# Physics Tutorial On Magnetic Fields

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question
Compasses line up with magnetic fields. A compass will line up a) Perpendicular to magnetic field lines, in a direction defined by the right-hand rule. b) Parallel to magnetic field lines, with the north pole pointing in the direction of the field. c) Perpendicular to magnetic field lines. d) Parallel to magnetic field lines, with the south pole pointing in the direction of the field.
b) Parallel to magnetic field lines, with the north pole pointing in the direction of the field.
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Magnetic field lines form closed loops a) Around and through bar magnets. b) Through current-carrying loops. c) Around current-carrying straight wires. d) All of the above.
d) All of the above.
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A magnet will exert a force on a) A piece of steel. b) A beam of electrons. c) A current-carrying wire. d) All of the above.
d) All of the above.
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A bar magnet is oriented perpendicular to a uniform magnetic field as shown in (Figure 1) . Describe the force and/or torque on the magnet. a) There is a net force on the magnet in the direction of the magnetic field (towards the bottom of screen). b) There is a net torque on the magnet in a clockwise direction. c) There is no net force or torque on the magnet. d) There is a net force on the magnet opposite to the direction of the magnetic field (towards the top of screen). e) There is a net torque on the magnet in a counterclockwise direction.
e) There is a net torque on the magnet in a counterclockwise direction.
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Two concentric loops of wire are carrying currents in opposite directions as shown in (Figure 2) . Describe the net force and the torque on either of the current loops. a) There is a net force on the loops that causes them to attract each other. b) There is no net force or torque on the loops. c) There is a net force on each loop that causes them to repel each other. d) There is a net torque on each loop that causes them to rotate in opposite directions. e) There is a net torque on each loop that causes them to rotate in the same direction
c) There is a net force on each loop that causes them to repel each other.
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The magnetic dipole moment (μ) of a coil depends on the current (I), the area of the coil (A), and the number of loops (N). Rank the three different coils of wire shown in (Figure 3) in order by the magnitude of their magnetic dipole moment. Rank the coils from greatest to least magnetic dipole moment. If the coils have the same magnetic dipole moment, place them on top of each other. Coil 1 = single coil with area 10cm2 and I=6A. Coil 2 = single coil with area 20cm2 and I=3A. Coil 3 = coil with 6 loops each with an area of 10cm2 and I=1A.
All have equal magnetic dipole moment and are stacked on top of each other
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The torque (τ) that is exerted on a magnetic dipole moment (μ) depends on the orientation of the magnetic dipole moment. Rank the three loops of wire carrying current shown in (Figure 4) in order by the magnitude of the torque exerted on them. Rank the magnitude of the torque exerted on the loops from greatest to least. If two loops have the same magnitude of torque, place one on top of the other. a) Normal to loop is parallel to magnetic field b) Normal to loop is perpendicular to magnetic field c) Normal to loop is at 45 degrees with respect to magnetic field
(Left) Greatest Torque: b) Normal to loop is perpendicular to magnetic field (Middle) Middle Torque: c) Normal to loop is at 45 degrees with respect to magnetic field (Right) Least Torque: a) Normal to loop is parallel to magnetic field
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The figure shows the path of a charged particle moving in magnetic field B directed into the screen. The particle moves clockwise along the circular path. What is the particle's charge? a) negative b) positive c) neutral
a) negative
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The figure shows a wire that is connected to a power supply and suspended between the poles of a magnet. When the switch is closed, the wire deflects in the direction shown. The figure shows a circular cross section of a wire placed between four dashed boxes, labeled from A to D. The wire is located at the same distance from all the boxes. Box A is to the left of the wire. Box B is above the wire. Box C is to the right of the wire. Pole D is below the wire. A current in the wire is directed out of the screen. An arrow pointing downward shows the direction, in which the wire deflects. Which of the dashed boxes A-D represents the position of the north magnetic pole? a) A b) B c) C d) D
c) C
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A 65-cm segment of conducting wire carries a current of 0.35 A. The wire is placed in a uniform magnetic field that has a magnitude of 1.24 T. What is the angle between the wire segment and the magnetic field if the force on the wire is 0.26 N? a) 37° b) 67° c) 43° d) 23° e) 53°
b) 67°
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Two long straight wires are parallel to each other and are separated by 78 mm. The current in wire 1 is 3.55 A and the current in wire 2 is 2.75 A. What is the force per unit length between the two wires? a) 1.9 × 10-5 N/m b) 2.5 × 10-5 N/m c) 3.2 × 10-5 N/m d) 2.5 × 10-8 N/m
b) 2.5 × 10-5 N/m
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A circular coil of conducting wire, with a radius of 4.48 cm and 25 turns, is in a 1.67-T magnetic field. When the coil's dipole moment vector makes an angle of 34° with the magnetic field, a 0.537-N•m torque is exerted on the coil. What is the current in the coil? a) 2.46 A b) 0.365 mA c) 2.04 A d) 3.65 A e) 81.8 mA
d) 3.65 A
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A uniform magnetic field points upward, parallel to the page, and has a magnitude of 7.85 mT. A negatively charged particle (q = -3.32 µC, m = 2.05 pg) moves through this field with a speed of 67.3 km/s at a 42° with respect to the magnetic field, parallel to the page as shown. What is the magnitude of the magnetic force on this particle? The figure shows a negative charge q moving in a uniform magnetic field B. The field B is directed vertically upwards. The charge q moves with the velocity v rightwards and upwards at an angle of 42 degrees from the vertical. a) 1.75 mN b) 1.30 mN c) 1.61 mN d) 1.17 mN
d) 1.17 mN
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A uniform magnetic field points upward, parallel to the page, and has a magnitude of 7.85 mT. A negatively charged particle (q = -3.32 µC, m = 2.05 pg) moves through this field with a speed of 67.3 km/s at a 42° with respect to the magnetic field, parallel to the page as shown. What is the direction of the magnetic force on this particle? The figure shows a negative charge q moving in a uniform magnetic field B. The field B is directed vertically upwards. The charge q moves with the velocity v rightwards and upwards at an angle of 42 degrees from the vertical. a) Downward, parallel to the page b) To the left, parallel to the page c) Out of the page d) To the right, parallel to the page e) Upward, parallel to the page f) Into the page
f) Into the page
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A uniform magnetic field points upward, parallel to the page, and has a magnitude of 7.85 mT. A negatively charged particle (q = -3.32 µC, m = 2.05 pg) moves through this field with a speed of 67.3 km/s perpendicular to the magnetic field, as shown. The magnetic force on this particle is a centripetal force and causes the particle to move in a circular path. What is the radius of the particle's circular path? The figure shows a negative charge q moving in a uniform magnetic field B. The field B is directed vertically upwards. The charge q moves with the velocity v horizontally rightwards. a) 5.29 mm b) 7.91 mm c) 0.118 mm d) 0.189 mm
a) 5.29 mm
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In this video, we see that a _____ magnetic field can create an electric current. a) Strong b) Changing c) Dipole d) Perpendicular
b) Changing
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Lenz's law states that an induced magnetic field in a conductor a) Opposes the change in flux through the conductor. b) Opposes the applied magnetic field. c) Induces a matching electric field that causes a current. d) Opposes the applied flux through the conductor.
a) Opposes the change in flux through the conductor.
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Considering light at the two ends of the visible light spectrum, violet light has a _____ wavelength and a _____ photon energy than red light. a) Shorter, lower b) Longer, lower c) Shorter, higher d) Longer, higher
c) Shorter, higher
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A conducting rod is being dragged along conducting rails, as shoawn in (Figure 1). The magnetic field is directed out of the screen. In what direction does the induced current flow through the light bulb? A conducting rod is being dragged along conducting rails, as shown in A uniform magnetic field is directed out of the screen. The rod is on the bottom portion of the circuit, moving upward along the vertical rails. The light bulb is on the upper leg of the circuit.. The magnetic field is directed out of the screen. In what direction does the induced current flow through the light bulb? a) There is no induced current. b) The induced current flows through the bulb from the left to the right. c) The induced current flows through the bulb from the right to the left.
c) The induced current flows through the bulb from the right to the left.
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A circular loop of conducting wire is moving through a uniform magnetic field, as shown in A circular loop is moving to the right within a uniform magnetic field that is directed into the screen.. Is a non-zero emf induced in the loop? a) Yes b) No
b) No
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A wire loop is rotating at a constant angular frequency within a uniform magnetic field B⃗ . The orientation of the magnetic field is shown at three moments in time below. [Hint: you may want to watch the video again starting at 05:55] Rank the orientations from lowest to highest magnitude of the instantaneous induced emf. a) 90 degree angle b) 45 degree angle c) 0 degree angle
Lowest (Left): a) 90 degree angle Middle (Middle): b) 45 degree angle Highest (Right): c) 0 degree angle
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Two loops of wire, each having a different radius, encircle an infinitely long solenoid, as shown in A magnetic field B is directed into the interior of the solenoid along its long axis. Loops 1 and 2 encircle the solenoid such that both loops are perpendicular to B. The radius of loop 2 is less than the radius of loop 1, and both radii are greater than the radius of the solenoid.. The magnetic field is zero outside the solenoid. The current through the solenoid is increasing with time, causing the magnetic field inside the solenoid to increase with time. Which statement is true? a) The emf around wire loop 2 is greater than that of loop 1. b) The emf around wire loops 1 and 2 is zero since there is no magnetic field outside the solenoid. c) The emf around the two wire loops is the same and is non-zero.
c) The emf around the two wire loops is the same and is non-zero.
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Electric rail cars often use magnetic braking. The brake consists of a set of electromagnets that are held just above the rails. To brake the train, the electromagnets are switched on, creating a magnetic field that induces eddy currents in the metal rails passing beneath them. In the figure, which of the choices correctly represents the eddy currents induced in the rails? The diagrams represent a view from above, looking down at the rail through the electromagnet. The electromagnet moves to the right, and the magnetic field points into the screen. The figure contains an illustration of a rail car moving to the right along the horizontal rails and four diagrams, labeled from A to D. In the illustration an electromagnet is held on the bottom side of the rail car just above the rails. Each diagram represents the view from above looking down at the rail through the electromagnet. The electromagnet moves to the right in each diagram. Magnetic field B points into the screen in each diagram. In diagram A eddy currents are flowing clockwise in areas beneath both left and right ends of the electromagnet. In diagram B eddy currents are flowing clockwise in the area beneath the left end of the electromagnet and counterclockwise in the area beneath the right end of the electromagnet. In diagram C eddy currents are flowing counterclockwise in areas beneath both left and right ends of the electromagnet. In diagram D eddy currents are flowing counterclockwise in the area beneath the left end of the electromagnet and clockwise in the area beneath the right end of the electromagnet. a) A b) B c) C d) D
b) B B shows the left rotation going clockwise and the right rotation going counter clockwise. The magnetic flux increases under the leading edge of the electromagnet and decreases under its trailing edge. Therefore, by Lenz's law, the induced magnetic field will point out of the screen under the leading edge and into it under the trailing edge. The eddy currents in choice B have the right directions to induce such magnetic fields, according to the right-hand rule.
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A 144-Ω light bulb is connected to a conducting wire that is wrapped into the shape of a square with side length of 83.0 cm. This square loop is rotated within a uniform magnetic field of 454 mT. What is the change in magnetic flux through the loop when it rotates from a position where its area vector makes an angle of 30° with the field to a position where the area vector is parallel to the field? a) 41.9 mWb b) 313 mWb c) 50.5 mWb d) 271 mWb
a) 41.9 mWb
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A 144-Ω light bulb is connected to a conducting wire that is wrapped into the shape of a square with side length of 83.0 cm. This square loop is rotated within a uniform magnetic field of 454 mT. The loop rotates from a position where its area vector makes an angle of 30° with the field to a position where the area vector is parallel to the field in 56.3 ms. What is the induced current through the light bulb? a) 744 mA b) 107 A c) 5.17 mA d) 16.4 µA
c) 5.17 mA
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A 144-Ω light bulb is connected to a conducting wire that is wrapped into the shape of a square with side length of 83.0 cm. This square loop is rotated with a frequency of 60 Hz within a uniform magnetic field of 454 mT. This means the loop makes half a revolution in 8.33 ms. What is the induced current in the light bulb when the loop rotates from a position where its area vector is opposite the magnetic field to a position where its area vector is parallel to the magnetic field? a) 261 mA b) 75.1 A c) 521 mA d) 626 mA
c) 521 mA
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Faraday's law of induction deals with how a changing magnetic flux induces an emf in a circuit. Recall that magnetic flux depends on magnetic field strength and the effective area the field is passing through. We'll start our investigation by looking at the field strength around a bar magnet. Position the magnet around the coil so that the region labeled A in the figure below is inside the coil. Move the magnet slowly back and forth and observe the effect on the brightness of the bulb and the needle of the voltmeter. Repeat the same process for the other two regions. The figure shows a bar magnet placed horizontally on a plane. The south pole of the magnet is to the right of the north pole of the magnet. Points A, B, and C are shown. Point A is to the left of the north pole of the magnet on the same horizontal line. Point B is a bit above the middle of the magnet very close to it. Point C is very close to the south pole of the magnet. For which of the regions shown in the figure is the observed effect the strongest? a) Region A b) Region C c) The observed effect is the same for all three regions. d) Region B
b) Region C
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Click on the button with two coils in it in the lower part of the window. The circuit should now have two sets of coils. Place the bar magnet inside the coil containing two loops. Try to find a location where the stationary magnet induces a current in the coil and causes the light bulb to shine. Which of the following is correct? a) The light bulb shines due to an induced current if one pole of the magnet is near the middle of the coil. b) There is no induced current in the coil, so the light bulb does not shine, if the magnet is stationary (for any location of the magnet). c) The light bulb shines due to an induced current if the magnet is inside the coil.
b) There is no induced current in the coil, so the light bulb does not shine, if the magnet is stationary (for any location of the magnet).
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Now, let's look at a situation with changing flux. Starting from the far left of the screen, move the magnet to the right so it goes through the middle of the two loops coil at a constant speed and out to the right of the coil. The figure shows two pickup coils, one with two loops above another coil with four loops, both connected to a lightbult and a voltmeter. A bar magnet, with its north pole to the left of its south pole, is moving to the right through the two loops coil. The brightness of the bulb and the needle of the voltmeter deflected to the right are shown. Roughly where is the magnet when the light bulb is the brightest? (The brightness of the light bulb correlates with how much the needle of the voltmeter gets deflected away from the middle.) a) The light bulb is brightest when the middle of the magnet is in the middle of the coil. b) The brightness of the light bulb is the same, regardless of the location of the magnet (as long as it is moving). c) The light bulb does not shine since the magnet is moving at a constant speed. d) The light bulb is brightest when either end of the magnet is in the middle of the coil.