Q&A Hero

Q: A long straight cylindrical shell has an inner radius Riand an outer radius Ro. It carries a current i, uniformly distributed over its cross section. A wire is parallel to the cylinder axis, in the hollow region (r<Ri). The magnetic field is zero everywhere outside the shell (r>Ro). We conclude that the wire: A) is on the cylinder axis and carries current iin the same direction as the current in the shell B) may be anywhere in the hollow region but must be carrying current iin the direction opposite to that of the current in the shell C) may be anywhere in the hollow region but must be carrying current iin the same direction as the current in the shell D) is on the cylinder axis and carries current iin the direction opposite to that of the current in the shell E) does not carry any current

Q: A long straight cylindrical shell carries current iuniformly distributed over its cross section. The magnitude of the magnetic field is greatest: A) at the inner surface of the shell B) at the outer surface of the shell C) inside the shell near the middle D) in hollow region near the inner surface E) near the center of the hollow region

Q: A hollow cylindrical conductor (inner radius = a, outer radius = b) carries a current iuniformly spread over its cross section. Which graph below correctly gives Bas a function of the distance rfrom the center of the cylinder? A) I B) II C) III D) IV E) V

Q: The magnetic field at any point in the xyplane is given by , where is the position vector of the point,Ais a constant, and is a unit vector in the +zdirection. The net current through a circle of radius R, in the xyplane and centered at the origin is given by:

Q: If the magnetic field is uniform over the area bounded by a circle with a radius R, the net current through the circle is:A) 0

Q: Two long straight wires enter a room through a window. One carries a current of 3.0 A into the room while the other carries a current of 5.0 A out. The magnitude of the path integral around the window frame is:A) 2.5 x10-6 T.mB) 3.8 x10-6T.mC) 6.3 x10-6 T.mD) 1.0 x10-5 T.mE) none of these

Q: In the figure, there are three wires carrying currents in the indicated directions, and an Amperian loop. The sign of the contribution of the current in each wire to the path integral in Ampere's Law is:A) i1+, i2-, i30B) i1-, i2+, i30C) i1+, i2+, i3-D)i1-, i2-, i3+E)i1+, i2-, i3-

Q: A long straight wire carrying a 3.0 A current enters a room through a window 1.5 m high and 1.0 m wide. The path integral around the window frame has the value:A) 0.20 T.mB) 2.5 x10-7T.mC) 3.0 x10-7T.mD) 3.8 x10-6T.mE) none of these

Q: In Ampere's law, the direction of the integration around the path: A) must be clockwise B) must be counterclockwise C) must be such as to follow the magnetic field lines D) must be along the wire in the direction of the current E) none of the above

Q: In Ampere's law, the symbol is: A) an infinitesimal piece of the wire that carries current i B) in the direction of C) perpendicular to D) a vector whose magnitude is the length of the wire that carries current i E) none of the above

Q: In Ampere's law, , the integration must be over any: A) surface B) closed surface C) path D) closed path E) closed path that surrounds all the current producing

Q: Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is: A) north B) east C) south D) west E) zero

Q: A constant current is sent through a helical coil made of flexible wire. The coil: A) tends to get shorter B) tends to get longer C) tends to rotate about its axis D) produces zero magnetic field at its center E) none of the above

Q: Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in opposite directions. The force per unit length of one wire on the other is:A) 1x10-3N/m, repulsiveB) 1 x10-3N/m, attractiveC) 4 x10-5N/m, repulsiveD) 4 x10-5N/m, attractiveE) none of these

Q: Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in the same direction. The force per unit length of one wire on the other is:A) 1 x10-3N/m, repulsiveB) 1 x10-3N/m, attractiveC) 4 x10-5N/m, repulsiveD) 4 x10-5N/m, attractiveE) none of these

Q: Two parallel long wires carry the same current and repel each other with a force Fper unit length. If both these currents are doubled and the wire separation tripled, the force per unit length becomes: A) 2F/9 B) 4F/9 C) 2F/3 D) 4F/3 E) 6F

Q: Two parallel wires carrying equal currents of 10 A attract each other with a force of 1 mN. If both currents are doubled, the force of attraction will be: A) 0.25mN B) 0.5mN C) 1mN D) 2mN E) 4mN

Q: The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank the wires according to the magnitudes of the magnetic forces on them, from least to greatest. A) 1, 2, 3 B) 2, then 1 and 3 tie C) 2 and 3 tie, then 1 D) 1 and 3 tie, then 2 E) 3, 2, 1

Q: A coulomb is:A) one ampere per secondB) the quantity of charge which will exert a force of 1 N on a similar charge at a distance of 1 mC) the amount of current in each of two long parallel wires separated by 1 m, which produces a force of 2 x10-7N per meterD) the amount of charge which flows past a point in one second when the current is 1 AE) an abbreviation for a certain combination of kilogram, meter and second

Q: The diagrams show three circuits consisting of concentric circular arcs (either half or quarter circles of radii r, 2r, and 3r) and radial lengths. The circuits carry the same current. Rank them according to the magnitudes of the magnetic fields they produce at C, least to greatest. A) 1, 2, 3 B) 3, 2, 1 C) 1, 3, 2 D) 2, 3, 1 E) 2, 1, 3

Q: The magnitude of the magnetic field at point P, at the center of the semicircle shown, is given by:

Q: A semi-infinite wire runs along the positive yaxis, beginning at the origin (x= y= 0) and extending infinitely far in the +ydirection. If the current in the wire is 12 A, what magnetic field does it create on the xaxis at the point x= 3.5 cm? A) 0 T B) 3.4 x 10-5T C) 6.9 x 10-5T D) 1.4 x 10-4T E) 4.3 x 10-4T

Q: Lines of the magnetic field produced by a long straight wire carrying a current are: A) in the direction of the current B) opposite to the direction of the current C) leave the wire radially D) circles concentric with the wire E) lines similar to those produced by a bar magnet

Q: An ordinary magnetic compass is placed flat on the ground. A wire carrying a large current ifrom east to west is placed directly above the compass.The end of the compass needle marked "N" will point: A) north B) south C) east D) west E) the compass will act as an electric motor, hence the needle will keep rotating

Q: In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is: A) east B) up C) north D) down E) west

Q: Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is: A) zero B) non-zero and along a line connecting the wires C) non-zero and parallel to the wires D) non-zero and perpendicular to the plane of the two wires E) none of the above

Q: Two long straight wires pierce the plane of the paper at vertices of an equilateral triangle as shown below. They each carry 2.0 A, out of the paper. The magnetic field at the third vertex (P) has magnitude:A) 5.0 x10-6TB) 8.7 x10-6TC) 1.0 x10-5TD) 1.7 x10-5TE) 2.0 x10-5T

Q: Two long straight current-carrying parallel wires cross the xaxis and carry currents I and 3I in the same direction, as shown. At what value of xis the net magnetic field zero? A) 0 B) 1 C) 3 D) 5 E) 7

Q: Two long straight wires are parallel and carry current in opposite directions. The currents are 8.0 A and 12 A and the wires are separated by 0.40 cm. The magnetic field at a point midway between the wires is:A) 0 TB) 4.0 x10-4TC) 8.0 x10-4TD) 12 x10-4TE) 20 x10-4T

Q: Two long straight wires are parallel and carry current in the same direction. The currents are 8.0 and 12 A and the wires are separated by 0.40 cm. The magnetic field at a point midway between the wires is:A) 0 TB) 4.0 x10-4TC) 8.0 x10-4TD) 12 x10-4TE) 20 x10-4T

Q: The magnetic field a distance 2 cm from a long straight current-carrying wire is 2 x10-5T. The current in the wire is:A) 0.16 AB) 1.0 AC) 2.0 AD) 4.0 AE) 25 A

Q: Which graph correctly gives the magnitude of the magnetic field outside an infinitely long, very thin, straight current-carrying wire as a function of the distance rfrom the wire?A) IB) IIC) IIID) IVE) V

Q: The magnetic field outside a long straight current-carrying wire depends on the distance Rfrom the wire axis according to: A) R B) 1/R C) 1/R2 D) 1/R3 E) 1/R3/2

Q: In the figure, if the current element has a length of 1.0 mm, carries a current of 2.5 A, and is a distance of 4.8 cm from the point P, what is the magnitude of the magnetic field at point P?A) 0 TB) 2.6x 10-9TC) 5.4 x 10-8TD) 9.4 x 10-8TE) 1.1 x 10-7T

Q: In the figure, the current element , the point P, and the three vectors (1, 2, 3) are all in the plane of the page. The direction of , due to this current element, at the point P is: A) in the direction marked "1" B) in the direction marked "2" C) in the direction marked "3" D) out of the page E) into the page

Q: Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is: A) into the page B) out of the page C) to the left D) to the right E) zero

Q: Suitable units for are:A) teslaB) newton/ampere2C) weber/meterD) kilogram.ampere/meterE) tesla.meter/ampere

Q: A loop of current-carrying wire has a magnetic dipole moment of 5.0x10-4A.m2. The moment initially is aligned with a 0.50-T magnetic field.To rotate the loop so its dipole moment is perpendicular to the field and hold it in that orientation, you must do work of:A) 0 JB) 2.5 x10-4JC) -2.5 x10-4JD) 1.0 x10-3JE) -1.0 x10-3J

Q: The diagrams show five possible orientations of a magnetic dipole in a uniform magnetic field . For which of these is the potential energy the greatest? A) I B) II C) III D) IV E) V

Q: For a loop of current-carrying wire in a uniform magnetic field the potential energy is a minimum if the magnetic dipole moment of the loop is: A) in the same direction as the field B) in the direction opposite to that of the field C) perpendicular to the field D) at an angle of 45ï‚°to the field E) none of the above

Q: A loop of current-carrying wire has a magnetic dipole moment of 5.0 x10-4A.m2. If the dipole moment makes an angle of 57 with a magnetic field of 0.35 T, what is its potential energy?A) -9.5 x 10-5JB) -1.5 x 10-4JC) -1.8 x 10-4JD) +1.5 x 10-4JE) +9.5 x 10-5J

Q: The diagrams show five possible orientations of a magnetic dipole in a uniform magnetic field . For which of these does the magnetic torque on the dipole have the greatest magnitude? A) I B) II C) III D) IV E) V

Q: A coil of 1000 turns of wire has a radius of 12 cm and carries a counterclockwise current of 15A. If it is lying flat on the ground, and the Earth's magnetic field points due north,has a magnitude of 5.8 x 10-5T, and makes a downward angle of 25 with the vertical, what is the torque on the loop? A) 1.7 x 10-2Nm west B) 3.6 x 10-2Nm west C) 1.7 x 10-2Nm east D) 3.6 x 10-2Nm east E) 3.6 x 10-2Nm south

Q: The magnetic dipole moment of a current-carrying loop of wire is in the positive zdirection. If a uniform magnetic field is in the positive xdirection the magnetic torque on the loop is: A) zero B) in the positive ydirection C) in the negative ydirection D) in the positive zdirection E) in the negative zdirection

Q: A circular loop of wire with a radius of 20 cm lies in the xyplane and carries a current of 2 A, counterclockwise when viewed from a point on the positive zaxis. Its magnetic dipole moment is:A) 0.25 A.m2, in the positive zdirectionB) 0.25 A.m2, in the negative zdirectionC) 2.5 A.m2, in the positive zdirectionD) 2.5 A.m2, in the negative zdirectionE) 0.25 A.m2, in the xyplane

Q: You are facing a loop of wire which carries a clockwise current of 3.0 A and which surrounds an area of 5.8 x 10-2m2. The magnetic dipole moment of the loop is:A) 3.0 A.m2, into the pageB) 3.0 A.m2, out of the pageC) 0.17 A.m2, into the pageD) 0.17 A.m2, out of the pageE) 0.17 A.m2, left to right

Q: The units of magnetic dipole moment are:A) ampereB) ampere .meterC) ampere .meter2D) ampere/meterE) ampere/meter2

Q: The magnetic torque exerted on a flat current-carrying loop of wire by a uniform magnetic field is: A) maximum when the plane of the loop is perpendicular to B) maximum when the plane of the loop is parallel to C) dependent on the shape of the loop for a fixed loop area D) independent of the orientation of the loop E) such as to rotate the loop around the magnetic field lines

Q: A square loop of wire lies in the plane of the page and carries a current I as shown. There is a uniform magnetic field directed towards the top of the page, as indicated. The loop will tend to rotate: A) about PQ with KL coming out of the page B) about PQ with KL going into the page C) about RS with MK coming out of the page D) about RS with MK going into the page E) about an axis perpendicular to the page

Q: A current is clockwise around the outside edge of this page and a uniform magnetic field is directed parallel to the page, from left to right. If the magnetic force is the only force acting on the page, the page will rotate so the right edge: A) moves toward you B) moves away from you C) moves to your right D) moves to your left E) does not move

Q: A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is in the plane of the triangle and is perpendicular to the hypotenuse. The resultant magnetic force on the two equal sides has a magnitude of: A) 0 N B) 0.21 N C) 0.30 N D) 0.41 N E) 0.51 N

Q: A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is parallel to the hypotenuse. The total magnetic force on the two equal sides has a magnitude of: A) 0 N B) 0.21 N C) 0.30 N D) 0.41 N E) 0.51 N

Q: The figure shows a uniform magnetic field directed to the left and a wire carrying a current into the page. The magnetic force acting on the wire is: A) toward the top of the page B) toward the bottom of the page C) toward the left D) toward the right E) zero

Q: The figure shows the motion of electrons in a wire which is near the N pole of a magnet. The wire will be pushed: A) toward the magnet B) away from the magnet C) downward D) upward E) along its length

Q: The diagram shows a straight wire carrying current iin a uniform magnetic field. The magnetic force on the wire is indicated by an arrow but the magnetic field is not shown. Of the following possibilities, the direction of the magnetic field is: A) to the right B) opposite the direction of C) in the direction of D) into the page E) out of the page

Q: The diagram shows a straight wire carrying a flow of electrons into the page. The wire is between the poles of a permanent magnet. The direction of the magnetic force exerted on the wire is: A)  B)  C)  D)  E) into the page

Q: Which is NOT one of the differences between a cyclotron and a synchrotron? A) Orbits in a cyclotron are spirals, while in a synchrotron they are circles B) Conventional cyclotrons fail above energies of about 50 MeV because the proton speeds get too close to the speed of light, while synchrotrons are designed to accommodate all proton energies C) Large cyclotrons would require extremely large magnets, since they must cover all possible orbital radii, while synchrotrons only need a thin ring D) In general, synchrotrons are much smaller than cyclotrons E) Both cyclotrons and synchrotrons require electrical oscillators to accelerate the protons

Q: A cyclotron operates with a given magnetic field and at a given frequency. If Rdenotes the radius of the final orbit, the final particle energy is proportional to: A) 1/R B) R C) R2 D) R3 E) R4

Q: The resonance condition in a cyclotron states that: A) the time it takes the protons to make one cycle equals the natural frequency of the proton B) the protons oscillate on a vertical axis once per cycle C) the proton spin changes direction once per cycle D) the frequency of the proton orbits equals the frequency of the electrical oscillator E) the frequency of the proton orbits is an integer multiple of 60 Hz

Q: An electron is launched with velocity in a uniform magnetic field . The angle between and is between 0 and 90o. As a result, the electron follows a helical path. The pitch of the helix is:A) the angle the helix makes with the magnetic fieldB) the angle the helix makes with the electron's velocity vectorC) the radius of the circular motionD) the distance between adjacent turns of the helixE) the time it takes the electron to move from one turn of the helix to the next

Q: An electron is launched with velocity in a uniform magnetic field . The angle between and is between 0 and 90o. As a result, the electron follows a helix, its velocity vector returning to its initial value in a time interval of:

Q: An electron and a proton both each travel with equal speeds around circular orbits in the same uniform magnetic field, as shown in the diagram (not to scale). The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged: A) the electron travels clockwise around the smaller circle and the proton travels counterclockwise around the larger circle. B) the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle C) the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle D) the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle E) the electron travels counterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle

Q: In a certain mass spectrograph, an ion beam passes through a velocity filter consisting of mutually perpendicular fields and . The beam then enters a region of another magnetic field perpendicular to the beam. The radius of curvature of the resulting ion beam is proportional to: A) EB'/B B) EB/B' C) BB'/E D) B/EB' E) E/BB'

Q: Electrons (mass m, charge "e) are accelerated from rest through a potential difference Vand are then deflected by a magnetic field that is perpendicular to their velocity. The radius of the resulting electron trajectory is: A) B) C) D) E) none of these

Q: A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius as shown. A reasonable explanation is: A) the charge is positive and slowing down B) the charge is negative and slowing down C) the charge is positive and speeding up D) the charge is negative and speeding up E) none of the above

Q: At one instant an electron is moving in the positive xdirection along the xaxis in a region where there is a uniform magnetic field in the positive zdirection. When viewed from a point on the positive zaxis, it subsequent motion is: A) straight ahead B) counterclockwise around a circle in the xyplane C) clockwise around a circle in the xyplane D) in the positive zdirection E) in the negative zdirection

Q: A strip 1.2 mm wide is moving at a speed of 25cm/s through a uniform magnetic field of 5.6 T. What is the maximum Hall voltage across the strip? A) 1.7 mV B) 8.5 mV C) 27 mV D) 1.2 V E) 17 V

Q: The Hall effect can be used to calculate the charge-carrier number density in a conductor. If a conductor carrying a current of 2.0 A is 0.5 mm thick, and the Hall effect voltage is 4.5 V when it is in a uniform magnetic field of 1.2 T, what is the density of charge carriers in the conductor? A) 1.0 x 1028/m3 B) 6.7 x 1027/m3 C) 4.6 x 1027/m3 D) 1.7 x 1027/m3 E) 1.2 x 1027/m3

Q: A conducting strip of width 1.5 mm is in a magnetic field. As a result, there is a potential difference of 4.3 mV across the width of the strip. What is the magnitude of the electric field in the strip? A) 0.35 V/m B) 1.2 V/m C) 1.9 V/m D) 2.9 V/m E) 6.4 V/m

Q: The current is from left to right in the conductor shown. The magnetic field is into the page and point S is at a higher potential than point T. The charge carriers are: A) positive B) negative C) neutral D) absent E) moving near the speed of light

Q: An ion with a charge of +3.2x10-19C is in region where a uniform electric field of 5 x104. V/m is perpendicular to a uniform magnetic field of 0.8 T. If its acceleration is zero then its speed must be:A) 0 m/sB) 1.6 x10-5m/sC) 4.0 x105m/sD) 6.3 x105m/sE) any value but 0 m/s

Q: An electron is travelling in the positive xdirection. A uniform electric field is in the negative ydirection. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magnetic field is: A) the positive ydirection B) the negative ydirection C) into the page D) out of the page E) the negative xdirection

Q: A uniform magnetic field is in the positive zdirection. A positively charged particle is moving in the positive xdirection through the field. The net force on the particle can be made zero by applying an electric field in what direction? A) Positive y B) Negative y C) Positive x D) Negative x E) Positive z

Q: An electron enters a region of uniform perpendicular and fields. It is observed that the velocity of the electron is unaffected. A possible explanation is: A) is parallel to and has magnitude E/B B) is parallel to C) is perpendicular to both and and has magnitude B/E D) is perpendicular to both and and has magnitude E/B E) the given situation is impossible

Q: A charged particle is projected into a region of uniform, parallel, and fields. The force on the particle is: A) zero B) at some angle < 90 with the field lines C) along the field lines D) perpendicular to the field lines E) unknown (need to know the sign of the charge)

Q: J. J. Thomson's experiment, involving the motion of an electron beam in mutually perpendicular and fields, gave the value of: A) the mass of an electron B) the charge of an electron C) the Earth's magnetic field D) the charge/mass ratio for an electron E) Avogadro's number

Q: At any point the magnetic field lines are in the direction of: A) the magnetic force on a moving positive charge B) the magnetic force on a moving negative charge C) the velocity of a moving positive charge D) the velocity of a moving negative charge E) none of the above

Q: A static magnetic field CANNOT: A) exert a force on a charge B) accelerate a charge C) change the momentum of a charge D) change the kinetic energy of a charge E) exist

Q: The magnetic force on a charged particle is in the direction of its velocity if: A) it is moving in the direction of the field B) it is moving opposite to the direction of the field C) it is moving perpendicular to the field D) it is moving in some other direction E) never

Q: In the formula : A) must be perpendicular to but not necessarily to B) must be perpendicular to but not necessarily to C) must be perpendicular to but not necessarily to D) all three vectors must be mutually perpendicular E) must be perpendicular to both and