• 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 online … posted at homework site (5 pts each… we use your feedback!).

Results from Content survey

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Normalized Gain = 45%Traditional lectures ~20%

Wavelength of E/M radiation

Radiowaves are longer wavelength than

Microwaves which are longer wavelength than

Visible light waves which are longer wavelength than

Gamma rays

Holes in microwavedoor are smaller than wavelength of microwaves!

Wavelength of visible light is ~400 nm … really small!

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!

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- 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

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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

Circuits – Think like an electronlots of energyat start.

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

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Rtotal = Rlight + Rwires

Hopefully very small

If decrease resistance of light bulb to ½ of original, what happens to current through wire? a. ~Unchanged b. ~2x c. ~4x d. ~1/2 e. ~1/4

Rlight

Ans is b. I = Vdiff/R (voltage diff is set by battery, total R ~ 1/2 so I ~2 )

What happens to power loss in wire? a. Unchanged b. ~2x c. ~4x d. ~½ e. ~¼

Answer is c. P = IVdiff. I and Vdiff both 2x greater. Current is 2x (above). V_drop in wires = I*R_wires, so V_drop_wires = 2 x . Electrons moving faster …MORE electrons per sec (bigger I)

… MORE energy loss per electron (bigger V-drop-wire)

Primary coil (in)

Secondary coil (out)

CORE

You have the transformer below plugged into your outlet at home. You are investigating how transformers work and decide to try the following changes and see what happens.

I. Hook it up to a 12 Volt batteryII. Remove the core

a. Transformer will still work after either change I or II just not as well.

b. Transformer will not work at all after either change I or IIc. Transformer will not work at all after I, but will still work after II

just not as well. d. Transformer will not work at all after II, but will still work after I

just not as well.

Primary coil (in)

Secondary coil (out)

CORE

You have the transformer below plugged into your outlet at home. You try the following changes and see what happens.

I. Hook it up to a 12 Volt battery

II. Remove the core

Answer is c. Transformer will not work at all after I, but will still work after II just not as well.

Transformer: transfers power from primary to secondary coil 1. current through primary coil makes magnetic field that extends into secondary2. core keeps field from spreading out so much (without it just less concentrated … still works but not as well)3. changing magnetic field in secondary induces voltage (push on electrons)

I

Won’t work b/c makes constant magnetic field

Still works because still have changing magnetic field

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.

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?

Electron flow

Electron flow

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). Both pointing in same direction. ADD at center point.

S

N

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.

1) Material conductivity & semiconductors- control current by controlling number available to move.(copiers, transistors)

2) Generation of electromagnetic waves- oscillating electrons up and down. different frequencies= different types (radio, microwave, visible light, x-rays) Examples: the sun, moving in broadcasting antenna, move in receiving antenna.

3) Change energy levels in atoms- give off light- florescent lights with mercury and phosphors, lasers

4).TVs- beams of electrons precisely controlled by magnetic fields.hit phosphors on pixels on screen to give off light to make image.

5) X-ray machines- high energy electrons smash into cathodes.give off very high energy EM waves, goes through body and absorbedby electrons in different types of atoms in body. Heavier atoms with moreelectrons absorb best.

Lots of other places where electrons are important:

Electrons and Material conductivity:

1

2

3

Model of allowed electron energy levels in solid material: Bands of very, very close levels with energy gaps between levels.

The way this is filled with electrons determines behavior of material… i.e. semiconductor, conductor, insulator.

Consider this case. We can conclude that this material is likely:

a. a conductor always, b. an insulator always, c. a semiconductor, electrically insulatingd. a semiconductor currently exposed to

heat or light, electrically conductivee. a N-type doped semiconductor neither

exposed to light or heat.

Answer is d. Notice not all electrons in lowest state. Some electrons excited across energy band gap between 2 and 3.

Electrons in both 2 and 3 have empty energy levels just above them, so these electrons can move freely. Conductive.

Electrons in Atoms:

Model: Electrons can only have certain energies (Quantum mechanics). Only 1 electron in each energy level .

Electrons can move between these specific energy levels, to move to higher energy requires energy input to move to lower energy releases energy …. As light photon

OR electrons can be kicked out of the atom/molecule if energy input is high

Electron in atom is excited. What will happen? a. It will stay in excited state until a random photon

comes along to prompt it to fall to lower stateb. It will spontaneous transition to lower energy state

and release a single photon with a random energyc. It will spontaneously transition to a lower energy

state and release some combination of photons whose energies add up to the energy between the levels

d. It will spontaneously transition to a lower energy state and release one photon whose energy is equal to the energy between the levels

Energy

Electrons in Atoms:

Model: Electrons can only have certain energies (Quantum mechanics). Only 1 electron in each energy level .

Electrons can move between these specific energy levels, to move to higher energy requires energy input to move to lower energy releases energy …. As light photon

OR electrons can be kicked out of the atom/molecule if energy input is high

Electron in atom is excited. What will happen? d. It will spontaneously transition to a lower energy

state and release one photon whose energy is equal to the energy between the levels

Energy

Applications: Lasers, Atomic Discharge Lamps, TV phosphors

(LEDs … electron transition in material)

Methods of Exciting Electrons in Atoms:

Method 1: Use Photons (Photon is absorbed)-Hit atom with photon matching transition energy (Laser, MRI (proton spins), CCDs (in semiconductor, each pixel))

-Hit atom with a photon with enough energy to knock electron out (UV light, skin cancer)

In a TV, we excite an electron within the phosphor atom using: a. electronsb. lightc. Heatd. Magnetic energye. Current through wires

a. electrons. Method 2: Excite by hitting with electrons. Create beam of free electrons and hit atoms hard. Kinetic energy of free flying electron excites an electron in atom. (TV, X-ray generation, (also sometimes to excite atoms in laser medium))

Electron beam in magnetic field:

Case 1 Case 2 Case 3

How do these change electron beam? a. Case 1 deflects vertically, Case 2 speeds beam up, Case 3 deflects horizontallyb. Case 1 deflects vertically, Case 2 does nothing, Case 3 deflects horizontallyc. Case 1 deflects horizontally, Case 2 speeds beam up, Case 3 deflects verticallyd. Case 1 deflects horizontally, Case 2 slows beam down, Case 3 deflects verticallye. Case 1 deflects horizontally, Case 2 does nothing, Case 3 deflects vertically

Answer is e. If magnetic field is perpendicular to direction in which electrons are moving, magnetic field will exert a force on electrons that is perpendicular to both direction of velocity of electrons and direction of magnetic field.

Used in TVs

From tiles to photons

60 inches wideI want to retile my shower area and the wall is 60 inches across. How many tiles can I fit across one row if each tile is 3 inch square (neglect caulking space)? a. 10 tile b. 0.05 tiles c. 20 tiles

Answer is c… 20 tiles. Look how you did this … no different than many of the problems we’ve given you: # of tiles = Total length or in general # of widgets = Total Quantity

length per tile quantity per widget

How many photons are emitted each second by 1mW green laser pointer (530 nm)? (HINT: Quantity = energy (J))a. 1 x 10^-15, b. 2.7 x 10^-15, c. 2.7 x 10^15, d. 2.7 x 10^6, e. none of the above

From tiles to photonsHow many photons are emitted each second by 1mW green laser pointer (530 nm)? (HINT: Quantity = energy (J))a. 1 x 10^-15, b. 2.7 x 10^-15, c. 2.7 x 10^15, d. 2.7 x 10^6, e. none of the above

Total quantity = Energy emitted in 1 second. For 1 mW = 1 mJ/s, then 1 mJ in 1 sec, or 0.001 J.

Quantity per widget (here photon) = energy per green photon E = hc/ = 6.6 x 10^-34 Js x 3 x 10^8 m/s / (530 x 10^-9 m)

= 3.75 x 10^-19 J

# of widgets (photons) = Total Quantity = 0.001 J/3.75x10^-19 J Quantity per widget

= 2.7 x 10^15 photons

Hint… examine your answer… is it reasonable, on last exam about 1/3 of you chose answers that were much, much less than 1 photon…. Photons come in integer quantities… 1,2,3 not fractions… no ½ photons!

Manipulating Light: Lens/Cameras

If have only the parallel light rays shown entering the lens of my eye, what do I see? a. Blackness b. A uniformly light wall of light, like a white wallc. A single spot of light surrounded by blacknessd. Many scattered points of light, like stars in the night sky.e. None of these/don't know

Manipulating Light: Lens/Cameras

Big distance

f f

If have only the parallel light rays shown entering the lens of my eye, what do I see? c. A single spot of light surrounded by blackness… Parallel light rays focus to a

single spot in the focal plane. Like looking at a star far away. If another star some distance away from first, parallel rays enter at different angle creating another spot.

Manipulating Light: Lens/Cameras

f

f

f

f

Object Distance

Image Distance

If I move object closer to lens, focused image will appear: a. Closer to lens, smallerb. Further from lens, biggerc. Closer to lens, biggerd. Further from lens, smallere. Same spot, same size

Answer is b. Rays from point on closer object diverging more when hit lens. Lens can only bend light by a fixed amount, so after lens the rays are converging but not as strongly so must travel further to point where they come together (focus).

Manipulating Light: Lens/Cameras

f

f

f

f

Object Distance

Image Distance

If I block top ½ of lens, focused image will appear: a. Complete image, dimmer, and closerb. Only Top ½ of image, and dimmerc. Complete image, unchangedd. Complete image, but dimmere. Only Top ½ of image will appear

Answer is d. Blocking ½ of lens, blocks ½ of rays from each point on object that are focused by lens. But still ½ of rays from each point are collected and focused just as before. Image in same spot, just dimmer.

Manipulating LightWhen a ray of light passes from air into a lens made of glass, what

changes? a. Speed only. b. Speed and energy per photon.c. Speed, color, and energy per photon.d. frequency, wavelength, speed, color, energy per photon.e. Wavelength, speed

Answer is e. 1. Energy per photon is fixed .... Same in air and in glass. 2. Energy per photon determined by frequency of light … frequency

unchanged (EM wave oscillates up and down at same rate inside and outside of glass). E = h*frequency

3. Energy per photon and frequency determines color also, so color is the same.

4. Speed slows down (smaller), and since for waves (sound or light) frequency * wavelength = speed; then wavelength smaller.

Of course direction will change too if light ray comes in at an angle.

Common Math errors onApplying Lens Equation

If object distance = 20 cm and image distance = 20 cm, what is f?

Equation:

iof

111

cmf

.vide by f, and dihrough by multiply t

cm^f

cmcmcmcmf

101.0

1

10

1 1.01

1^ 1.020

2

20

1

20

11

cmfcm^f

also

cmcmcmf

1.0 1 1.01

40

1

20

1

20

11

a. 0.1 cmb. 10 cmc. 10 mmd. 40 cme. 0.025 cm

For a balloon to rise, there must be a force acting upward on the balloon. This force arises from:

a. the gas inside the balloon b. the gas surrounding the balloon c. both the gas surrounding the balloon and the gas inside the balloond. the anti-gravity force

Answer is b. The gas surrounding the balloon. The air molecules hitting the bottom of the balloonare pushing up slightly harder than the air molecules hitting the top are pushing down, because the pressure of air at the bottom is slightly higher than the pressure at the top of the balloon.

The force upwards is equal to the weight of the displaced air.

Transitors & Diodes+-

Gate

Add transistor to light bulb, battery circuit. How will this circuit behave? a. Identical to circuit without transistorb. The more positive charge on the gate, the larger the current, the brighter the bulb…

until bulb so bright it fries itself.c. The more positive charge on the gate, the larger the current, the brighter the bulb…

until bulb reaches same brightness as without transistor there.d. The more positive charge on the gate, the smaller the current, the dimmer the bulbe. The more positive charge on the gate, the larger the current, the dimmer the bulb

Transitors & Diodes+-

Gate

Add transistor to light bulb, battery circuit. How will this circuit behave? Answer is c. The more positive charge on the gate, the larger the current, the brighter

the bulb… until bulb reaches same brightness as without transistor there.

Small signal on gate … allows to control large current through main circuit. Tiny CD signal controls speaker output.

Power system and transformers

5000 V AC Primary coil

Iron Core

HomeSecondary coil

Why do transformers only work with AC (alternating current) power? a. Because only AC current can create a magnetic field in the core.b. Because only AC current can push electrons back and forth from the primary

coil, through the core, to the secondary coil so that we create AC power at the home.

c. Because only AC current can create a magnetic field in the core whose strength and direction changes with time.

Answer is c. Transformers allow us to transfer power from primary to secondary coil. And power is transferred by creating a changing magnetic field in the core. No electron flow from primary to secondary coil!