Deck 24: Magnetic Fields and Forces

Question 1

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 1

C

Question 2

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 2

C

Question 3

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 3

C

Question 4

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

Question 5

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.

Question 6

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.

Question 7

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

Question 8

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.

Question 9

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.

Question 10

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.

Question 11

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.

Question 12

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.

Question 13

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.

Question 14

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

Question 15

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

Question 16

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.

Question 17

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

Question 18

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.

Question 19

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.

Question 20

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.

Question 21

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

Question 22

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.

Question 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.

Question 24

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.

Question 25

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.

Question 26

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.

Question 27

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

Question 28

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.

Question 29

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.

Question 30

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.

Question 31

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.

Question 32

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.

Question 33

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.

Question 34

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.

Question 35

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.

Question 36

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.

Question 37

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.

Question 38

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

Question 39

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.

Question 40

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

Question 41

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.

Question 42

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

Question 43

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.

Question 44

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

Question 45

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

Question 46

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

Question 47

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

Question 48

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.

Question 49

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

Question 50

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

Question 51

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.

Question 52

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

Question 53

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.

Question 54

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.

Question 55

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.

Question 56

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.

Question 57

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

Question 58

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°

Question 59

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.

Question 60

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.

Question 61

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.

Question 62

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

Question 63

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.

Question 64

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

Question 65

An electron traveling toward the magnetic north with speed 400 km/s enters a region where the earth's magnetic field has the magnitude 5.0 × 10^-5 T and is directed downward at 45° below horizontal. What magnitude magnetic force acts on the electron? (e = 1.60 × 10^-19C)

Question 66

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^-19C)

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

Question 67

A geophysicist measures the magnetic force on a proton that is moving vertically downward at a point 1.996 km/s at the earth's equator. At that location, the earth's magnetic magnetic field is horizontal and has a strength of 0.40 × 10^-4 T. What are the magnitude and direction of the force she will measure? (e = 1.60 × 10^-19
C)

Question 68

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.

Question 69

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

Question 70

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.

Question 71

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.

Question 72

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.

Question 73

A proton moving at 5.0 × 10⁴ m/s horizontally enters a region where a magnetic field of 0.12 T is present, directed vertically downward. What magnitude force acts on the proton due to this field?
(e = 1.60 × 10^-19
C)

Question 74

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.

Question 75

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.

Question 76

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.

Question 77

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

Question 78

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.

Question 79

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

Question 80

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.