electrons
combine
C) Release of energy when the nucleus of an atom breaks apart
D) Release of energy when gamma rays combine with the protons in the
nucleus
Correct Answer is C. Fission (splitting) of the nucleus releases
massive amounts of energy as the positively charged nuclei
repel each other and go speeding off.
5) When light with a wavelength of 620 nm (reddish-orange color) shines
on a green filter, what comes through the filter is?
A) a little bit of reddish-orange light
B) a little bit of green light
C) no light will come through
D) most of light will come through but it will now all be green.
E) most of light will come through and it will be reddish-orange.
Correct Answer is A. The filter cannot change the color of the
photons it can only absorb them or pass them. Most of these
reddish-orange photons are absorbed and so only a little bit of
reddish-orange comes through.
1
2
Correct Answer is C. Electrons proceed round the circuit from
the negative terminal, attracted to the positive terminal.
Remember electron man: no electron deaths, no passing.
Current going through 2 same as through 1.
7) Why do the filaments in your lights begin heating up almost instantaneously
when you turn on the switch?
(A) When the circuit is completed, there is a rapid rearrangement of surface
charges in the circuit.
(B) Charges store energy. When the circuit is completed, the energy is
released.
(C) Charges in the wire travel very fast.
(D) The circuits in a home are wired in parallel. Thus, a current is already
flowing.
(E) Charges in the wire are like marbles in a tube. When the circuit is
completed, the charges push each other through the wire.
Correct Answer is E. There are electrons in the whole wire.
When the voltage is applied, the electrons near the negative
terminal push on the ones in front of it which push on the ones in
front of it and almost instantaneously all of the electrons are
parading around.
8) Immediately after the switch is opened, what happens to the resistance of
the bulb?
(A) The resistance of the bulb goes to infinity.
(B) The resistance of the bulb increases.
(C) The resistance of the bulb decreases.
(D) The resistance of the bulb stays the same.
(E) The resistance of the bulb goes to zero.
Switch
closed
Correct Answer is D. The resistance of the bulb is a property of
the bulb itself.
9) Below is the diagram of 2 coils of wire wrapped around a piece of
material. The primary coil is connected to a voltage source like the power
lines outside your house.
Secondary coil (out)
Primary coil (in)
CORE
In order for power to transfer from the primary coil to the secondary coil
efficiently as happens in a transformer, it is important to choose the core
material wisely. In this selection, it is important for
A)
the core to be a conductor so that it allows electrons to pass from the
primary to the secondary coil.
B)
the core to have magnetic properties so that it can concentrate the
magnetic field produced by the primary coil
C)
the core to be a permanent magnet so that a constant magnetic field is
always present
D)
both a and b
Correct Answer is B. The core concentrates the magnetic field
that is produced by the current passing through the primary coil.
10) An electron moves horizontally with a velocity (V) towards a TV screen.
The electron moves along the path that is shown because of a force caused
by a magnetic field. In what direction does that magnetic field point?
V
e-
e
A) Vertically towards the top of the page
B) Towards the screen
C) Out of the page (towards you)
D) Always parallel to the path
Correct Answer is C. Magnetic fields exert a force on moving
electrons. The direction of that force is perpendicular to both the
direction of the magnetic field and the direction of the velocity.
In this case the force is upwards so the magnetic field must be
C.
11) You have a battery hooked up to a bulb. If you double the current
through the circuit by decreasing the resistance of the bulb, will the voltage
difference across the battery double?
(A) Yes, because Ohm's law says V=IR
(B) Yes, because as you decrease the resistance, you increase the voltage
difference.
(C) No, because as you double the current, you reduce the voltage
difference by half.
(D) No, because the voltage difference is a property of the battery.
(E) No, because the voltage difference is a property of everything in the
circuit.
Correct Answer is D. The battery supplies a fixed voltage
difference between its two ends. Electrons traveling all the way
around the circuit will lose all of the energy and will thus
experience a voltage drop equal to the battery voltage
difference. If resistance decrease, the electrons will go faster
(higher current) and run into the “stuff” in the wires and bulbs
harder such that they still lose all of their energy.
12) Below is a bulb with a coil of wire attached to it. Two permanent
magnets are aligned as shown. Under which scenario would the bulb
light and be the brightest.
B
.
A
.
s
Holding steady
with coil centered
as shown.
N
s
C
.
s
Moving rapidly
side to side
N
s
N
N
Moving rapidly
up and down
D
.
Rotating rapidly
as shown
s
N
s
N
s
N
s
N
Correct Answer is C. Rapidly changing the strength/direction of
the magnetic field through the coil produce the largest current.
13) In the cases below an electron is moving through a uniform magnetic field.
In which case is the force exerted by the magnetic field on the moving electron
greatest? If it is not possible for the magnetic field to exert a force on the
electron chose E.
Direction of Magnetic Field:
A)
C)
B)
V
e-
V
e-
e-
V
D) The electrons in A, B, and C all experience the same size force.
E) Constant magnetic fields cannot exert forces on charges.
Correct Answer is A. The amount that a magnetic field exerts a
force on a moving electron is proportional to the amount of the
velocity that is perpendicular to the direction of the magnetic
field.
14) The color emitted from a light-emitting diode (LED) is determined by:
A) the temperature of the filament inside the LED.
B) the change in energy of a electron as it moves through the LED.
C) the amount of current flowing through the LED.
D) the voltage of the battery used to run the LED.
Correct Answer is B. As the electron moves from N to P type
semiconductor, it drops down in energy and gives off a photon.
The color or wavelength is determined by the energy of the
photon which is equal to this change in energy of the electron.
15) Compare the brightness of the bulb connected between A and B to the
brightness of the second bulb in the circuit pictured below.
A
B
(A) The bulb between A and B is brighter than the 2nd bulb.
(B) The bulb between A and B is just as bright as the 2nd bulb.
(C) The bulb between A and B is dimmer than the 2nd bulb.
Correct Answer is B. Both bulbs have the same amount of
current passing through them because the electrons can choose
either path and see the same resistance. Since the bulbs have
the same amount of current passing through and since each
electron is loosing the same amount of voltage, they have the
same power or brightness.
16)
Correct Answer is D. Need to have a complete circuit for the
electrons to flow round.
17) Compare the brightness of the bulb connected between A and B to the
brightness of the second bulb in the circuit pictured below.
A
B
(A) The bulb between A and B is brighter than the 2nd bulb.
(B) The bulb between A and B is just as bright as the 2nd bulb.
(C) The bulb between A and B is dimmer than the 2nd bulb.
Correct Answer is B. Both bulbs have the same amount of
current passing through them current flows through one and
then the other sequentially. Since the bulbs have the same
current and since the voltage drop across each bulb is the same,
they have the same power.
18). You run a long thin extension cord from your house to your electric
mower. The resistance in the cord is small, but not negligible. As current
flows from your house to your mower along this cord,
(A) current and voltage both gradually decrease along the length of the
cord
(B) voltage gradually decreases along the length of the cord.
(C) current gradually decreases along the length of the cord.
(D) voltage gradually decreases along the length of the cord and current
gradually increases to partly compensate.
(E) neither voltage nor current change along the length of the cord.
Correct Answer is B. The current is the same at any point along
the cord. But as the electrons move through they lose some
energy because the cord has some resistance. This energy loss
is a loss of voltage… a voltage drop along the cord.
19) Inside a TV are loops of wires with current passing through them as
shown below. Which arrow best represents the direction of the magnetic
field between the loops at point P?
A
.
B
.
Current
C
.
D
Current
.
E
. A or C is acceptable. If the electrons are flowing round in
Either
the direction of the arrow, the magnetic field will point
downwards (A).
20) Below is a drawing of the electronic energy bands in a material, showing
both the energy levels that are filled with electrons and those that are empty.
Energy levels filled with electrons =
Energy levels with no electrons =
Material #1
empty
Material #2
Material #3
Which of these materials acts as a conductor in the sense that electrons would
easily flow through it?
A) Only material #1 would conduct electrons because the lower energy band is
full of electrons while the upper energy band has no electrons.
B) Materials #2 and #3 would conduct electrons because each material has an
energy band that is partially filled with electrons.
C) Materials #1 and #2 would conduct electrons because each have electrons
only in the lower energy band.
D) Only material #3 would conduct electrons because it is the only material to
have electrons in the upper energy band.
21) If you wanted to choose an insulator for coating the outside of your
wires, which of the following material is the best choice; that is, which
one is most likely to be an insulator?
A
Material A
B
Material B
Material D
Material E
D
E
C
Material C
Correct Answer is
D. Lower band is
full.. no empty
levels nearby.
And it has a large
gap in energy up
to the higher
band.
22)Comparing a microwave photon, a radiowave photon, and a photon of
red visible light. Rank them from MOST energy to LEAST energy.
A) Radiowave then microwave then red visible light.
B) Microwave then red visible light then radiowave.
C) Red visible light then radiowave then microwave
D) Red visible light then microwave then radiowave
E) It depends on how fast each photon is moving.
Correct Answer is D. Visible light has the highest energy
(smallest wavelength), then microwaves, then radiowaves.
23) What will happen at the screen if the iris in front of lens is closed down
part of the way?
Iris
2
2
Screen and
image of
candle
a. Image of the candle will appear brighter.
b.The iris will cut off part of the image of the candle.
c.The image of the candle will be dimmer.
d.The image will go out of focus
e.Both b and c
Correct Answer is C. The image will dim because with the iris
the lens is capturing less of the light off of the candle, but the
whole image will still be there because there are still some light
rays from each point on the candle that are focused by the lens.
24) Red light has a lower frequency than blue light, so you know that, in
empty space,
A) blue light travels faster than red light.
B) red light has a shorter wavelength than blue light.
C) blue light has a shorter wavelength than red light.
D) red light travels faster than blue light.
Correct Answer is C. The lower (smaller) the frequency, the
longer (larger) the wavelength.
Velocity = wavelength * frequency.
Velocity = (distance per cycle) * (cycles per second)
25) The leaves of nearly all plants look green, what colors are the leaves
absorbing most?
A) blue
B) green
C) red
D) b and c
E) a and c
Correct Answer is E. If leaves look green then they must be
reflecting green to your eye and absorbing blue and red.
26) When an x-ray is taken, an image of bone is created on a sheet of
photographic film because
A) bones produce light which makes an image on the film.
B) tissues are good at absorbing x-rays but bones are not.
C) bones are good at absorbing x-rays but tissue is not.
D) x-rays reflect off of the bones.
Correct Answer is C. As a large atom calcium absorbs x-rays
well. So where there is just tissue a lot of x-rays pass through
and hit the film, turning it black. Where there is bone, few x-rays
hit the film and it stays white.
27) Consider the following photo-opportunity.
To change the focus of the camera from the candle (#1) to the flower (#2),
you would need to:
A. move the lens towards the film
B. move the lens away from the film
C. there is no need to move the lens, the candle and the flower will be in
focus at the same time.
Correct Answer is A. Because the rays from the flower will be
diverging less rapidly when they hit the lens, the focus of the
flower will be closer to the lens.
Exam 1 hard short problems.
1. (Multiple Choice) You are working on a construction site and are powering your tools using a long extension
cord with a plug strip at the end. You are running an electric drill that draws about 5 amps of current. The sun is
setting so you turn on a bank of lights that are also plugged into the plug strip as you continue to drill. The lights
use the same amount of current as the drill so that when the lights are on there is twice as much current in the
extension cord as with only the drill. When the lights are turned on, what happens to the voltage drop along the
wires of the extension cord and what happens to the amount of electrical power being deposited in the wires of
the cord, compared to the situation with the drill by itself?
a.The voltage drop along the wires of the cord and the power loss in the cord remain the same
b.The voltage drop along the wires of the cord doubles and the power loss doubles.
c.The voltage drop along the wires of the cord doubles and the power loss quadruples.
d.The voltage drop along the wires of the cord quadruples and the power loss quadruples
2-3. (True/False) The spacing of energy levels within a solid material and how the electrons occupy those
energy levels tells us a lot about the electrical conductivity of the material. In this context, consider whether the
following statements are true or false.
3. True (a) or False (b)
In a conductor, when a voltage is applied and the electrons are conducting
electricity, the electrons jump into energy levels that are at a much higher energy than the levels they occupy
with no voltage applied. .
10-14. (True/False) I hook up a battery and a bulb with two wires as shown below. Which of the following
statements are True and which are False?
10. True (a) or False (b)
The light bulb will light up
12. True (a) or False (b)
The battery will drain slowly
15 – 20. Our electrical power distribution system uses many transformers. Consider whether the following
statements about this system are true or false.
19. True (a) or False (b)
A transformer is used to turn the high voltage direct current coming from
the generating plants into alternating current used in the houses.
1020, Spring 2004, Exam 2. hard short problems.
1. True (a) or False (b) You open a book and there is a photograph of a bird that appears to have perfectly
white feathers. True or false; the only way for the photograph to look white is if the distribution of colors of
the photons coming from the page to your eye matches the distribution emitted by the sun.
3. A light emitting diode (LED) is made up of N-type and P-type semiconductor materials that join together at
the P-N junction. As individual materials, N-type and P-type semiconductors are both electrical
conductors.
True (a) or False (b) The reason they are conductors is because the energy gap between the lower and
upper band of energy levels is small.
4. True (a) or False (b) If no electrons are flowing through a conducting wire, we can conclude that the two
ends of the wire are at the same voltage
6. Which modification of a TV would allow me to see more of the fine details of an image on a TV screen?
a. the pixels need to be bigger and further apart so that each pixel on the screen is easier to see
b. the pixels need to be smaller and closer together so that more pixels fit on the screen
c. the electron beam in the TV must complete one scan of the screen in less time so that it scans the whole
screen more times each second.
d. a and c,
e.
b and c
7. The large oil refinery in Denver starts venting into the air a lot of a pollutant molecule that scatters light with
wavelengths between 400 and 420 nm but does effect any other wavelengths of light. If you look toward
Denver from the base of the Boulder flatirons, what would the sky look like?
a. more reddish
b. more greenish
c. more bluish
d. more yellowish
e. more
purplish
8. If you increase the current through an LED, which of the following change?
the temperature of the filament inside the LED.
the color of the light emitted by the LED.
the brightness of the light emitted by the LED.
a and b., e. a, b, and c.
1020, Spring 2004, Exam 3. hard short problems.
Object #3
Shutter/Iris
Lens Film
Object #2 Object #1
1. We focus the camera on Object #1. If Object #1 is 1.20 meters from the camera and
the focal length of the lens is 35 mm, how far should the film be from the lens when
Object #1 is in focus? (Choose the closest to the correct value)
a. 36 mm
b. 0.083 mm
c. 35 mm
d. 28.1 mm
e. 83 mm
13. If red light (650 nm) in a glass optical fiber has an index of refraction of 1.53 and
blue light (440 nm) has an index of refraction of 1.56, how long does it take the red
light to travel 500 km?
a. 2.6 x 10-6 s b. 0.0026 s
c. 0.19 s
d. 0.0017 s
e. 0.000017 s
15. True (a) or False (b) The primary reason that UV radiation causes skin cancer is
because the wavelength of UV light is less than the size of our skin cells. This
allows the UV light to penetrate the skin cells.
16. Which of the following cases is most likely to lead to total internal
reflection of the incoming light ray? (Note that N indicates the index of
refraction of the substance). The color of the shading has no significance. It
is only to indicate the locations of two transparent materials.
N= N=
1.55 1.53
A.
N= N=
1.70 1.50
B.
N= N=
1.55 1.50
C.
N= N=
1.55 1.60
D.
N= N=
1.55 1.57
E.
Circuits – Think like an electron
Remember electron man!
If there is a reason and a way to flow …
electrons will find it!
e
Remember rules:
- No Passing
- No electron man
deaths on route
lots of energy Wires: glide down pretty easily, just a few bumps.
at start.
Lose a little bit of energy.
e
e
e
e
-e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
Hopefully
very small
Rtotal = Rlight + Rwires
e
V = IR or
I = V/R
e
e
e
+
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
Exhausted! energy used up getting
through course.
Remember to go faster (higher current) uses up
more energy (V) b/c bump into things harder… so
higher I leads to higher Power = I x V_drop
e
e
e
e
e
e
Light Bulb…
high resistance like trudging
through three feet of mud!
takes lots of energy to get
through. R (rottenness)