physics 1252 exam #2 part 1 version d - physics and ......physics 1252 exam #2 part 1 version d...

Physics 1252 Exam #2 Part 1 Version DThu/Fri/Sat, 09/10/11 April 2020

Physics 1252Exam #2 Part 1 Version D

Instructions:This is an open-book, open-notes exam. Each question on this exam is worth 1 point. Thereis no penalty for guessing a wrong answer. The first five questions (Problem 1 - Problem5) are conceptual, the remaining ones are numerical. In Part 2 of this exam (to be postedlater), there will be several bonus questions, also worth 1 point each.

You are allowed to use only your textbook and materials posted on the PHYS1252 coursewebsite

https://www.physast.uga.edu/classes/phys1212/schuttler

and on the Spring 2020 PHYS1252 course webpages on the eLC website, under

https://uga.view.usg.edu .

You are not allowed to use any other materials.

You are not allowed, by any means of communications, to discuss this exam with anyother person, except your course instructor, nor share copies of any part of this exam withanyone, prior to Tue. Apr. 14, 2020.

• You have 4.0 hours to complete this Part 1 of the exam, Part 2 to be postedand announced to you by email later.

• Your 4.0h time allocation starts at the time you press the ”Start Quiz”button on the eLC webpage for ”Exam 2”.

• Be Careful: If you leave the eLC website or otherwise interrupt taking the exam,after pressing the ”Start Quiz” button, the clock for your 4.0h exam time allocationwill continue to run down, even if you are disconnected from the eLC website.

• If you do get disconncted or log out from the eLC website after pressing the ”StartQuiz” button, you can always log back into eLC and resume taking the exam bypressing the ”Continue Quiz” button on the the eLC webpage for ”Exam 2”, as longas your 4.0h time allocation has not run out yet.

• Do not forget to press the ”Submit Quiz” button on eLC when you have completedthe exam.

• Beware: Check your answers to all your questions very carefully, before you press the”Submit Quiz” button on eLC. You cannot go back to make any more changes to youranswers after you have pressed the the ”Submit Quiz” button.

By pressing the ”Start Quiz” button for this exam on eLC, you indicate that you understandthe instructions for this exam and agree to abide by them. You also certify that you willpersonally uphold UGA’s standards of academic honesty for this exam, and will not tolerateany violations of these standards by others.

Copyright c© 2020 University of Georgia. Unauthorized duplication or distribution prohibited.


Problem 1: In the figure below, Q1 and Q2 are both a negative point charges, both ofcomparable magnitude, with

|Q1| > |Q2| .

The locations of Q1, Q2 and observation point P are at three corners of a square. Whicharrow drawn at P could correctly represent the total electric field vector ~E generated by Q1

and Q2 at P ?

Q1

Fig. 2.38

Q2

P (E)

(D)

(C)

(B)(A)

(A) (B) (C) (D) (E)



Problem 2: Four metal balls, A, B, C and D of equal mass carry non-zero charges QA,QB, QC and QD, respectively, where

QA > 0 .

When suspended in different combinations from rigid horizontal supports by insulatingthreads of equal lengths, the balls are deflected from the vertical by the electric forces theyexert on each other, as shown in the three figures below.

AB

C DB

D AFig. 2.41

Which of the following could be true?

(A) QC = −QA < 0, QB < 0, QD > 0

(B) QC = QA > |QB|, QB < 0, QD > 0

(C) QC = −QA < 0, QB > |QC | > 0, QD < 0

(D) QC = QA < |QB|, QB < 0, QD > 0

(E) QC = QA < |QB|, QB > 0, QD < 0



Problem 3: Two positive point charges, each of charge Q > 0, are located at the two uppercorners of a square; and two negative point charges, each of charge −Q < 0, are located atthe two lower corners of that square, in the x-y-plane, as shown in the figure below. Thepoints R, S, T , and U are the mid-points of the edges and M is the center of the square. Twoedges of the square are parellel to the x-axis, and two edges are parallel to the y-axis, withx- and y-axis oriented as shown below. The total electric field, generated at these five pointsby the four charges, is denoted by ~ER = 〈ER,x, ER,y〉, ~ES = 〈ES,x, ES,y〉, ~ET = 〈ET,x, ET,y〉,~EU = 〈EU,x, EU,y〉 and ~EM = 〈EM,x, EM,y〉, respectively. Which of the following is true?

−Q

Fig. 2.39Q

S

U

M T

+ +

− −

R

−Q

Q

z

y

x

(A) The ~E-field vanishes at M , i.e., | ~EM | = 0.

(B) The ~E-field y-component at R is positive, i.e., ER,y > 0.

(C) The ~E-field y-component vanishes at S, i.e., ES,y = 0.

(D) The ~E-field x-component vanishes at T , i.e., ET,x = 0.

(E) The ~E-field y-component at U is positive, i.e., ET,y > 0.



Problem 4: A laser beam of wavelength λ is incident upon a narrow single slit of widthW , with λ < W , as shown in figure below. Assume |∆y| is the distance (in cm) between thetwo dark fringes (intensity minima), observed closest to the central maximum on a screen ata distance L on the other side of the slit. This distance |∆y| will

L

Single Slit

Laser Beam

Fig. 2.28

Screen

1st dark fringes Δy

(A) increase if we decrease L (keeping λ and W fixed).

(B) decrease if we increase λ (keeping L and W fixed).

(C) increase if we decrease W (keeping λ and L fixed).

(D) decrease if we increase L (keeping λ and W fixed).

(E) increase if we decrease λ (keeping L and W fixed).



Problem 5: A diffaction grating is illuminated with coherent (laser) light with wavelengthλ < d and oscillation period τ = λ/c where d is the spacing between adjacent slits in thegrating. The 4th order intensity maximum, to the left of the central intensity maximumM , is located at point P , as shown in figure below. A wave crest A from slit R and a wavecrest B from the neighboring slit S, to the right of R, have both departed at the same timefrom their respective slits of origin. Therefore, at P ,

MP

Diffr. Grating

Laser Beam

Fig. 2.40

SR

(A) A will arrive 3 periods before B.

(B) A will arrive 4 periods before B.

(C) A and B will arrive at the same time.

(D) B will arrive 3 periods before A.

(E) B will arrive 4 periods before A.



Problem 6: If a double-slit is illuminated at normal incidence by coherent (laser) light itproduces, on a screen parallel to and far away from the slits, its 3rd dark fringe (≡intensityminimum), at an angle of ±36.0o, measured from the central bright fringe (≡intensity maxi-mum). At what angle, θ, measured from the central bright fringe, will the 2nd dark fringeappear?

(All dark fringes are counted here from the central bright one, going outward. So, the 1stdark fringe is the one closest to the central bright one.)

(A) ±23.1o

(B) ±13.6o

(C) ±20.7o

(D) ±61.8o

(E) ±78.4o



Problem 7: For the double-slit experiment described in Problem 6, how many bright finges,in total, will be obervable on the screen? (Include in your count the central bright fringe, aswell as every bright fringe to either side of the central one.)

(A) 9

(B) 8

(C) 10

(D) 7

(E) 5



Problem 8: Assume the double-slit experiment described in Problem 6 (with the 3rd darkfringe observed at ±36.0o from the central bright one) was actually perfomed when the entireapparatus (laser, double-slit, screen) was immersed in air. At what angle, θ, measured fromthe central bright fringe, will the 3rd dark fringe be observed if that same entire apparatusis now submerged in water, with an index of refraction nWater = 1.33?

Hints: For air, nAir = 1.00. The vacuum wavelength of the laser and the distance betweenthe two slits is the same in both experiments.

(A) ±28.0o

(B) A 3rd dark fringe cannot be observed under water.

(C) ±26.2o

(D) ±51.42o

(E) ±15.4o



Problem 9: Two point charges lie on the x-axis: Q1 = −4.0mC is at x = 0, and Q2 =+4.0mC is at x=3.0m. Find the x-component, Ex, of the electric field vector, produced byboth charges, Q1 and Q2, at a point P on the x-axis, at x=6.0m. Also find the x-component,Fx, of the electric force exerted on a test charge, q=−5mC, placed on the x-axis at point P .

(A) Ex = +3.00 MN/C and Fx = −15.0 kN

(B) Ex = −4.50 MN/C and Fx = +22.5 kN

(C) Ex = −3.00 MN/C and Fx = +15.0 kN

(D) Ex = +4.50 MN/C and Fx = −22.5 kN

(E) Ex = +3.00 MN/C and Fx = +15.0 kN



Problem 10: Two point charges are located on the x-axis: Q1 = +90µC is at x=0, and Q2

is at x= 20.0cm. The total electric field, ~E, produced by both charges, is found to vanish,| ~E|=0, at x=24.0cm. Therefore, Q2 is

(A) +2.5 µC.

(B) −15. µC.

(C) −2.5 µC.

(D) +15. µC.

(E) −10. µC.



Problem 11: Two point charges, Q and q, spaced 5.0m apart, repel each other with a forceof 225N. What is the amount of the larger point charge, |Q|, if the larger charge is 8 timesthe amount of the smaller charge, |q|?

(A) 9.50 mC

(B) 1.00 mC

(C) 2.83 mC

(D) 7.50 mC

(E) 2.24 mC



Problem 12: Recall that a cube has eight corners and six square facees. A point charge Q,placed at the center of a cube, produces an electric field strength of E = 150V/m, at eachof the eight corners of the cube. What is the strength of the electric field at the centers ofthe six square faces of the cube?

(A) 260 V/m

(B) 50 V/m

(C) 212 V/m

(D) 450 V/m

(E) 86.6 V/m



Physics 1252Exam #2 Part 2 Version D

Instructions:This is an open-book, open-notes exam. Each question on this exam is worth 1 point. Thereis no penalty for guessing a wrong answer. Out of the total of 18 questions on this exam(Parts 1+2), three questions will be counted as bonus questions, i.e., only 15 correct answersare required for a 100% score.

For this Part 2 of Exam #2, you are allowed to use only your textbook and materials on thePHYS1252 course website, https://www.physast.uga.edu/classes/phys1212/schuttler,on Spring 2020 PHYS1252 pages of the eLC website, under https://uga.view.usg.edu,and on the LONCAPA PHYS1252 Homework site, http://spock.physast.uga.edu.

You are not allowed to use any other materials. You are not allowed, by any means ofcommunications, to discuss this exam with any other person, except your course instructor,nor share copies of any part of this exam with anyone, prior to Tue. Apr. 14, 2020.

• You have 3.0 hours to complete this Part 2 of the exam. This 3.0h timeallocation is in addition to the 4.0h time allocated for Part 1.

• Your 3.0h time allocation starts at the time you press the ”Start Quiz”button on the eLC webpage for ”Exam 2 Part 2”.

• Be Careful: If you leave the eLC website or otherwise interrupt taking the exam,after pressing the ”Start Quiz” button, the clock for your 3.0h exam time allocationwill continue to run down, even if you are disconnected from the eLC website.

• If you do get disconncted or log out from the eLC website after pressing the ”StartQuiz” button, you can always log back into eLC and resume taking the exam bypressing the ”Continue Quiz” button on the the eLC webpage for ”Exam 2 Part2”, aslong as your 3.0h time allocation has not yet run out.

• Do not forget to press the ”Submit Quiz” button on eLC when you have completedthe exam.

• Beware: Check your answers to all your questions very carefully, before you pressthe ”Submit Quiz” button on eLC. You cannot go back to make any more changes toyour answers after you have pressed the ”Submit Quiz” button. And don’t forget toanswer ”Problem 19”, before pressing ”Submit Quiz”.

By pressing the ”Start Quiz” button for this exam on eLC, you indicate that you understandthe instructions for this exam and agree to abide by them. You also certify that you willpersonally uphold UGA’s standards of academic honesty for this exam, and will not tolerateany violations of these standards by others.



Problem 13: A point charge of −35.4nC is placed on the y-axis, at y = 12.00cm, andanother point charge, of 70.8nC, is placed on the y-axis, at y = 36.00cm.

There is a point in the x-y-plane where the total electric field, ~E, produced by these twopoint charges vanishes, i.e., a point where | ~E| = 0. Find the x- and y-coordinate of thatpoint in the x-y-plane

(A) (x, y) = ( −7.88cm, +127.94cm )

(B) (x, y) = ( 0, −7.88cm )

(C) (x, y) = ( +21.94cm, −45.94cm )

(D) (x, y) = ( 0, +21.94cm )

(E) (x, y) = ( 0, −45.94cm )



Problem 14: For the two point charges, Q1 and Q2, shown in the figure for Problem 1(see also below), and the electric field, ~E, generated by Q1 and Q2 at point P , assume thatQ2 = −3.0nC, the sidelength of the square is 6.0cm, and the electric field strength at Pis E := | ~E| = 12.5kV/m. Allowing for the possibility that Q1 could be either positive ornegative, find all possible values of Q1.

Q1

Fig. 2.38

Q2

P (E)

(D)

(C)

(B)(A)

(A) The problem has only one solution for Q1:

Q1 = −5.8nC.

(B) The problem has two solutions for Q1:

Q1 = −5.8nC or Q1 = +5.8nC.

(C) The problem does not have a solution for Q1.

(D) The problem has two solutions for Q1:

Q1 = −4.0nC or Q1 = +4.0nC.

(E) The problem has only one solution for Q1:

Q1 = +4.0nCs.



Problem 15: A point charge, Q, of −75nC is affixed to the y-axis at yQ = 4.0cm. Anelectron is placed at some point P ≡ (xP , yP ) in the x-y-plane, free to move subject to theelectric field generated by Q. Assuming xP = −18.0cm and yP = −20.0cm, what is theelectron’s acceleration vector ~a ≡ 〈 ax, ay 〉, at point P?

(A) 〈−0.79, −1.05 〉 × 1015 m/s2

(B) 〈+0.79, +1.05 〉 × 1015 m/s2

(C) 〈−1.05, −0.79 〉 × 1015 m/s2

(D) 〈+4.94, +6.59 〉 × 1033 m/s2

(E) 〈−7.20, −9.60 〉 × 10−16 m/s2



Problem 16: If two point charges q1 and q2 at some distance r attract each other with aforce of 60N, what force would they exert on each other if r is increased fivefold (×5), q1is tripled (×3); and q2 is quartered (×1/4) and the signs of both charges are reversed. Thetwo charges will

(A) attract each other with a force of 1.8N.

(B) repel each other with a force of 1.8N.

(C) attract each other with a force of 2000N.

(D) attract each other with a force of 9N.

(E) repel each other with a force of 9N.



Problem 17: Two small loudspeakers, L1 and L2, are oscillating in phase at a frequencyof 516 Hz, sending out soundwave crests and troughs in synchrony. The speakers are spaced4.1 m apart. The speed of sound propagation in air is 344 m/s.

If you move a small microphone, M , slowly, along a straight line from L1 to L2, you willdectect maxima and minima of sound intensity, i.e., in the loudness of the sound. How manysuch maxima and minima will you find between L1 and L2?

Hints: Consider the pathlength difference, ∆` := `2− `1, where `2 is the distance from L2 toM and and `1 is the distance from L1 to M . (1) What is the range of ∆`-values covered asM is moved from L1 to L2? In other words: What is the initial value of ∆` when M startsfrom L1? What is the final value of ∆` when M arrives at L2? (2) An intensity maximumoccurs whenever ∆` reaches the values ∆` = 0, ±1λ, ±2λ, ... . (3) An intensity minimumoccurs when ∆` = ±1

2λ, ±3

2λ, ... .

(A) 6 maxima, 5 minima

(B) 6 maxima, 7 minima

(C) 15 maxima, 14 minima

(D) 13 maxima, 14 minima

(E) 13 maxima, 12 minima



Problem 18: A car is traveling North on a straight road, towards two radio transmissiontowers, S1 and S2. Transmitter S1 is located very close to the road; S2 is located at somedistance d to the East of S1. (Draw this!) The two transmitters are oscillating in phase ata frequency of 91.7 MHz, sending out radio wave crests and troughs in synchrony.

The car radio is tuned to the 91.7 MHz radio station and receives a strong radio signalwhen the car is still several km away from S1 and S2. However, as the car approaches thetransmitters, the radio signal gets weaker, and the radio signal disappears altogeter, due todestructive interference, when the car reaches a distance of 690 m from S1.

What is the distance, d, between the two radio transmitters?

Hints: Consider the pathlength difference, ∆` := `2− `1, where `2 is the distance from S2 tothe car and and `1 is the distance from S1 to the car. (1) Initially, when the car is very, veryfar away from S1, i.e., in the limit `1 → ∞, the pathlength difference vanishes: ∆` → 0.(2) As the car approaches S1, the value of ∆` increases as `1 decreases. (3) Destructiveinterference occurs when whenever ∆` reaches the values ∆` = 1

2λ, 3

2λ, ... .

(A) 977.m

(B) 82.4m

(C) 67.3m

(D) 47.5m

(E) 23.8m



Problem 19:Enter your Exam Version Letter: A, B, C, D or E !!Your Exam Version Letter is: D


physics 1252 exam #2 part 1 version d - physics and ......physics 1252 exam #2 part 1 version d...

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