physics 72 samplex

8/10/2019 Physics 72 Samplex

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University of the Philippines

College of Science

Physics 72

Set A

Second Long Exam

Second Semester, AY 20132014

Name: Instructor:

Section/Class Schedule: Student Number:

Course: College:


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Second Long Exam Second Semester, AY 20132014 Physics 72

INSTRUCTIONS: Choose the best answer and shade the corresponding circle on your answer

sheet. To change your answer, cross-out and sign your original answer and then shade your

new answer. No computational devices allowed (e.g. calculators, mobile phones). Any form

of cheating in examinations or any act of dishonesty in relation to studies, such as plagiarism,

shall be subject to disciplinary action.Following instructions is part of the exam.

Useful formulas:

Area Volume

Sphere (radius=r) 4r2 43

r3

Cylinder (radius=r, height=h) 2r2 + 2rh r2h

sin(/3) = cos(/6) =

3/2cos(/3) = sin(/6) = 1/2

sin(/4) = cos(/4) = 2/2

Useful constants:

e 1.60 1019 Cme 9.1 1031 kg0 8.854 1012 C2/Nm2k 8.988 109 Nm2/C2

1. Broken magnet. A bar magnet with orientation of North (N) and South (S) poles as shown

is cut into three pieces. The W pole of the middle pieceATTRACTSthe Z pole of another

bar magnet. What is the polarity of the X and Y poles?

A. X: North, Y: North

B. X: North, Y: South

C. X: South, Y: North

D. X: South, Y: South

E. Cannot be determined

2. Magnetic force. An electron initially moving upward encounters a region with uniform

magnetic fieldBwith direction shown. Which of the following situations will produce a

magnetic force directedINTO THE PAGE?

A. I only

B. II only

C. III only

D. I and II only

E. I, II and III

A 2


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3. What motion? Charge X is in a region of uniform magnetic field while charge Y isseparately in a region of uniform electric field. If both charges are initially held at rest,

which of the following best describes the motion of the charges after they are released?

(Neglect gravitational effects)

A. Only chargeX accelerates

B. Only chargeY accelerates

C. Both charges accelerate

D. Neither of the charges accelerates

E. X will change direction, Y willaccelerate

4. Parallel. A positive charge moving with initial velocityv in a uniform magnetic field

B experiences a magnetic force F. Which of the following statements is/areALWAYS

TRUE?

I. vhas no component parallel toF.II. Bhas no component parallel tov.

III. Fhas no component parallel toB.

A. I only

B. II only

C. II and III only

D. I and III only

E. I, II and III

5. Field line. In the figure shown, a bar magnet is

placed in thexy-plane with its longer side par-allel to thex-axis. What is the direction of themagnetic field at point P due to the bar magnet?

A. i B.i C.j D.j E. k

6. Flux. A magnetic field B enters a loop of area A as

shown in the figure. What must be the value of suchthat the magnetic flux through the loop is 1

2BA?

A. 30

B. 45

C. 60

D. 0

E. 90

A 3


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7. Zorro. A wire, bent as shown is immersed in a uniform magnetic

field B = 1 Tdirected to the right. The horizontal segments are1 m long while the diagonal segment is

2 m long. CurrentI =

1 A flows in the direction shown. What is the net force on thewire?

A. Zero

B. 1 N, into the page

C. 1 N, out of the page

D. 3 N, into the page

E. 3 N, out of the page

8. Rainbow. Figure shows the path of three positively charged parti-

cles as they enter a region of uniform magnetic field directed out

of the page. The particles have the same initial speed and have the

same masses. Which of the following is the correct relationship

between the charge magnitudesQ of the particles?

A. QI=QII=QIII

B. QI> QII> QIII

C. QI< QII< QIII

D. QI> QII< QIII

E. QI< QII> QIII

9. Magnetic field due to point particle. Shown in the fig-

ure is the instantaneous position of an electron with ve-

locity v. What is the magnitude and direction of the mag-

netic field at point P?

A. 08

evr2 , outward

B. 038

evr2 , inward

C. 08

evr2 , inward

D. 03

8evr2 , outward

E. 04

evr2 , inward

A 4


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10. B due to infinitesimal current. What is the magnetic field at

point(a, a)due to the two identical current elements of infinitesi-mal lengthLas shown in the figure?

A. 04

ILa2 k

B. zero

C.04

ILa2

k

D. 02

ILa2 k

E. 02

ILa2

k

11. Two infinite straight wires. What is the magnetic field

at point P due to the two infinite straight wires with cur-

rent directed into the page as shown in the figure?

A. 02

2Ir B.

02

2Ir

C. 0Ir

D.0Ir

E. 04

Ir

12. Bdue to a loop. What is the magnetic field at the origin

(0, 0)due to the current loop shown in the figure?

A. 0I2 (

1

b 1

a)k

B. 0I8

(1a 1

b)k

C. 0I2

(1a 1

b)k

D. zeroE. 0I

8 (1b

1

a)k

13. vanishing force. Shown in the figure are three infinite

straight wires with the indicated currents. Which of the

wires will experience a vanishing force per unit length?

A. 1 only

B. 2 only

C. 3 only

D. 1 and 3

E. 2 and 3

A 5


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14. proton. A proton with chargeehas a velocity v= vx+vy. At the instant when the protonis at the origin(x,y,z) = (0, 0, 0), which of the following gives the correct magnetic fieldat point (x,y,0)?

A.

0e

4

(vx

vy)

x2 +y2 k

B. 0e

4

(vxy vyx)(x2 +y2)3/2

k

C. 0

e

28 (v

xi vy

j)x2 +y2 D.

0e

2

8

vzx2 +y2 k

E. 0e

4

vzx2 +y2

k

For the next two numbers, consider an infinitely long solid cylinder

cylinder (radiusa) coaxial but insulated with a hollow, infinitely longcylinder (inner radiusb, outer radiusc). Equal currentsIflow across

the cylinders, into the page as shown.

15. Middle. What is the magnitude of the magnetic field at the space

between the conductors (a < r < b)?

A. Zero

B. I/(r)

C. I/(3r)

D. I/(2r)

E. Ir/(2b2)

16. Outside. What is the magnitude of the magnetic field outside the conductors (r > c)?

A. Zero

B. I/(r)

C. I/(3r)

D. I/(2r)

E. Ir/(2b2)

17. Together.A pair of electrons a distancedapart lies in thex-axis. They move with identicalspeedvin the+xdirection. What is the magnetic field midway between the two electrons?

A. Zero

B.

04

ev

d2k

C.04

evd2 k

D.

04

ev

d2j

E.04

evd2j

A 6


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18. Lenz Law. Consider a loop lying in the xy-plane and immersed in a region of spatiallyuniform magnetic field whose magnitude is either increasing or decreasing. Which of the

following correctly depicts the direction of the induced current in the loop?

A.

B.

C.

D.

E.

19. New emf. A metal ring is oriented with the plane of its area perpendicular to a spatially

uniform magnetic field that increases at a steady rate. If the radiusRof the ring ishalved,by what factor does the induced emf in the ring change?

A. 12

B. 14

C. 2 D. 4 E. 1

20. CW. Consider a conducting loop whose axis coincides with that of a bar magnet. In case1, the loop moves downward with its plane still perpendicular to the axis of the magnet.In case2, on the other hand, the bar magnet moves downward along the axis of the loop.What must be P,Q,X andY so that in both cases a current is induced in theclockwisedirection as seen from above the loop?

A. P: South,Q: North,X: North,Y: South

B. P: North,Q: South,X: South,Y: North

C. P: South,Q: North,X: South,Y: North

D. P: North,Q: South,X: North,Y: South

E. P: South,Q: South,X: North,Y: North

A 7


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21. Rectangular coil. A rectangular coil with area 1 m2 and oriented in the horizontal xy-plane is located in a region of uniform magnetic field that varies with time according to

B(t) = (1 T/s2)t2. The magnetic field is at an angle of30with the plane of the coil. Whatis the magnitude of the induced emf in the coil at time t= 1 s?

A. 1 V

B.

3 V

C. 2 V

D.3

2 V

E. zero

22. Rectangle +Wire. A metal rectangle is close to a long, straight, current-carrying wire,

with two of its sides parallel to the wire as shown in the figure. Which of the following

is/areTRUE?

I. If the rectangle is moved downward, there isNO induced current in the rectangle.

II. If the current in the long wire is decreasing, there is an induced current in the rectangle

in the clockwise direction.

III. If the rectangle is moved to the left, there is an induced current in the rectangle in the

counterclockwise direction.

A. I only

B. II only

C. III only

D. I and II only

E. I and III only

A 8


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23. Max emf. A bar of length 1 m moves through a uniform magnetic field with magnitude1 T with speed1 m/s. In which direction should this bar move so that the emf across itsends is maximum?

A. 1or2B. 3or4

C. x- or x-directionD. y- or y-directionE. z- or z-direction

24. Function. Given an electric field described by the functionE(t) = 0.15 N/Csin2twheretis in seconds, what is theMAXIMUM displacement current through a 1 m2 area perpen-dicular toE?

A. zero

B. 0.30

C. 0.0750D. 0.60

E. 0.150

25. E-induced. A circular loop with radius of 0.1 m, is placed in a time

varying magnetic field as shown in the figure. If the magnitude of themagnetic field increases at a rate of 0.20 T/s, what is the magnitude of

the induced electric field in the loop?

A. 0.01 V/m

B. 0.02 V/m

C. 0.10 V/m

D. 0.20 V/m

E. 0.001 V/m

26. Injuiced.Two loops have mutual inductance of5 mH. A constant current of5 Ais present

in one of the coils. What is the induced emf on the other coil?

A. 25 mV

B. 10 mV

C. 5 V

D. 1 mV

E. Zero

27. Critical. IfR = 1.0 and L = 2.0 H,what value of capacitanceCwill make an L-R-Cseries circuit critically damped?

A. 0.5 F B. 1.0 F C. 2.0 F D. 4.0 F E. 8.0 F

A 9


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28. Giant. A giant coil has an area of8 m2 and16 min length. How many turns would createa self-inductance of2 H?

A. 40 B. 80

C. 60 D. 20

E. 160

29. Energy Catcher. A certain region in space contains a magnetic field of0.2T. You place acubical box of side length2m2 to catch some energy. What is the magnetic energy densityinside the box?

A. 0.08

B. 0.16

C. 0.02

D. 0.004

E. 0.1

For the next three numbers, consider AnLCCircuit containing a 5H inductor and a5Fca-pacitor. The current oscillates with amplitude of2A.

30. Max Out! What is the maximum charge on the capacitor?

A. 1.0C B. 7.5C C. 5C D. 2.5C E. 10.0C

31. Repeat. What is the oscillation frequency of the circuit?

A. 0.1rad/s

B. 0.2rad/s

C. 0.25rad/s

D. 0.5rad/s

E. 0.75rad/s

32. Start. If the capacitor has maximum charge at time t = 0, then what is the energy storedin the inductor att= 0?

A. 0.01J B. 0.02J C. 0.05J D. Zero E. 0.20J

For the next two numbers consider the phasor diagram. The values of resistance R, capacitivereactanceXC, and inductive reactanceXLof three circuits are given in the table.

33. Matching type. Which of the circuits match/es the phasor diagram?

A. I onlyB. II only

C. III onlyD. II and III only

E. I and III only

A 10


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34. Zzzzz. What is the impedance of circuit II?

A. 9 k B. 7 k C. 3 k D. 25k E. 5 k

35. Insta-avenue. Consider an RLC series circuit driven by an AC source with fixed voltage

amplitude. If the driving frequency is1, the phase angle is 10. If the driving frequencyis2, the phase angle is+10

. Which of the following is/are true?

I. The instantaneous power at any given timetis equal for the two frequencies.

II. The average power is equal for the two frequencies.

III. The rms current is the same for both frequencies.

A. I only

B. II only

C. III only

D. II and III only

E. I, II, and III

36. phasensyahan. Consider three independent LRC series circuits. At a driving frequency

, the phase angles for circuitsI,I I, andI II are 30,+30, and+60, respectively. Ifthe resistance are the same for all three circuits, which of the following best describes the

impedance?

A. I=II < IIIB. I=II > III

C. I < II < IIID. I > II > III

E. I=I I=I II

For the next two questions, consider three different LRC

circuits connected to an AC source with the same peak

amplitude. All inductors and capacitors are ideal and

the values of the resistanceR, inductanceL, and capac-itanceCare given in the table.

37. At resonance. Which of the following correctly describes the maximum current in the

circuit at resonance?

A. I=IIIII

38. Circuit II. If the amplitude of the AC source is 5.0 V and the frequency is 10, 000 rad/swhat is the average power dissipated in the resistor for circuit II?

A 11


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A. 1/50 W

B. 1/32 W

C. 1/40 W

D. 1/25 W

E. 1/16 W

39. Bagay! Figure shows the current versus time plot of an ideal AC source and the voltage

versus time plot of a resistor, ideal inductor, and ideal capacitor when they are connectedseparately to the source. Which of the following correctly matches the circuit element to

its corresponding voltage versus time plot?

A. X-Inductor, Y- Capacitor, Z-Resistor

B. X-Resistor, Y- Inductor, Z-Capacitor

C. X-Capacitor, Y- Resistor, Z-Inductor

D. X-Inductor, Y- Resistor, Z-Capacitor

E. X-Resistor, Y- Capacitor, Z-Inductor

40. Root. A 12.0 resistor is connected across a sinusoidal emf that has a peak value of 48.0 V.What is the rms value of the current passing through the circuit?

A. 2

2A B. 2

2 A

C. 2 A

D. 4 A

E. 4

2 A

A 12

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