DOC PREVIEW
UW-Madison PHYSICS 208 - Phy 208 Exam 3

This preview shows page 1-2-3-4 out of 11 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 11 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 11 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 11 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 11 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 11 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Name: ____________________________________________________ Student ID: ___________________________ Section #: _________ Physics 208 Exam 3 Apr. 23, 2008 Print your name and section above. If you do not know your section number, write your TA’s name. Your final answer must be placed in the box provided. You must show all your work to receive full credit. If you only provide your final answer (in the box), and do not show your work, you will not receive very many points. Problems will be graded on reasoning and intermediate steps as well as on the final answer. Be sure to include units, and also the direction of vectors. You are allowed one handwritten 8½ x 11” sheet of notes and no other references. The exam lasts exactly 90 minutes. Planck’s constant h = 6.626 x 10-34 m2 kg/s Planck’s constant x velocity of light hc = 1240 eV nm Reduced Planck constant: ! h =h2"= 1.055 #10$34J % s Bohr radius ao = 0.053 nm electron mass: 9.11 x 10-31 kg melectron c2 = 511,000 eV Speed of light in vacuum: ! c = 3 "108m /s Permittivity of free space ! "o= 8.85 #10$12C2/ N % m2 ! k =14"#0= 9 $109Nm2/C2 Permeability of free space ! µo= 4"#10$7 T % m / A Magnitude of electron charge ! e = 1.6 "10#19C Problem 1: _______ / 20 Problem 2: _______ / 20 Problem 3: _______ / 20 Problem 4: _______ / 20 Problem 5: _______ / 20 TOTAL: _______ / 1002 1) [20 points, 4 points each]. Explain your reasoning for full credit. Multiple choice/short answer questions. i) A wire of radius R carries a uniform current density of j=1 A/cm2. The magnitude of the magnetic field inside the wire at distance r from the center (r<R) a. increases with r b. decreases with r c. is equal to zero d. is a non-zero constant e. none of the above ii) A large coil of wire has a time-dependent current as shown below. A positive value in the plot means the current is flowing in the direction of the arrow, negative in the opposite direction. Plot the induced current in the small coil of wire at the center of the large coil. The vertical scale is arbitrary, but you need to show the correct sign and relative magnitudes. Explanation/work: I(t) time I(t) time Induced current 0 0 Explanation: Iinduced(t)3 iii) A helium-neon laser emits light of wavelength 632.8 nm ! 632.8 "10#9m( )and has a power output of 1.5 milli-Watts ! 1.5 "10#3W( ). How many photons per second does this laser emit? a. 1.34 x 1014 b. 2.56 x 1015 c. 3.09 x 1015 d. 4.22 x 1015 e. 4.78 x 1015 iv) Light is an electromagnetic wave. When visible light is linearly polarized, which of the following statements is/are true? Circle all that are true. a. The electric field vector is parallel to the magnetic field vector. b. The electric field vector is parallel to the direction of propagation. c. The electric field vector is perpendicular to the direction of propagation, but the magnetic field vector may be in any direction. d. The electric field vector is perpendicular to the direction of propagation, and the magnetic field vector is perpendicular to the electric field vector. e. The direction of propagation is ! r E "r B . v) Potassium has a work function of 2.3 eV for photoelectric emission. Which of the following wavelengths is the longest wavelength for which photoemission occurs? a. 400 nm b. 450 nm c. 500 nm d. 540 nm e. 600 nm Work/explanation Work/Explanation: Work/Explanation: .4 2) [20 pts, 4 pts each] Short calculations. i) The electric field in a capacitor with circular plates of radius R = 3 m varies linearly with time according to ! E = 2 "107N /C # s( )t with time t in seconds. Calculate the conduction current flowing onto the positive plate of the capacitor. ii) The Sun radiation of intensity I = 1350 W/m2 is incident normally on a perfectly reflecting sail of a spacecraft of total mass m=5 x 104 kg of area A = 104 m2. What is the acceleration in free space of the spacecraft? iii) The electric field of an electromagnetic wave is ! r E = 25 V / m( )sin kx "#t( )ˆ y . What is the amplitude and direction of magnetic field of the electromagnetic wave? I = Value Units ! r B = Amplitude Units Direction a = Value Units5 x ( nm ) 0 10 -10 5 -5 ! "x( )2 iv) A proton is accelerated through a potential difference of 10,000 V. How does its de Broglie wavelength compare to that of a football of mass 0.4 kg launched by Brett Favre (emeritus) at a velocity of 30 m/s? The mass of the proton is 1.67 x 10-27 kg = 938.27 MeV/c2 ! 938.27 "106eV /c2( ). v) The probability density of a quantum particle is shown in the figure. It is zero except in the region -10nm < x < 10 nm. Calculate the probability that the particle will be found in the region 5 nm < x < 10 nm Ratio λp/ λfootball = Value Probability = Value6 3) [20 pts, 5 pts each] Unpolarized light is incident on a series of polarizers as shown below. The transmission axis of each polarizer is indicated by a heavy line. a) In what direction(s) does the electric field vector of the electromagnetic wave in region 1 point? Explain. b) What is the ratio of the electric field amplitude in region 2 to that in region 1? Unpolarized light x y z Region 1 Region 2 90˚ 30˚ Region 3 Direction(s) = Ratio ! E2/ E1=7 c) What percentage of the power incident on polarizer 3 is absorbed? d) Suppose that the middle polarizer rotates while the first and last polarizers stay fixed at the angles shown in the figure. Draw the intensity after the last polarizer (in Region 3) as a function of middle polarizer angle (defined as in the figure). Explain MIDDLE POLARIZER ANGLE ( deg ) INTENSITY (arbitrary units) 0 360 180 90 270 % power absorbed =8 4) [20 pts, 4 pts each] A rectangular coil of 10,000 turns of total resistance 0.01Ω is below an infinitely long straight wire. The infinitely-long wire carries a constant current I =1000A, producing a field ! B =µoI /2"r with r the distance from the wire. a) Assume that the 10,000-turn coil is narrow enough that the magnetic field from the long wire can be approximated as constant everywhere across the area of the coil. Calculate the magnetic flux through one turn of the coil when its center


View Full Document

UW-Madison PHYSICS 208 - Phy 208 Exam 3

Documents in this Course
Lect 11

Lect 11

19 pages

EM Waves

EM Waves

23 pages

Exam 3

Exam 3

5 pages

Load more
Download Phy 208 Exam 3
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Phy 208 Exam 3 and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Phy 208 Exam 3 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?