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, 306but percentages on exams often lower.

0

1

2

3

4

5

6

7

20

5

22

0

23

5

25

0

26

5

28

0

29

5

31

0

32

5

34

0

35

5

37

0

38

5

40

0

41

5

Max possible = 443.5

C

BA

• 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-- review-

1) Sky not all bright, implies must be edge (in space or time).not covered so much in class2) Hubble expansion, see galaxies moving away. Speedproportional to distance. Implies started at one place15 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!

theoretical calc.of what shouldlook like.Thermal radiationfrom 2.7 K source.(really cold light bulb)

Observations of cosmic background radiation

Deuterium (PN)on Deuterium

Fusion bomb or “hydrogen bomb”Fusion bomb or “hydrogen bomb”Basic process is fusion of two deuterium nuclei, like in sun. PN + PN helium nucleus 2P2N.

Proton-proton big repulsive hill. If coming together withenough energy to overcome hill and get very close togetherthan fall into very deep nuclear-force pit, get lots of energy.Hard part-- sending the deuteriums towards each other withenough energy to get up the hill (~100 million degrees). solution: Heat in sun or in a fission bomb.

energyreleased

when fusion

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

B

e-

e-

e-

When this electron moves from energy level A to B, light is emitted. The greater the energy difference betweenthe electronic energy level A and B … A) the more photons emitted.B) 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.

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 atomsB) Release of electrostatic potential energy when protons and electrons combineC) Release of energy when the nucleus of an atom breaks apartD) 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 lightB) a little bit of green lightC) no light will come throughD) 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?

Switch closed

(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.

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.

Primary coil (in)

Secondary coil (out)

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 bCorrect Answer is B. The core concentrates the magnetic field that is produced by the current passing through the primary coil.

e

Ve-

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?

A) Vertically towards the top of the page

B) Towards the screenC) 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.

s N s N

Holding steady with coil centeredas shown.

A.

s N s N

Moving rapidly side to side

B.

s N s N

Moving rapidly up and down

C.

s N s N

D.

Rotating rapidly as shown

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.

VV

V

Direction of Magnetic Field:

A) B) C)

D) The electrons in A, B, and C all experience the same size force. E) Constant magnetic fields cannot exert forces on charges.

e- e- e-

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?

Current

Current

A.

B.

C.

D.E.Either A or C is acceptable. If the electrons are flowing round in

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 = empty

Material #1 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.

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?

Material AMaterial B Material C

Material DMaterial E

A

E

B C

D

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 microwaveD) 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?

Screen and image of candle

2

2

Iris

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 focuse.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) blueB) greenC) redD) b and cE) 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 becauseA) 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 filmB. move the lens away from the filmC. 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 quadruples2-3. (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 slowly15 – 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 screenc. 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 c7. 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 reddishb. 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.

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.

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

1020, Spring 2004, Exam 3. hard short problems.

Object #1Object #2

Object #3

LensShutter/Iris

Film

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=1.70

N=1.50

N=1.55

N=1.50

N=1.55

N=1.53

N=1.55

N=1.57

N=1.55

N=1.60

A. B. C. D. E.

Circuits – Think like an electron

lots of energyat start.

Exhausted! energy used up getting through course.

Light Bulb… high resistance like trudging through three feet of mud! takes lots of energy to getthrough. R (rottenness)

Wires: glide down pretty easily, just a few bumps. Lose a little bit of energy.

eRemember electron man!If there is a reason and a way to flow … electrons will find it!

+

- e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e ee e e e ee e e e ee e e e ee e e e ee e e

e

e

e

e

e

e

e

e

e

e

ee

e

e

Remember rules: - No Passing- No electron man deaths

on route

V = IR or I = V/R

Rtotal = Rlight + Rwires

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

Hopefully very small