Deck 20: Magnetic Fields and Forces

Question 1

If an electron is released at the equator and falls toward the Earth under the influence of gravity, the magnetic force on the electron will be toward the:

A) south.
B) east.
C) west.
D) north.

Answer 1

west.

Question 2

A deuteron, with the same charge but twice the mass of a proton, moves with a speed of 6.0 * 10⁵ m/s perpendicular to a uniform magnetic field of 0.20 T. Which of the paths described below would it follow? (q_p = 1.6 * 10 ^{- 19} C and m_d = 3.34 *10 ^{- 27} kg)

A) a circular path of 6.3-cm radius
B) a circular path of 1.6-cm radius
C) a circular path of 3.1-cm radius
D) a straight line path

Answer 2

a circular path of 6.3-cm radius

Question 3

A current in a long, straight wire produces a magnetic field. The magnetic field lines:

A) go out from the wire to infinity.
B) form circles that go around the wire.
C) come in from infinity to the wire.
D) form circles that pass through the wire.

Answer 3

form circles that go around the wire.

Question 4

A proton is released such that it has an initial speed of 5.0 *10⁵ m/s from left to right across the page. A magnetic field of 1.2 T is present at an angle of 30° to the horizontal direction (or positive x axis). What is the magnitude of the force experienced by the proton? (q_p = 1.6 *10^-19 C)

A) 3.8 *10^-14 N
B) 7.5 * 10³ N
C) 4.8 *10^-14 N
D) 1.3 * 10^-19 N

Question 5

Electrical charges and magnetic poles have many similarities, but one difference is:

A) a magnetic pole cannot be isolated.
B) one magnetic pole cannot create magnetic poles in other materials.
C) opposite magnetic poles repel.
D) magnetic poles do not produce magnetic fields.

Question 6

Two singly ionized isotopes, X and Y, of the same element move with the same speed perpendicular to a uniform magnetic field. Isotope X follows a path of radius 3.35 cm while isotope Y moves along a path 3.43 cm in radius. What is the ratio of the two isotope masses, m_Y/m_X?

A) 0.977
B) 0.954
C) 1.05
D) 1.02

Question 7

A bar magnet is attached solidly to a frictionless surface and its length is aligned with the x axis. To the right of the first magnet a short distance away is a second bar magnet with its center placed on the x axis and its length perpendicular to the x axis. The second magnet is free to move. Once placed in position at rest, which best describes the initial motion of the second magnet?

A) The magnet will move away from the fixed magnet.
B) The magnet will not move.
C) The magnet will move toward the fixed magnet.
D) The magnet will start to rotate.

Question 8

A proton moves across the Earth's equator in a northwesterly direction. At this point the Earth's magnetic field has a direction due north and is parallel to the surface. What is the direction of the force acting on the proton at this instant?

A) toward the northeast
B) into the Earth's surface
C) toward the northwest
D) out of the Earth's surface

Question 9

A proton, which moves perpendicular to a magnetic field of 0.60 T in a circular path of radius 0.080 m, has what speed? (q_p = 1.6 *10^-19 C and m_p = 1.67 *10^-27 kg)

A) 9.2 * 10⁶ m/s
B) 4.6 * 10⁶ m/s
C) 9.6 * 10⁶ m/s
D) 3.4 * 10⁶ m/s

Question 10

An electron which moves with a speed of 7.5 *10³ m/s parallel to a uniform magnetic field of 0.40 T experiences a force of what magnitude? (e = 1.6 *10^-19 C)

A) 1.9 *10^-15 N
B) zero
C) 2.2 - 10^-24 N
D) 4.8 *10^-14 N

Question 11

The force on a charged particle created by its motion in a magnetic field is maximum at what angle between the particle velocity and field?

A) 90°
B) 180°
C) zero
D) 45°

Question 12

There is a magnetic force on a particle. It is possible that the particle is:

A) moving in the direction of the magnetic field.
B) stationary.
C) not part of a wire.
D) uncharged.

Question 13

An electron is released such that its initial velocity is from right to left across this page. The electron's path, however, is deflected in a direction toward the bottom edge of the page due to the presence of a uniform magnetic field. What is the direction of this field?

A) from bottom edge to top edge of the page
B) into the page
C) from right to left across the page
D) out of the page

Question 14

A proton moving at a speed of 3.8 *10⁶ m/s cuts across the lines of a magnetic field at an angle of 40°. The strength of the field is 0.25 *10^-4 T. What is the magnitude of the force acting on the proton? (q_p = 1.6 *10^-19 C)

A) 1.4 *10^-17 N
B) 2.3 * 10^-17 N
C) 9.8 *10^-18 N
D) 5.1 *10^-18 N

Question 15

A stationary positive charge +Q is located in a magnetic field B, which is directed toward the right as indicated. The direction of the magnetic force on Q is:

A) toward the right.
B) up.
C) There is no magnetic force.
D) down.

Question 16

Assume that a uniform magnetic field is directed into this page. If a proton is released with an initial velocity directed from the bottom edge to the top edge of the page, which of the following describes the direction of the resultant force acting on the proton?

A) into the page
B) to the left
C) to the right
D) out of the page

Question 17

A proton moving with a speed of 6.0 *10⁵ m/s perpendicular to a uniform magnetic field of 0.20 T will follow which of the paths described below? (q_p = 1.6 *10^-19 C and m_p = 1.67 * 10^-27 kg)

A) a circular path of 1.6-cm radius
B) a circular path of 3.1-cm radius
C) a circular path of 6.3-cm radius
D) a straight line path

Question 18

The right-hand rule allows one to find a property of the interaction of a magnetic field with a charged particle. The right-hand rule applied to moving charges:

A) can be used for positive charges only.
B) results in negative charges moving clockwise.
C) gives the direction of the force on a charge moving in a magnetic field.
D) results in positive charges moving clockwise.

Question 19

Three identical bar magnets, each 2 cm long, are placed in a straight line on a frictionless surface with their ends equally spaced with 2 cm between ends. They are constrained so they cannot rotate, but they can slide along the straight line. The left magnet has its north pole nearest the north pole of the middle magnet, and the right magnet has its south pole toward the middle magnet's south pole. Once they are in place and released from rest, which of the following best describes the motion of the magnets?

A) The end magnets will move away from the center magnet.
B) The magnets will not move since they are equally spaced.
C) The magnets will snap together.
D) None of the above would happen.

Question 20

The path of a charged particle moving perpendicular to a uniform magnetic field will be a:

A) ellipse.
B) circle.
C) straight line.
D) parabola.

Question 21

The magnetic force on a charged particle moving at a constant speed in a uniform magnetic field will be halved when the angle between the velocity of the particle and the direction of the magnetic field changes from 25.0° to

A) 50.0°.
B) 12.5°.
C) 57.7°.
D) 12.2°.

Question 22

The direction of the force on a current carrying wire located in an external magnetic field is which of the following?

A) perpendicular to the field
B) perpendicular to the current
C) Both choices A and B are valid.
D) None of the above are valid.

Question 23

At the Fermilab accelerator in Weston, Illinois, singly-charged ions with momentum 3.2 *10 ^{- 16} kg.m/s are held in a circular orbit of radius 1 km by an upward magnetic field. What B-field must be used to maintain the ions in this orbit? (q_ion = 1.6 *10 ^{- 19} C)

A) 4 T
B) 2 T
C) 1 T
D) 3 T

Question 24

A wire is lying horizontally in the north-south direction and the horizontal magnetic field is toward the east. Some positive charges in the wire move south and an equal number of negative charges move north. The direction of the force on the wire will be:

A) up, out of the page.
B) down, into the page.
C) east.
D) There is no magnetic force.

Question 25

A proton, mass 1.67 * 10 ^{- 27} kg and charge +1.6 *10 ^{- 19} C, moves in a circular orbit perpendicular to a uniform magnetic field of 0.50 T. Find the time for the proton to make one complete circular orbit.

A) 8.7 *10 ^{- 8} s
B) 1.3 *10 ^{- 7} s
C) 9.8 *10 ^{- 7} s
D) 4.3 *10 ^{- 8} s

Question 26

In a mass spectrometer, an ion will have a smaller radius for its circular path if:

A) its mass is greater.
B) the magnetic field is weaker.
C) its charge is greater.
D) its speed is greater.

Question 27

A copper wire of length 10 cm is in a magnetic field of 0.30 T. If it has a mass of 20 g, what minimum current through the wire would cause a magnetic force equal to its weight?

A) 4.9 A
B) 6.5 A
C) 2.0 A
D) 1.3 A

Question 28

There is a current I flowing in a clockwise direction in a square loop of wire that is in the plane of the paper. If the magnetic field B is toward the right, and if each side of the loop has length L, then the net magnetic force acting on the loop is:

A) 2ILB.
B) ILB.
C) IBL².
D) zero.

Question 29

A current-carrying wire of length 50 cm is positioned perpendicular to a uniform magnetic field. If the current is 10.0 A and it is determined that there is a resultant force of 7.5 N on the wire due to the interaction of the current and field, what is the magnetic field strength?

A) 1.8 * 10^-3 T
B) 1.5 T
C) 6.7 * 10^-3 T
D) 0.60 T

Question 30

A horizontal wire of length 3.6 m carries a current of 6.0 A and is oriented so that the current direction is 50° S of W. The Earth's magnetic field is due north at this point and has a strength of 0.14 * 10^-4 T. What is the size of the force on the wire?

A) 1.6 * 10^-4 N
B) 0.28 *10^-4 N
C) 1.9 * 10^-4 N
D) 2.5 *10^-4 N

Question 31

A proton with initial kinetic energy E is moving in circular motion in a uniform magnetic field. When it has completed one fourth of a revolution, what is its kinetic energy?

A) 1.4 E
B) 0.71 E
C) E
D) The value is not given.

Question 32

The force exerted on a current-carrying wire located in an external magnetic field is directly proportional to which of the following?

A) field strength
B) current strength
C) both A and B
D) None of the above are valid.

Question 33

A proton and an alpha particle are moving perpendicular to a uniform magnetic field with the same speeds. If the velocity of the proton is at an angle of 20° with respect to the magnetic field direction, at what angle to the magnetic field direction would the alpha particle be moving if it experiences the same magnitude magnetic force as the proton? The alpha particle has 4 times the mass of the proton and twice its charge.

A) 10°
B) 62°
C) 43°
D) 9.8°

Question 34

A proton and an alpha particle are moving perpendicular to a uniform magnetic field with the same speed. If the velocity of the proton is at an angle of 20° with respect to the magnetic field direction, at what angle to the magnetic field direction would the alpha particle be moving if it experiences the same magnitude acceleration as the proton? The alpha particle has 4 times the mass of the proton and twice its charge.

A) 43°
B) 10°
C) 62°
D) 9.8°

Question 35

A 2.0-m wire segment carrying a current of 0.60 A oriented parallel to a uniform magnetic field of 0.25 T experiences a force of what magnitude?

A) 0.30 N
B) 6.7 N
C) 0.15 N
D) zero

Question 36

An electron is moving at a speed of 12 *10⁶ m/s at an angle of 30° with respect to a uniform magnetic field of 8.0 * 10^-4 T. What is the radius of the resulting helical path? (m_e = 9.11 * 10^-31 kg, q_e = 1.6 *10^-19 C)

A) 2.1 cm
B) 3.7 cm
C) 4.3 cm
D) 8.5 cm

Question 37

If a charged particle is moving in a uniform magnetic field, its path can be:

A) a straight line.
B) a circle.
C) a helix.
D) any of these choices.

Question 38

A horizontal wire of length 3.6 m carries a current of 6.0 A and is oriented so that the current direction is 50° S of W. The Earth's magnetic field is due north at this point and has a strength of 0.14 *10 ^{- 4} T. What is the direction of the force on the wire?

A) 40° S of E
B) out of the Earth's surface
C) toward the Earth's surface
D) due east

Question 39

The magnetic force on a charged particle moving at a constant speed in a uniform magnetic field will be doubled when the angle between the velocity of the particle and the direction of the magnetic field changes from 25.0° to

A) 57.7°.
B) 12.5°.
C) 12.2°.
D) 50.0°.

Question 40

When a magnetic field causes a charged particle to move in a circular path, the only quantity listed below which the magnetic force changes significantly as the particle goes around in a circle is the particle's:

A) energy.
B) time to go around the circle once.
C) momentum.
D) radius for the circle.

Question 41

Two long parallel wires 40 cm apart are carrying currents of 10 A and 30 A in the opposite direction. What is the magnitude of the magnetic field halfway between the wires?

A) 3.0 * 10^-5 T
B) 2.0 * 10^-5 T
C) 1.0 * 10^-5 T
D) 4.0 *10^-5 T

Question 42

Two parallel conductors are carrying currents in the same direction. The currents are non-zero and not necessarily equal. The magnitude of the magnetic field midway between them is 40 $\mu$ T. If one of the currents then has its direction reversed, what is the resulting magnitude of the magnetic field midway between them?

A) It could be any value.
B) 40 $\mu$ T
C) a value less than 40 $\mu$ T
D) a value greater than 40 $\mu$ T

Question 43

Two parallel conductors are carrying currents in the opposite direction. The currents are non-zero and not necessarily equal. The magnitude of the magnetic field midway between them is 40 $\mu$ T. If one of the currents then has its direction reversed, what is the resulting magnitude of the magnetic field midway between them?

A) a value greater than 40 $\mu$ T
B) a value less than 40 $\mu$ T
C) 40 $\mu$ T
D) It could be any value.

Question 44

Two parallel wires are separated by 0.25 m. Wire A carries 4.1 A and Wire B carries 10 A, both currents in the same direction. The magnetic permeability in empty space µ₀ = 4 $\pi$ * 10 ^{- 7} T.m/A. The magnitude of the force on 1.4 m of Wire A is:

A) 2.6 * 10^-5 N.
B) 4.6 * 10^-5 N.
C) less than 1.0 * 10^-5 N.
D) 3.2 *10^-5 N.

Question 45

An incredible amount of electrical energy passes down the funnel of a large tornado every second. Measurements taken in Oklahoma at a distance of 9.00 km from a large tornado showed an almost constant magnetic field of 1.50 *10^-8 T associated with the tornado. What was the average current going down the funnel? (µ₀ = 4 $\pi$ *10 ^{- 7} T.m/A)

A) 450 A
B) 1500 A
C) 950 A
D) 675 A

Question 46

A circular current loop is placed in an external magnetic field. How is the torque related to the radius of the loop?

A) directly proportional to radius squared
B) directly proportional to radius
C) inversely proportional to radius squared
D) inversely proportional to radius

Question 47

Two long parallel wires 40 cm apart are carrying currents of 10 A and 30 A in the same direction. What is the magnitude of the magnetic field halfway between the wires?

A) 4.0 * 10^-5 T
B) 1.0 * 10^-5 T
C) 3.0 * 10^-5 T
D) 2.0 *10^-5 T

Question 48

A circular loop carrying a current of 3.0 A is oriented in a magnetic field of 0.35 T. The loop has an area of 0.24 m² and is mounted on an axis, perpendicular to the magnetic field, which allows the loop to rotate. If the plane of the loop is oriented parallel to the field, what torque is created by the interaction of the loop current and the field?

A) 0.017 N.m
B) 0.084 N.m
C) 0.25 N.m
D) 5.8 N.m

Question 49

The current in a long wire creates a magnetic field in the region around the wire. How is the strength of the field at distance r from the wire's center related to the magnitude of the field?

A) field directly proportional to r
B) field inversely proportional to r²
C) field directly proportional to r²
D) field inversely proportional to r

Question 50

Two long parallel wires 15 cm apart carry currents of 5.0 A and 8.0 A in the same direction. Is there any point between the two wires where the magnetic field is zero?

A) yes, midway between the wires
B) yes, 9.2 cm from the 5 A wire
C) no
D) yes, 5.8 cm from the 5 A wire

Question 51

There is a current I flowing in a clockwise direction in a square loop of wire that is in the plane of the paper. If the magnetic field B is toward the right, and if each side of the loop has length L, then the net magnetic torque acting on the loop is:

A) IBL².
B) 2ILB.
C) ILB.
D) zero.

Question 52

Niobium metal becomes a superconductor (with electrical resistance equal to zero) when cooled below 9 K. If superconductivity is destroyed when the surface magnetic field exceeds 0.100 T, determine the maximum current a 2.00-mm diameter niobium wire can carry and remain superconducting. (µ₀ = 4 $\pi$ *10^-7 T.m/A)

A) 2000 A
B) 1000 A
C) 125 A
D) 500 A

Question 53

Two parallel wires carry the a current I in the same direction. Midway between these wires is a third wire, also parallel to the other two, which carries a current 0.5 I, but in the direction opposite from the first two wires. In which direction are the net forces on the outer wires?

A) Both forces are toward the center wire.
B) Since the net forces are zero, there is no direction.
C) Both forces are away from the center wire.
D) Both forces on the two wires are in the same direction in space.

Question 54

A 100-m long wire carrying a current of 4.0 A will be accompanied by a magnetic field of what strength at a distance of 0.040 m from the wire? (magnetic permeability in empty space µ₀ = 4 $\pi$ *10 ^{- 7} T.m/A)

A) zero
B) 1.6 *10^-5 T
C) 2.0 *10 ^{- 5} T
D) 4.0 *10 ^{- 5} T

Question 55

An electron moves through a region of crossed electric and magnetic fields. The electric field E = 3200 V/m and is directed straight down. The magnetic field B = 0.80 T and is directed to the left. For what velocity v of the electron into the paper will the electric force exactly cancel the magnetic force?

A) 2500 m/s
B) 5000 m/s
C) 4000 m/s
D) 8000 m/s

Question 56

Consider two long, straight parallel wires, each carrying a current I. If the currents are flowing in the same direction:

A) the two wires will attract each other.
B) the two wires will repel each other.
C) neither wire will exert a force on the other.
D) the two wires will exert a torque on each other.

Question 57

Solenoid #1 has a length L, cross-sectional area A, and N turns. Solenoid #2 has a length 2L, cross-sectional area 2A, and 2N turns. Which long solenoid has the greater magnetic field at its center when equal currents are going through them?

A) Both have the same magnetic field.
B) #1
C) #2
D) Since it depends on the values of A and L, none of the above are correct.

Question 58

A circular loop carrying a current of 3.0 A is oriented in a magnetic field of 0.35 T. The loop has an area of 0.24 m² and is mounted on an axis, perpendicular to the magnetic field, which allows the loop to rotate. What is the torque on the loop when its plane is oriented at a 50° angle to the field?

A) 4.6 N.m
B) 0.16 N.m
C) 0.076 N.m
D) 0.23 N.m

Question 59

A beam of electrons is sent in the positive x direction in a region with a uniform magnetic field B in the positive y direction and a uniform electric field E in the positive z direction. At which of the following speeds would the electrons be deflected in the positive z direction?

A) v = E/B
B) There is no speed for which this will happen.
C) v < E/B
D) v > E/B

Question 60

Two parallel conductors each of 0.50-m length, separated by 5.0 *10 ^{- 3} m and carrying 3.0 A in the same direction, will experience what type and magnitude of mutual force? (magnetic permeability in empty space µ₀ = 4 $\pi$ *10 ^{- 7} T.m/A)

A) attractive, 1.8 * 10 ^{- 4} N
B) repulsive, 1.8 /10 ^{- 4} N
C) repulsive, 0.60 *10 ^{- 4} N
D) attractive, 0.06 *10 ^{- 4} N

Question 61

The magnetic field of the Earth is believed responsible for which of the following?

A) deflection of charged cosmic rays
B) ozone in the upper atmosphere
C) solar flares
D) deflection of both charged and uncharged cosmic rays

Question 62

A superconducting solenoid is to be designed to generate a magnetic field of 2.50 T. If the solenoid winding has 1000 turns/m, what is the required current? (µ₀ = 4 $\pi$ *10 ^{- 7} T.m/A)

A) 5000 A
B) 1990 A
C) 3980 A
D) 1000 A

Question 63

Geophysicists today generally attribute the existence of the Earth's magnetic field to which of the following?

A) solar flares
B) iron ore deposits in the crust
C) currents within the liquid interior
D) nickel-iron deposits in the crust

Question 64

Which of the following devices makes use of an electromagnet?

A) relay
B) motor
C) both A and B
D) None of the above.

Question 65

Magnetic reversals of the Earth's field have occurred on the average every 200,000 years over the past 5 million years. When was the last magnetic reversal?

A) about 200,000 years ago
B) 1792
C) about 990,000 years ago
D) about 780,000 years ago

Question 66

Two insulated current-carrying straight wires of equal length are arranged in the lab so that Wire A carries a current northward and Wire B carries a current eastward, the wires crossing at their midpoints separated only by their insulation. Which of the statements below is true?

A) There are no forces in this situation.
B) The net force on Wire B is southward.
C) There are forces, but the net force on each wire is zero.
D) The net force on Wire A is westward.

Question 67

The magnetic domains in a magnetized piece of iron are characterized by which orientation?

A) anti-parallel (opposite direction) to the magnetic axis
B) perpendicular to the magnetic axis
C) parallel to the magnetic axis
D) random

Question 68

The magnetic pole of the Earth nearest the geographic South Pole corresponds to which of the following?

A) a magnetic south pole
B) a magnetic Antarctic pole
C) a magnetic north pole
D) a magnetic arctic pole

Question 69

A current in a solenoid with N turns creates a magnetic field at the center of that loop. The field strength is directly proportional to:

A) current strength.
B) number of turns in the loop.
C) Both choices A and B are valid.
D) None of these choices are valid.

Question 70

Four long wires are perpendicular to the x-y plane. Each wire carries the same magnitude current. Wire 1 goes through point (a, a, 0) on the x-y plane, wire 2 goes through the point (-a, a, 0), wire 3 goes through the point (-a, -a, 0), and wire 4 goes through the point (a, -a, 0). Wires 1 and 2 carry their currents in the positive z direction and wires 3 and 4 carry their currents in the -z direction. What is the direction of the resulting magnetic field at the origin?

A) the -x direction
B) the -y direction
C) the +x direction
D) There is no direction since the field is zero at this position.

Question 71

Magnetic blood flow meters function using which of the following?

A) magnetic relays
B) magnetic bacteria
C) a Hall-effect type arrangement
D) radioactive decay

Question 72

Four long wires are perpendicular to the x-y plane. Each wire carries the same magnitude current. Wire 1 goes through point (a, a, 0) on the x-y plane, wire 2 goes through the point (-a, a, 0), wire 3 goes through the point (-a, -a, 0), and wire 4 goes through the point (a, -a, 0). Wires 1 and 3 carry their currents in the positive z direction and wires 2 and 4 carry their currents in the -z direction. What is the direction of the resulting magnetic field at the origin?

A) in the +x direction
B) in the -y direction
C) in the -x direction
D) There is no direction since the field is zero at this point.

Question 73

A solenoid with 500 turns, 0.10 m long, carrying a current of 2.0 A and with a radius of 10 ^{- 2} m will have what strength magnetic field at its center? (magnetic permeability in empty space µ₀ = 4 $\pi$ * 10 ^{- 7} T.m/A)

A) 31*10 ^{- 4} T
B) 1.3 *10 ^{- 2} T
C) 2.5 *10 ^{- 2} T
D) 62 *10 ^{- 4} T

Question 74

A charged particle moving perpendicular to a magnetic field has a magnet force on it. But when an observer moves with the particle, so that the particle has a relative velocity of zero with respect to the observer, one would expect there would be no magnetic force. Yet there is still a force. If it is not a magnetic force, then what is the source of this force?

A) It is an electrical force.
B) It is due to the ether.
C) It is a gravitational force.
D) It is something other than any of the above.

Question 75

Superconductors can carry very large currents with no resistance. If a superconducting wire is formed into a solenoid of length 50.0 cm with 250 turns, what is the magnetic field inside the solenoid when the current is 10⁴ A? (µ₀ = 4 $\pi$ *10 sup>- ⁷ T.m/A)

A) 12.6 T
B) 2.50 T
C) 1.25 T
D) 6.28 T

Question 76

A current in a solenoid coil creates a magnetic field inside that coil. The field strength is directly proportional to:

A) Both A and B are valid choices.
B) the current.
C) the coil area.
D) None of the above choices are valid.

Question 77

A square loop of wire has a current going around it. The magnetic field from the current on each side of the loop results in a magnetic force on the wire on the opposite side. Each resulting magnetic force on the opposite side wire

A) pushes inward on the loop.
B) is actually zero since a current cannot push on itself.
C) depends on whether the current is clockwise of counterclockwise.
D) pushes outward on the loop.