Physics Chapter 22
30) A particle carrying a charge of +e travels in a circular path of radius R in a uniform magnetic field. If instead the particle carried a charge of +2e the radius of the circular path would have been A) 2R. B) 4R. C) 8R. D) R/2. E) R/4.
Answer: D
31) A wire in the shape of an "M" lies in the plane of the paper. It carries a current of 2.0 A, flowing from A to E. It is placed in a uniform magnetic field of 0.85 T in the same plane, directed as shown on the right side of the figure. The figure indicates the dimensions of the wire. Note that AB is parallel to DE and to the baseline from which the magnetic field direction is measured. What are the magnitude and direction of the net force acting on this wire? A) 0.40 N perpendicular out of the page B) 0.10 N perpendicular into the page C) 0.40 N perpendicular into the page D) 0.10 N perpendicular out of the page E) 0.20 N perpendicular out of the page
Answer: D
32) If a calculated quantity has units of , that quantity could be A) an electric field. B) an electric potential. C) μ0. D) a magnetic field. E) a magnetic torque.
Answer: D
38) For the horseshoe magnet shown in the figure, the left end is a north magnetic pole and the right end is a south magnetic pole. When the switch is closed in the circuit, which way will the wire between the poles of the horseshoe magnet initially deflect? A) to the right B) to the left C) upward D) downward
Answer: D
40) The magnetic force on a current-carrying wire in a magnetic field is the strongest when A) the current is in the direction of the magnetic field lines. B) the current is in the direction opposite to the magnetic field lines. C) the current is at a 180° angle with respect to the magnetic field lines. D) the current is perpendicular to the magnetic field lines. E) the current is at a 0° angle with respect to the magnetic field lines.
Answer: D
41) A vertical wire carries a current straight up in a region where the magnetic field vector points toward the north. What is the direction of the magnetic force on this wire? A) downward B) toward the north C) toward the east D) toward the west E) upward F) toward the south
Answer: D
43) Two long parallel wires are placed side-by-side on a horizontal table. If the wires carry current in opposite directions, A) one wire is lifted slightly while the other wire is forced downward against the table's surface. B) both wires are lifted slightly. C) the wires pull toward each other. D) the wires push away from each other.
Answer: D
47) Two long, parallel wires carry currents of different magnitudes. If the amount of current in one of the wires is doubled, what happens to the magnitude of the force that each wire exerts on the other? A) It is increased by a factor of 8. B) It is increased by a factor of 4. C) It is increased by a factor of 3. D) It is increased by a factor of 2. E) It is increased by a factor of
Answer: D
58) A long, straight wire carrying a current is placed along the y-axis. If the direction of the current is in the +y direction, what is the direction of the magnetic field due to this wire as you view it in such a way that the current is coming directly toward you? A) clockwise, around the x-axis B) counterclockwise, around the x-axis C) counterclockwise, around the z-axis D) counterclockwise, around the y-axis E) clockwise, around the y-axis
Answer: D
59) A negatively charged particle -Q is moving to the right, directly above a wire having a current I flowing to the right, as shown in the figure. In what direction is the magnetic force exerted on the particle due to the current? A) into the page B) out of the page C) downward D) upward E) The magnetic force is zero because the velocity is parallel to the current.
Answer: D
59) Two long parallel wires carry currents of 20 A and 5.0 A in opposite directions. The wires are separated by 20 cm. At what point between the two wires do they produce the same strength magnetic field? A) 4.0 cm from the 20 A wire B) 8.0 cm from the 20 A wire C) 12 cm from the 20 A wire D) 16 cm from the 20 A wire E) 18 cm from the 20 A wire
Answer: D
60) A wire lying in the plane of the page carries a current toward the bottom of the page, as shown in the figure. What is the direction of the magnetic force it produces on an electron that is moving to the left directly toward the wire, as shown? A) straight into the page B) straight out of the page C) directly toward the top of the page D) directly toward the bottom of the page E) directly to the left away from the wire
Answer: D
63) The magnetic field at a distance of 2 cm from a long straight current-carrying wire is 4 μT. What is the magnetic field at a distance of 1 cm from this wire? A) 2 μT B) 4 μT C) 6 μT D) 8 μT E) 10 μT
Answer: D
67) A horizontal wire carries a current straight toward you. From your point of view, the magnetic field caused by this current A) points directly away from you. B) points to the left. C) circles the wire in a clockwise direction. D) circles the wire in a counter-clockwise direction.
Answer: D
67) An ideal solenoid having a coil density of 5000 turns per meter is 10 cm long and carries a current of 4.0 A. What is the strength of the magnetic field at its center? A) 3.1 mT B) 6.2 mT C) 13 mT D) 25 mT
Answer: D
7) An electron, moving south, enters a magnetic field. Because of this field, the electron curves upward. We may conclude that the magnetic field must have a component A) downward. B) towards the east. C) upward. D) towards the west. E) towards the north.
Answer: D
70) A current-carrying loop of wire lies flat on a horizontal tabletop. When viewed from above, the current moves around the loop in a counterclockwise sense. For points on the tabletop outside the loop, the magnetic field lines caused by this current A) circle the loop in a clockwise direction. B) circle the loop in a counterclockwise direction. C) point straight up. D) point straight down
Answer: D
72) Consider two current-carrying circular loops. Both are made from one strand of wire and both carry the same current, but one has twice the radius of the other. If the magnetic field strength at the center of the smaller loop is B, what is the magnetic field strength at the center of the larger loop? A) 8B B) 4B C) 2B D) B/2 E) B/4
Answer: D
13) Three particles travel through a region of space where the magnetic field is out of the page, as shown in the figure. What are the signs of the charges of these three particles? A) 1 is neutral, 2 is negative, and 3 is positive. B) 1 is neutral, 2 is positive, and 3 is negative. C) 1 is positive, 2 is neutral, and 3 is negative. D) 1 is positive, 2 is negative, and 3 is neutral. E) 1 is negative, 2 is neutral, and 3 is positive.
Answer: E
14) An electron moving in the +y direction, at right angles to a magnetic field, experiences a magnetic force in the -x direction. The direction of the magnetic field is in the A) -x direction. B) +x direction. C) +y direction. D) -z direction. E) +z direction.
Answer: E
17) A proton is to orbit Earth at the equator using Earth's magnetic field to supply part of the necessary centripetal force. In what direction should the proton move? A) upward B) northward C) southward D) eastward E) westward
Answer: E
38) A flat rectangular loop of wire is placed between the poles of a magnet, as shown in the figure. It has dimensions w
0.60 m and L = 1.0 m, and carries a current I = 2.0 A in the direction shown. The magnetic field due to the magnet is uniform and of magnitude 0.80 T. The loop rotates in the magnetic field and at one point the plane of the loop is parallel to the field. At that instant, what is the magnitude of the torque acting on the wire due to the magnetic field? A) 0.00 N ∙ m B) 0.40 N ∙ m C) 0.48 N ∙ m D) 0.83 N ∙ m E) 0.96 N ∙ m= Answer: E
39) A flat rectangular loop of wire is placed between the poles of a magnet, as shown in the figure. It has dimensions w
0.60 m and L = 1.0 m, and carries a current I = 2.0 A in the direction shown. The magnetic field due to the magnet is uniform and of magnitude 0.80 T. The loop rotates in the magnetic field and at one point the plane of the loop is perpendicular to the field. At that instant, what is the magnitude of the torque acting on the wire due to the magnetic field? A) 0.00 N ∙ m B) 0.40 N ∙ m C) 0.48 N ∙ m D) 0.83 N ∙ m E) 0.96 N ∙ m= Answer: A
37) A flat rectangular loop of wire is placed between the poles of a magnet, as shown in the figure. It has dimensions w
0.60 m and L = 1.0 m, and carries a current I = 2.0 A in the direction shown. The magnetic field due to the magnet is uniform and of magnitude 0.80 T. The loop rotates in the magnetic field and at one point the plane of the loop makes a 30° angle with the field. At that instant, what is the magnitude of the torque acting on the wire due to the magnetic field? A) 0.30 N ∙ m B) 0.40 N ∙ m C) 0.48 N ∙ m D) 0.83 N ∙ m E) 0.96 N ∙ m= Answer: C
4) A proton moving eastward with a velocity of 5.0 km/s enters a magnetic field of 0.20 T pointing northward. What are the magnitude and direction of the force that the magnetic field exerts on the proton? (e
1.60 × 10-19 C) A) 0 N B) 1.6 × 10-16 N upwards C) 1.6 × 10-16 N downwards D) 1.1 × 10-16 N eastwards E) 4.4 × 10-16 N westwards= Answer: B
3) A proton is projected with a velocity of 7.0 km/s into a magnetic field of 0.60 T perpendicular to the motion of the proton. What is the magnitude of the magnetic force that acts on the proton? (e
1.60 × 10-19 C) A) 0 N B) 3.4 × 10-16 N C) 4.2 × 10-16 N D) 13 × 10-16 N E) 6.7 × 10-16 N= Answer: E
17) Alpha particles, each having a charge of +2e and a mass of 6.64 × 10-27 kg, are accelerated in a uniform 0.80-T magnetic field to a final orbit radius of 0.30 m. The field is perpendicular to the velocity of the particles. How long does it take an alpha particle to make one complete circle in the final orbit? (e
1.60 × 10-19 C) A) 0.15 μs B) 0.25 μs C) 0.33 μs D) 0.40 μs E) 0.49 μs= Answer: A
5) A proton moving with a velocity of 4.0 × 104 m/s enters a magnetic field of 0.20 T. If the angle between the velocity of the proton and the direction of the magnetic field is 60°, what is the magnitude of the magnetic force on the proton? (e
1.60 × 10-19 C) A) 1.8 × 10-15 N B) 0.60 × 10-15 N C) 1.1 × 10-15 N D) 2.2 × 10-15 N E) 3.3 × 10-15 N= Answer: C
14) A doubly charged ion with speed 6.9 × 106 m/s enters a uniform 0.80-T magnetic field, traveling perpendicular to the field. Once in the field, it moves in a circular arc of radius 30 cm. What is the mass of this ion? (e
1.60 × 10-19 C) A) 11 × 10-27 kg B) 6.7 × 10-27 kg C) 3.3 × 10-27 kg D) 8.2 × 10-27 kg= Answer: A
2) A proton travels at a speed of 5.0 × 107 m/s through a 1.0-T magnetic field. What is the magnitude of the magnetic force on the proton if the angle between the proton's velocity and the magnetic field vector is 30°? (e
1.60 × 10-19 C) A) 2.0 × 10-14 N B) 4.0 × 10-14 N C) 2.0 × 10-12 N D) 4.0 × 10-12 N= Answer: D
8) An electron moves with a speed of 8.0 × 106 m/s along the +x-axis. It enters a region where there is a magnetic field of 2.5 T, directed at an angle of 60° to the +x-axis and lying in the xy-plane. Calculate the magnitude of the magnetic force on the electron. (e
1.60 × 10-19 C) A) 2.8 × 10-10 N B) 3.2 × 10-10 N C) 2.8 × 10-12 N D) 3.2 × 10-12 N E) 0 N= Answer: C
22.2 Problems 1) An electron moves with a speed of 5.0 × 104 m/s perpendicular to a uniform magnetic field of magnitude 0.20 T. What is the magnitude of the magnetic force on the electron? (e
1.60 × 10-19 C) A) 4.4 × 10-14 N B) 1.6 × 10-15 N C) 5 × 10-20 N D) 2.6 × 10-24 N E) zero= Answer: B
7) An electron moves with a speed of 3.0 × 104 m/s perpendicular to a uniform magnetic field of 0.40 T. What is the magnitude of the magnetic force on the electron? (e
1.60 × 10-19 C) A) 4.8 × 10-14 N B) 1.9 × 10-15 N C) 5 × 10-20 N D) 2.2 × 10-24 N E) 0 N= Answer: B
9) An electron moves with a speed of 8.0 × 106 m/s along the +x-axis. It enters a region where there is a magnetic field of 2.5 T, directed at an angle of 60° to the +x-axis and lying in the xy-plane. Calculate the magnitude of the acceleration of the electron. (e
1.60 × 10-19 C, me1 = 9.11 × 10-31 kg) A) 1.3 × 1018 m/s2 B) 3.0 × 1018 m/s2 C) 1.3 × 10-18 m/s2 D) 3.0 × 10-18 m/s2 E) 0 m/s2= Answer: B
13) An electron is accelerated from rest through a potential difference of 3.75 kV. It enters a region where a uniform 4.0-mT magnetic field is perpendicular to the velocity of the electron. Calculate the radius of the path this electron will follow in the magnetic field. (e
1.60 × 10-19 C, melectron = 9.11 × 10-31 kg) A) 1.2 cm B) 2.2 cm C) 3.2 cm D) 4.2 cm E) 5.2 cm= Answer: E
11) An electron moving perpendicular to a uniform magnetic field of 3.2 × 10-2 T moves in a circle of radius 0.40 cm. How fast is this electron moving? (e
1.60 × 10-19 C, melectron = 9.11 × 10-31kg) A) 2.2 × 107 m/s B) 1.9 × 107 m/s C) 1.9 × 106 m/s D) 3.0 × 107 m/s E) 0.80 × 107 m/s= Answer: A
12) An electron moving perpendicular to a uniform magnetic field of 0.22 T moves in a circle with a speed of 1.5 × 107 m/s. What is the radius of the circle? (e
1.60 × 10-19 C, melectron = 9.11 × 10-31kg) A) 1.5 mm B) 0.22 mm C) 2.2 mm D) 0.39 mm E) 3.9 mm= Answer: E
15) A proton, starting from rest, accelerates through a potential difference of 1.0 kV and then moves into a magnetic field of 0.040 T at a right angle to the field. What is the radius of the proton's resulting orbit? (e
1.60 × 10-19 C, mproton = 1.67 × 10-27 kg) A) 0.080 m B) 0.11 m C) 0.14 m D) 0.17 m= Answer: B
18) Alpha particles, each having a charge of +2e and a mass of 6.64 × 10-27 kg, are accelerated in a uniform magnetic field to a final orbit radius of The field is perpendicular to the velocity of the particles. What is the kinetic energy of an alpha particle in the final orbit? (1 eV
1.60 × 10-19 J, A) 3.0 MeV B) 2.6 MeV C) 3.4 MeV D) 3.9 MeV E) 4.3 MeV= Answer: A
35) A rectangular coil, with corners labeled ABCD, has length L and width w. It is placed between the poles of a magnet, as shown in the figure If there is a current I flowing through this coil in the direction shown, what is the direction of the force acting on section BC of this coil? A) perpendicular to and into the page B) perpendicular to and out of the page C) in the direction of the magnetic field D) in the opposite direction of the magnetic field E) The force is zero.
Answer: E
4) A positive charge is moving to the right and experiences an upward magnetic force, as shown in the figure. In which direction must the magnetic field have a component? A) to the right B) to the left C) upward D) out of the page E) into the page
Answer: E
44) A flat rectangular loop of wire carrying a 4.0-A current is placed in a uniform 0.60-T magnetic field. The magnitude of the torque acting on this loop when the plane of the loop makes a 30° angle with the field is measured to be 1.1 N ∙ m. What is the area of this loop? A) 0.20 m2 B) 0.40 m2 C) 0.26 m2 D) 0.80 m2 E) 0.53 m2
Answer: E
54) Two long parallel wires placed side-by-side on a horizontal table carry identical current straight toward you. From your point of view, the magnetic field at a point exactly between the two wires A) points upward. B) points downward. C) points toward you. D) points away from you. E) is zero.
Answer: E
65) A long straight wire on the z-axis carries a current of 8.0 A in the +z direction (out of the paper). A circular loop of radius 10 cm lies in the xy-plane and carries a 3.0-A current, as shown in the figure. Point P, at the center of the loop, is 25 cm from the z-axis. An electron is projected from P with a velocity of 3.0 × 106 m/s in the -x direction. What is the y component of the magnetic force on the electron? (e
1.60 × 10-19, μ0 = 4π × 10-7 T ∙ m/A) A) -9.0 × 10-18 N B) 9.0 × 10-18 N C) -4.5 × 10-18 N D) 4.5 × 10-18 N E) zero= Answer: A
57) A wire lying in the plane of this page carries a current directly toward the top of the page. What is the direction of the magnetic force this current produces on an electron that is moving perpendicular to the page and outward from it on the left side of the wire? A) perpendicular to the page and towards you B) perpendicular to the page and away from you C) toward the top of the page D) toward the bottom of the page E) The force is zero.
Answer: E
49) A long straight wire carrying a 4-A current is placed along the x-axis as shown in the figure. What is the magnitude of the magnetic field at a point P, located at y
2 cm, due to the current in this wire? A) 20 μT B) 30 μT C) 40 μT D) 50 μT E) 60 μT= Answer: C
6) A proton moving with a velocity of 4.0 × 104 m/s along the +y-axis enters a magnetic field of 0.20 T directed towards the -x-axis. What is the magnitude of the magnetic force acting on the proton? A) 8.0 × 10-15 N B) 3.9 × 10-15 N C) 2.6 × 10-15 N D) 0 N E) 1.3 × 10-15 N
Answer: E
64) A circular metal loop of radius 10 cm and three long straight wires carry currents of and as shown in the figure. Each of the straight wires is 20 cm from the center of the loop. The axes are shown in the figure, with the +z-axis coming out of the paper. What is the z component of the resultant magnetic field at the center of the loop? (μ0
4π × 10-7 T ∙ m/A) A) -360 μT B) +360 μT C) -40 μT D) +40 μT E) -170 μT= Answer: A
63) A circular metal loop of radius 10 cm and three long straight wires carry currents of and as shown in the figure. Each of the straight wires is 20 cm from the center of the loop. The axes are shown in the figure, with the +z-axis coming out of the paper. What is the y component of the resultant magnetic field at the center of the loop? (μ0
4π × 10-7 T ∙ m/A) A) -54 μT B) -60 μT C) -50 μT D) +60 μT E) +50 μT= Answer: A
73) An ideal solenoid is wound with 470 turns on a wooden form that is 4.0 cm in diameter and 50 cm long. The windings carry a current in the sense shown in the figure. The current produces a magnetic field of magnitude at the center of the solenoid. What is the current I in the solenoid windings? (μ0
4π × 10-7 T ∙ m/A) A) 3.5 A B) 3.0 A C) 2.6 A D) 4.3 A E) 3.9 A= Answer: A
61) Three very long, straight, parallel wires each carry currents of 4.0 A, directed out of the page as shown in the figure. These wires pass through the vertices of a right isosceles triangle as shown. Assume that all the quantities shown in the figure are accurate to two significant figures. What is the magnitude of the magnetic field at point P at the midpoint of the hypotenuse of the triangle? (μ0
4π × 10-7 T ∙ m/A) A) 4.4 µT B) 18 µT C) 57 µT D) 130 µT E) 1.8 µT= Answer: C
66) An ideal solenoid 20 cm long is wound with 5000 turns of very thin wire. What strength magnetic field is produced at the center of the solenoid when a current of 10 A flows through it? (μ0
4π × 10-7 T ∙ m/A) A) 0.0063 T B) 0.20 T C) 3.2 T D) 4.8 T E) 0.31 T= Answer: E
70) How much current must pass through a 400-turn ideal solenoid that is 4.0 cm long to generate a 1.0-T magnetic field at the center? (μ0
4π × 10-7 T ∙ m/A) A) 0.013 A B) 13 A C) 22 A D) 40 A E) 80 A= Answer: E
51) The magnetic field due to the current in a long, straight wire is 8.0 μT at a distance of 4.0 cm from the center of the wire. What is the current in the wire? (μ0
4π × 10-7 T ∙ m/A) A) 0.20 A B) 0.40 A C) 0.80 A D) 3.2 A E) 1.6 A= Answer: E
48) A long wire carrying a 2.0-A current is placed along the y-axis. What is the magnitude of the magnetic field at a point that is 0.60 m from the origin along the x-axis? (μ0
4π × 10-7 T ∙ m/A) A) 0.67 µT B) 1.3 µT C) 0.12 µT D) 6.7 T E) 12 T= Answer: A
54) A very long thin wire produces a magnetic field of at a distance of from the wire. What is the magnitude of the current? (μ0
4π × 10-7 T ∙ m/A) A) 1.0 mA B) 2.0 mA C) 4,000 mA D) 3,100 mA= Answer: A
56) At what distance from a long straight wire carrying a current of 5.0 A is the magnitude of the magnetic field due to the wire equal to the strength of Earth's magnetic field of about 5.0 × 10-5 T? (μ0
4π × 10-7 T ∙ m/A) A) 1.0 mm B) 1.0 cm C) 2.0 cm D) 3.0 cm E) 4.0 cm= Answer: C
58) Two long parallel wires carry currents of 20 A and 5.0 A in opposite directions. The wires are separated by 0.20 m. What is the strength of the magnetic field midway between the two wires? (μ0
4π × 10-7 T ∙ m/A) A) 1.0 × 10-5 T B) 2.0 × 10-5 T C) 3.0 × 10-5 T D) 4.0 × 10-5 T E) 5.0 × 10-5 T= Answer: E
60) Three long parallel wires each carry 2.0-A currents in the same direction. The wires are oriented vertically, and they pass through three of the corners of a horizontal square of side 4.0 cm. What is the magnitude of the magnetic field at the fourth (unoccupied) corner of the square due to these wires? (μ0
4π × 10-7 T ∙ m/A) A) 1.2 µT B) 2.1 µT C) 12 µT D) 21 µT E) 0 T= Answer: D
50) At point P the magnetic field due to a long straight wire carrying a current of 2.0 A is 1.2 µT. How far is P from the wire? (μ0
4π × 10-7 T ∙ m/A) A) 11 cm B) 22 cm C) 33 cm D) 44 cm E) 55 cm= Answer: C
71) How many turns should a 10-cm long ideal solenoid have if it is to generate a 1.5-mT magnetic field when 1.0 A of current runs through it? (μ0
4π × 10-7 T ∙ m/A) A) 12 B) 15 C) 120 D) 1200 E) 3200= Answer: C
52) The magnetic field at point P due to a 2.0-A current flowing in a long, straight, thin wire is 8.0 μT. How far is point P from the wire? (μ0
4π × 10-7 T ∙ m/A) A) 2.0 cm B) 2.5 cm C) 4.0 cm D) 5.0 cm E) 10 cm= Answer: D
33) Two long parallel wires that are 0.30 m apart carry currents of 5.0 A and 8.0 A in the opposite direction. Find the magnitude of the force per unit length that each wire exerts on the other wire and indicate if the force is attractive or repulsive. (μ0
4π × 10-7 T ∙ m/A) A) 2.7 × 10-5 N repulsive B) 7.2 × 10-5 N repulsive C) 3.4 × 10-5 N attractive D) 2.7 × 10-5 N attractive E) 7.2 × 10-5 N attractive= Answer: A
57) Two long parallel wires that are 0.40 m apart carry currents of 10 A in opposite directions. What is the magnetic field strength in the plane of the wires at a point that is 20 cm from one wire and 60 cm from the other? (μ0
4π × 10-7 T ∙ m/A) A) 3.3 µT B) 6.7 µT C) 33 µT D) 67 µT= Answer: B
53) The magnitude of the magnetic field that a long and extremely thin current-carrying wire produces at a distance of 3.0 µm from the center of the wire is 2.0 × 10-3 T. How much current is flowing through the wire? (μ0
4π × 10-7 T ∙ m/A) A) 30 mA B) 190 mA C) 19 mA D) 380 mA= Answer: A
47) A high power line carries a current of 1.0 kA. What is the strength of the magnetic field this line produces at the ground, 10 m away? (μ0
4π × 10-7 T ∙ m/A) A) 4.7 µT B) 6.4 µT C) 20 µT D) 56 µT= Answer: C
55) How much current must flow through a long straight wire for the magnetic field strength to be 1.0 mT at 1.0 cm from a wire? (μ0
4π × 10-7 T ∙ m/A) A) 50 mA B) 9.2 A C) 16 A D) 50 A E) 5.0 mA= Answer: D
62) A wire carrying a current is shaped in the form of a circular loop of radius 4.0 mm. If the magnetic field strength at its center is with no external magnetic fields present, what is the magnitude of the current flowing through the wire? (μ0
4π × 10-7 T ∙ m/A) A) 7.0 A B) 28 A C) 14 A D) 17 A= Answer: A
69) An ideal solenoid that is 34.0 cm long is carrying a current of 2.00 A. If the magnitude of the magnetic field generated at the center of the solenoid is 9.00 mT, how many turns of wire does this solenoid contain? (μ0
4π × 10-7 T ∙ m/A) A) 860 B) 1590 C) 2320 D) 3180 E) 1220= Answer: E
72) An ideal solenoid with 400 turns has a radius of 0.040 m and is 40 cm long. If this solenoid carries a current of 12 A, what is the magnitude of the magnetic field at the center of the solenoid? (μ0
4π × 10-7 T ∙ m/A) A) 16 mT B) 4.9 mT C) 15 mT D) 6.0 mT E) 9.0 mT= Answer: C
34) Two long parallel wires are 0.400 m apart and carry currents of 4.00 A and 6.00 A. What is the magnitude of the force per unit length that each wire exerts on the other wire? (μ0
4π × 10-7 T ∙ m/A) A) 2.00 μN/m B) 5.00 μN/m C) 12.0 μN/m D) 16.0 μN/m E) 38.0 μN/m= Answer: C
32) Two parallel straight wires are 7.0 cm apart and 50 m long. Each one carries a 18-A current in the same direction. One wire is securely anchored, and the other is attached in the center to a movable cart. If the force needed to move the wire when it is not attached to the cart is negligible, with what magnitude force does the wire pull on the cart? (μ0
4π × 10-7 T ∙ m/A) A) 46 mN B) 37 mN C) 66 mN D) 93 mN= Answer: A
11) A proton, moving east, enters a magnetic field. Because of this magnetic field the proton curves downward. We may conclude that the magnetic field must have a component A) towards the south. B) towards the north. C) towards the west. D) upward. E) downward.
Answer: A
12) An electron is moving to the right, as shown in the figure. Suddenly it encounters uniform magnetic field pointing out of the page. Which one of the three paths shown will it follow in the field? A) path a B) path b C) path c
Answer: A
16) A charged particle of mass 0.0040 kg is subjected to a magnetic field which acts at a right angle to its motion. If the particle moves in a circle of radius at a speed of what is the magnitude of the charge on the particle? A) 0.020 C B) 50 C C) 0.00040 C D) 2,500 C
Answer: A
19) A 2.0-m straight wire carrying a current of 0.60 A is oriented parallel to a uniform magnetic field of 0.50 T. What is the magnitude of the magnetic force on it? A) zero B) 0.15 N C) 0.30 N D) 0.60 N
Answer: A
20) A straight wire carries a current of 10 A at an angle of 30° with respect to the direction of a uniform 0.30-T magnetic field. Find the magnitude of the magnetic force on a 0.50-m length of the wire. A) 0.75 N B) 1.5 N C) 3.0 N D) 6.0 N
Answer: A
23) A rigid rectangular loop, measuring 0.30 m by 0.40 m, carries a current of 9.9 A, as shown in the figure. A uniform external magnetic field of magnitude 1.8 T in the -x direction is present. Segment CD is in the xz-plane and forms a 19° angle with the z-axis, as shown. What is the y component of the magnetic force on segment AB of the loop? A) +5.1 N B) -5.1 N C) +1.7 N D) -1.7 N E) 0.0 N
Answer: A
25) An electron has an initial velocity to the south but is observed to curve upward as the result of a magnetic field. This magnetic field must have a component A) to the west. B) to the east. C) upward. D) downward. E) to the north.
Answer: A
28) A wire in the shape of an "M" lies in the plane of the paper. It carries a current of 2.0 A, flowing from A to E, as shown in the figure. It is placed in a uniform magnetic field of 0.65 T in the same plane, directed as shown on the right side of the figure. The figure indicates the dimensions of the wire. Note that AB is parallel to DE and to the baseline from which the magnetic field direction is measured. What are the magnitude and direction of the force acting on section BC of this wire? A) 0 N B) 0.090 N perpendicular out of the page C) 0.090 N perpendicular into the page D) 0.060 N perpendicular out of the page E) none of the above
Answer: A
29) A charged particle is observed traveling in a circular path of radius R in a uniform magnetic field. If the particle were traveling twice as fast the radius of the circular path would be A) 2R. B) 4R. C) 8R. D) R/2. E) R/4.
Answer: A
33) A rectangular coil, with corners labeled ABCD, has length L and width w. It is placed between the poles of a magnet, as shown in the figure If there is a current I flowing through this coil in the direction shown, what is the direction of the force acting on section AB of this coil? A) perpendicular to and into the page B) perpendicular to and out of the page C) in the direction of the magnetic field D) in the opposite direction of the magnetic field E) The force is zero.
Answer: A
36) A rigid rectangular loop, measuring 0.30 m by 0.40 m, carries a current of 5.5 A, as shown in the figure. A uniform external magnetic field of magnitude 2.9 T in the -x direction is present. Segment CD is in the xz-plane and forms a 35° angle with the z-axis, as shown. What is the magnitude of the torque that the magnetic field exerts on the loop? A) 1.1 N ∙ m B) 0.73 N ∙ m C) 1.3 N ∙ m D) 1.4 N ∙ m E) 1.6 N ∙ m
Answer: A
40) A flat circular coil of wire having 200 turns and diameter 6.0 cm carries a current of 7.0 A. It is placed in a magnetic field of with the plane of the coil making an angle of 30° with the magnetic field. What is the magnitude of the magnetic torque on the coil? A) 2.1 N ∙ m B) 1.2 N ∙ m C) 3.9 N ∙ m D) 5.4 N ∙ m E) 1.0 N ∙ m
Answer: A
45) A flat circular loop of wire of radius 0.50 m that is carrying a 2.0-A current is in a uniform magnetic field of 0.30 T. What is the magnitude of the magnetic torque on the loop when the plane of its area is perpendicular to the magnetic field? A) 0.00 N ∙ m B) 0.41 N ∙ m C) 0.47 N ∙ m D) 0.52 N ∙ m E) 0.58 N ∙ m
Answer: A
48) A ring with a clockwise current (as viewed from above the ring) is situated with its center directly above another ring, which has a counter-clockwise current, as shown in the figure. In what direction is the net magnetic force exerted on the top ring due to the bottom ring? A) upward B) downward C) to the left D) to the right E) The net force is zero.
Answer: A
49) An object is hung using a metal spring. If now a current is passed through the spring, what will happen to this system? A) The spring will contract, raising the weight. B) The spring will extend, lowering the weight. C) The weight will not move. D) The spring will begin to swing like a pendulum. E) None of these are true.
Answer: A
50) Which of the following are units for the magnetic moment? (There could be more than one correct choice.) A) B) A ∙ m2 C) N ∙ m/T D) T/m2 Answer: B, C Var: 1 51) The maximum torque on a flat current-carrying loop occurs when the angle between the plane of the loop's area and the magnetic field vector is A) 0°. B) 45°. C) 90°. D) 135°.
Answer: A
55) A long, straight, horizontal wire carries current toward the east. A proton moves toward the east alongside and just south of the wire. What is the direction of the magnetic force on the proton? A) toward the north B) toward the south C) upward D) downward E) toward the east.
Answer: A
62) The magnetic field at a distance of 2 cm from a long straight current-carrying wire is 4 μT. What is the magnetic field at a distance of 4 cm from this wire? A) 2 μT B) 4 μT C) 6 μT D) 8 μT E) 10 μT
Answer: A
68) An ideal solenoid of length 10 cm consists of a wire wrapped tightly around a wooden core. The magnetic field strength is inside the solenoid. If the solenoid is stretched to by applying a force to it, what does the magnetic field become? A) 1.6 T B) 10.0 T C) 20 T D) 4.0 T
Answer: A
73) Consider an ideal solenoid of length L, N windings, and radius b (L is much longer than b). A current I is flowing through the wire windings. If the radius of the solenoid is doubled to 2b, but all the other quantities remain the same, the magnetic field inside the solenoid will A) remain the same. B) become twice as strong as initially. C) become one-half as strong as initially. D) become four times as strong as initially. E) become one-fourth as strong as initially.
Answer: A
75) When you double the number of windings in an ideal solenoid while keeping all other parameters (radius, length and current) fixed, the magnetic field at the center of the solenoid will A) double. B) triple. C) quadruple. D) be reduced by a factor of one-half. E) be reduced by a factor of one-fourth.
Answer: A
9) An electron, moving west, enters a magnetic field. Because of this field the electron curves upward. We may conclude that the magnetic field must have a component A) towards the north. B) towards the south. C) upward. D) downward. E) towards the west.
Answer: A
10) A proton having a speed of 3.0 × 106 m/s in a direction perpendicular to a uniform magnetic field moves in a circle of radius 0.20 m within the field. What is the magnitude of the magnetic field? , A) 0.080 T B) 0.16 T C) 0.24 T D) 0.32 T E) 0.36 T
Answer: B
16) A charged particle moving along the +x-axis enters a uniform magnetic field pointing along the +z-axis. A uniform electric field is also present. Due to the combined effect of both fields, the particle does not change its velocity. What is the direction of the electric field? A) along the -y-axis B) along the +y-axis C) along the -x-axis D) along the +x-axis E) along the -z-axis
Answer: B
21) What is the force per meter on a straight wire carrying 5.0 A when it is placed in a magnetic field of 0.020 T so that the wire makes an angle of 27° with respect to the magnetic field lines. A) 0.022 N/m B) 0.045 N/m C) 0.17 N/m D) 0.26 N/m
Answer: B
25) A straight wire is carrying a current of 2.0 A. It is placed at an angle of 60° with respect to a magnetic field of strength 0.20 T. If the wire experiences a force of 0.40 N, what is the length of the wire? A) 1.0 m B) 1.2 m C) 1.4 m D) 1.6 m E) 1.8 m
Answer: B
26) A proton has an initial velocity to the south but is observed to curve upward as the result of a magnetic field. This magnetic field must have a component A) to the west. B) to the east. C) upward. D) downward. E) to the north.
Answer: B
30) A wire in the shape of an "M" lies in the plane of the paper. It carries a current of 2.0 A, flowing from A to E, as shown in the figure. It is placed in a uniform magnetic field of 0.85 T in the same plane, directed as shown on the right side of the figure. The figure indicates the dimensions of the wire. Note that AB is parallel to DE and to the baseline from which the magnetic field direction is measured. What are the magnitude and direction of the force acting on section DE of this wire? A) 0.30 N perpendicular out of the page B) 0.12 N perpendicular into the page C) 0.30 N perpendicular into the page D) 0.12 N perpendicular out of the page E) 0.20 N perpendicular out of the page
Answer: B
34) A rectangular coil, with corners labeled ABCD, has length L and width w. It is placed between the poles of a magnet, as shown in the figure. If there is a current I flowing through this coil in the direction shown, what is the direction of the force acting on section CD of this coil? A) perpendicular to and into the page B) perpendicular to and out of the page C) in the direction of the magnetic field D) in the opposite direction of the magnetic field E) The force is zero.
Answer: B
35) What is the magnetic moment of a rectangular loop having 120 turns that carries 6.0 A if its dimensions are 4.0 cm × 8.0 cm? A) 0.23 A ∙ m2 B) 2.3 A ∙ m2 C) 23 A ∙ m2 D) 230 A ∙ m2
Answer: B
37) When the switch is closed in the circuit shown in the figure, the wire between the poles of the horseshoe magnet deflects upward. From this you can conclude that the left end of the magnet is A) a north magnetic pole. B) a south magnetic pole. C) There is not enough information given to answer the question.
Answer: B
41) A flat circular coil has 200 identical loops of very thin wire. Each loop has an area of 0.12 m2 and carries 0.50 A of current. This coil is placed in a magnetic field of 0.050 T oriented at 30° to the plane of the loop. What is the magnitude of the magnetic torque on the coil? A) 0.25 N ∙ m B) 0.52 N ∙ m C) 2.5 N ∙ m D) 5.2 N ∙ m
Answer: B
46) Two long, parallel wires carry currents of different magnitudes. If the current in one of the wires is doubled and the current in the other wire is halved, what happens to the magnitude of the magnetic force that each wire exerts on the other? A) It is doubled. B) It stays the same. C) It is tripled. D) It is quadrupled. E) It is reduced by a factor of two.
Answer: B
53) Two long parallel wires placed side-by-side on a horizontal table carry identical size currents in opposite directions. The wire on your right carries current directly toward you, and the wire on your left carries current directly away from you. From your point of view, the magnetic field at a point exactly midway between the two wires A) points upward. B) points downward. C) points toward you. D) points away from you. E) is zero.
Answer: B
56) A long, straight, horizontal wire carries current toward the east. An electron moves toward the east alongside and just south of the wire. What is the direction of the magnetic force on the electron? A) toward the north B) toward the south C) upward D) downward E) toward the west.
Answer: B
6) An electron moving along the +x-axis enters a magnetic field. If the electron experiences a magnetic deflection in the -y direction, then the magnetic field must have a component A) along the +z-axis. B) along the -z-axis. C) along the -x-axis. D) along the +y-axis. E) along the -y-axis.
Answer: B
64) A long straight wire carrying a 4-A current is placed along the x-axis as shown in the figure. What is the direction of the magnetic field at a point P due to this wire? A) into the plane of the page B) out of the plane of the page C) along the -x-axis D) along the +x-axis E) along the +y-axis
Answer: B
65) Which one of the following statements is correct? A) When a current-carrying wire is in your right hand, with your thumb in the direction of the current, your fingers point opposite to the direction of the magnetic field lines. B) When a current-carrying wire is in your right hand, with your thumb in the direction of the current, your fingers point in the direction of the magnetic field lines. C) When a current-carrying wire is in your left hand, with your thumb in the direction of the current, your fingers point in the direction of the magnetic field lines.
Answer: B
69) A single current-carrying circular loop of radius R is placed next to a long, straight wire, as shown in the figure. The current I in the wire flows to the right. In which direction must current flow in the loop to produce a net magnetic field of zero at its center? A) counterclockwise B) clockwise C) It could be either clockwise or counterclockwise. D) The current in the loop should be zero.
Answer: B
2) A straight bar magnet is initially 4 cm long, with the north pole on the right and the south pole on the left. If you cut the magnet in half, the right half will A) contain only a north pole. B) contain a north pole on the right and a south pole on the left. C) contain only a south pole. D) no longer contain any poles.
Answer: B Var: 1
10) A proton, moving west, enters a magnetic field. Because of this magnetic field the proton curves upward. We may conclude that the magnetic field must have a component A) towards the west. B) towards the east. C) towards the south. D) towards the north. E) downward.
Answer: C
15) A charged particle moving along the +x-axis enters a uniform magnetic field pointing along the +z-axis. Because of an electric field along the +y-axis, the charge particle does not change velocity. What is the sign of this particle? A) positive B) negative C) The particle could be either positive or negative. D) None of the above choices is correct.
Answer: C
19) If a calculated quantity has units of T ∙ m/A, that quantity could be A) an electric field. B) an electric potential. C) μ0. D) a magnetic field. E) a magnetic torque.
Answer: C
21) A charged particle moves with a constant speed through a region where a uniform magnetic field is present. If the magnetic field points straight upward, the magnetic force acting on this particle will be strongest when the particle moves A) straight upward. B) straight downward. C) in a plane parallel to Earth's surface. D) upward at an angle of 45° above the horizontal.
Answer: C
22) A negatively-charged particle moves across a constant uniform magnetic field that is perpendicular to the velocity of the particle. The magnetic force on this particle A) causes the particle to speed up. B) causes the particle to slow down. C) causes the particle to accelerate. D) is in the same direction as the particle's velocity. E) is opposite the direction of the particle's velocity.
Answer: C
22) A thin copper rod 1.0 m long has a mass of 0.050 kg and is in a magnetic field of 0.10 T. What minimum current in the rod is needed in order for the magnetic force to balance the weight of the rod? A) 1.2 A B) 2.5 A C) 4.9 A D) 9.8 A
Answer: C
23) At a particular instant, a proton moves toward the east in a uniform magnetic field that is directed straight downward. The magnetic force that acts on it is A) zero. B) upward. C) toward the north. D) toward the south. E) downward.
Answer: C
24) A straight wire that is 0.60 m and carrying a current of 2.0 A is placed at an angle with respect to the magnetic field of strength 0.30 T. If the wire experiences a force of magnitude 0.18 N, what angle does the wire make with respect to the magnetic field? A) 20° B) 25° C) 30° D) 35° E) 60°
Answer: C
26) A straight 1.0-m long wire is carrying a current. The wire is placed perpendicular to a magnetic field of strength 0.20 T. If the wire experiences a force of 0.60 N, what is the current in the wire? A) 2.0 A B) 1.0 A C) 3.0 A D) 4.0 A E) 5.0 A
Answer: C
27) A wire in the shape of an "M" lies in the plane of the paper. It carries a current of 2.0 A, flowing from A to E, as shown in the figure. It is placed in a uniform magnetic field of 0.75 T in the same plane, directed as shown on the right side of the figure. The figure indicates the dimensions of the wire. Note that AB is parallel to DE and to the baseline from which the magnetic field direction is measured. What are the magnitude and direction of the force acting on section AB of this wire? A) 0.20 N perpendicular out of the page B) 0.40 N perpendicular out of the page C) 0.11 N perpendicular out of the page D) 0.20 N perpendicular into the page E) 0.11 N perpendicular into the page
Answer: C
27) After landing on an unexplored Klingon planet, Spock tests for the direction of the magnetic field by firing a beam of electrons in various directions and by recording the following observations: Electrons moving upward feel a magnetic force in the northwest direction. Electrons moving horizontally toward the north are pushed downward. Electrons moving horizontally toward the southeast are pushed upward. Mr. Spock therefore concludes that the magnetic field at this landing site is in which direction? A) toward the east B) toward the northeast C) toward the southwest D) toward the southeast E) toward the west
Answer: C
29) A wire in the shape of an "M" lies in the plane of the paper. It carries a current of 2.0 A, flowing from A to E, as shown in the figure. It is placed in a uniform magnetic field of 0.55 T in the same plane, directed as shown on the right side of the figure. The figure indicates the dimensions of the wire. Note that AB is parallel to DE and to the baseline from which the magnetic field direction is measured. What are the magnitude and direction of the force acting on section CD of this wire? A) 0.066 N perpendicular into the page B) 0.40 N perpendicular out of the page C) 0.066 N perpendicular out of the page D) 0.40 N perpendicular into the page E) 0.20 N perpendicular out of the page
Answer: C
3) Which one of the following statements is correct? A) Earth's geographic north pole is the north pole of Earth's magnetic field. B) Earth's geographic south pole is the south pole of Earth's magnetic field. C) The north pole of a magnet points towards Earth's geographic north pole. D) The north pole of a magnet points towards Earth's geographic south pole. E) None of the above statements is correct.
Answer: C
31) A proton, moving in a uniform magnetic field, moves in a circle perpendicular to the field lines and takes time T for each circle. If the proton's speed tripled, what would now be its time to go around each circle? A) 9T B) 3T C) T D) T/3 E) T/9
Answer: C
36) A wire is carrying current vertically downward. What is the direction of the force on this wire due to Earth's magnetic field? A) horizontally towards the north B) horizontally towards the south C) horizontally towards the east D) horizontally towards the west E) vertically upward
Answer: C
39) The direction of the force on a current-carrying wire in a magnetic field is A) perpendicular only to the current. B) perpendicular only to the magnetic field. C) perpendicular to both the current and the magnetic field. D) in the direction opposite to the current. E) in same direction as the current.
Answer: C
42) A flat circular loop carrying a current of 2.0 A is in a magnetic field of 3.5 T. The loop has an area of 0.12 m2 and its plane is oriented at a 37° angle to the field. What is the magnitude of the magnetic torque on the loop? A) 0.10 N ∙ m B) 0.51 N ∙ m C) 0.67 N ∙ m D) 46 N ∙ m
Answer: C
42) Two long parallel wires are placed side-by-side on a horizontal table. If the wires carry current in the same direction, A) one wire is lifted slightly while the other wire is forced downward against the table's surface. B) both wires are lifted slightly. C) the wires pull toward each other. D) the wires push away from each other.
Answer: C
43) A flat circular loop of wire is in a uniform magnetic field of 0.30 T. The diameter of the loop is 1.0 m, and a 2.0-A current flows in it. What is the magnitude of the magnetic torque on the loop when the plane of the loop is parallel to the magnetic field? A) 0.00 N ∙ m B) 0.41 N ∙ m C) 0.47 N ∙ m D) 0.52 N ∙ m
Answer: C
44) When two long parallel wires carry unequal currents, the magnitude of the magnetic force that one wire exerts on the other is F. If the current in both wires is now doubled, what is the magnitude of the new magnetic force on each wire? A) 16F B) 8F C) 4F D) 2F E) F
Answer: C
45) Two long parallel wires are placed side-by-side on a horizontal table and carry current in the same direction. The current in one wire is 20 A, and the current in the other wire is 5 A. If the magnetic force on the 20-A wire has magnitude F, what is the magnitude of the magnetic force on the 5-A wire? No external magnetic fields are present. A) 4F B) 2F C) F D) F/2 E) F/4
Answer: C
46) A flat circular wire loop of area 0.25 m2 carries a current of 5.0 A. This coil lies on a horizontal table with the current flowing in the counterclockwise direction when viewed from above. At this point, the earth's magnetic field is 1.2 × 10-5 T directed 60° below the horizontal. What is the magnitude of the torque that the earth's magnetic field exerts on this loop? A) 2.5 × 10-6 N ∙ m B) 5.0 × 10-6 N ∙ m C) 7.5 × 10-6 N ∙ m D) 1.0 × 10-5 N ∙ m
Answer: C
5) A charged particle that is moving in a static uniform magnetic field A) will always experience a magnetic force, regardless of its direction of motion. B) may experience a magnetic force which will cause its speed to change. C) may experience a magnetic force, but its speed will not change. D) may experience a magnetic force, but its direction of motion will not change. E) None of the above statements are true.
Answer: C
52) A flat circular wire loop lies in a horizontal plane on a table and carries current in a counterclockwise direction when viewed from above. At this point, the earth's magnetic field points to the north and dips below the horizontal. Which side of the coil tends to lift off of the table due to the magnetic torque on the loop? A) the north side B) the east side C) the south side D) the west side E) The entire loop lifts straight up.
Answer: C
61) A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to a distance from the wire of A) 16d. B) 8d. C) 4d. D) 2d. E) d .
Answer: C
66) A vertical wire carries a current vertically downward. To the east of this wire, the magnetic field points A) north. B) east. C) south. D) down.
Answer: C
71) A current-carrying loop of wire lies flat on a horizontal tabletop. When viewed from above, the current moves around the loop in a counterclockwise sense. For points on the tabletop inside the loop, the magnetic field lines caused by this current A) circle the loop in a clockwise direction. B) circle the loop in a counterclockwise direction. C) point straight up. D) point straight down.
Answer: C
74) Consider an ideal solenoid of length L, N windings, and radius b (L is much longer than b). A current I is flowing through the wire windings. If the length of the solenoid becomes twice as long (to 2L), but all other quantities remained the same, the magnetic field inside the solenoid will A) remain the same. B) become twice as strong as initially. C) become one-half as strong as initially. D) become four times as strong as initially. E) become one-fourth as strong as initially.
Answer: C
76) When a ferromagnetic material is placed in an external magnetic field, the net magnetic field of its magnetic domains becomes A) smaller. B) zero. C) larger.
Answer: C
8) A proton, moving north, enters a magnetic field. Because of this field, the proton curves downward. We may conclude that the magnetic field must have a component A) downward. B) upward. C) towards the east. D) towards the west. E) towards the north.
Answer: C
1) If you were to cut a small permanent bar magnet in half, A) one piece would be a magnetic north pole and the other piece would be a south pole. B) neither piece would be magnetic. C) each piece would in itself be a smaller bar magnet with both north and south poles. D) None of these statements is true.
Answer: C Var: 1
18) If a calculated quantity has units of , that quantity could be A) an electric field. B) an electric potential. C) μ0. D) a magnetic field. E) a magnetic torque.
Answer: D
20) We observe that a moving charged particle experiences no magnetic force. From this we can definitely conclude that A) no magnetic field exists in that region of space. B) the particle must be moving parallel to the magnetic field. C) the particle is moving at right angles to the magnetic field. D) either no magnetic field exists or the particle is moving parallel to the field. E) either no magnetic field exists or the particle is moving perpendicular to the field.
Answer: D
24) At a particular instant, an electron moves toward the east in a uniform magnetic field that is directed straight downward. The magnetic force that acts on it is A) zero. B) upward. C) toward the north. D) toward the south. E) downward.
Answer: D
28) A charged particle is injected into a uniform magnetic field such that its velocity vector is perpendicular to the magnetic field lines. Ignoring the particle's weight, the particle will A) move in a straight line. B) follow a spiral path. C) move along a parabolic path. D) follow a circular path.
Answer: D
68) A long straight wire has a constant current flowing to the right. A rectangular metal loop is situated above the wire, and also has a constant current flowing through it, as shown in the figure. Which one of the following statements is true? A) The net magnetic force on the rectangle is upward, and there is also a nonzero torque on the rectangle. B) The net magnetic force on the rectangle is zero, and the net torque on it is zero. C) The net magnetic force on the rectangle is downward, and there is also a nonzero torque on the rectangle. D) The net magnetic force on the rectangle is zero, but there is a nonzero torque on the rectangle. E) The net magnetic force on the rectangle is downward, and the net torque on it is zero.
Answer: E