Deck 19: Magnetism

Question 1

Do like magnetic poles repel or attract?

Answer 1

repel

Question 2

An electron enters the velocity selector of a mass spectrometer in the direction shown Figure 19-1.

If it emerges along the path shown, which plate (A or B) has the higher potential?

Answer 2

Plate B

Question 3

A 14. Coulomb charge is moving straight toward you at 4800. m/s. When it is 70. meters away from you, what is the magnitude of the magnetic field it produces at YOUR location?

Answer 3

Zero. (The field is zero directly in front of and behind the charge.)

Question 4

If you find yourself in a locality where the magnetic declination is "15.° East," in which direction is true North in relation to a compass needle?

Question 5

Magnetic field lines come out of the south pole of a magnet and enter at the north pole.

Question 6

Breaking a bar magnet in half will produce

A) a single South magnetic monopole.
B) one North and one South monopole.
C) two bar magnets.
D) a single North magnetic monopole.

Question 7

A charged particle moves and experiences no magnetic force. From this we can conclude that

A) either no magnetic field exists or the particle is moving parallel to the field.
B) the particle is moving parallel to the magnetic field.
C) the particle is moving at right angles to the magnetic field.
D) the particle must be massive.
E) no magnetic field exists in that region of space.

Question 8

Consider a magnetic field pointing out of this page, as shown in Figure 19-1.

An electron moving on the page toward the right will

A) curve upward (path a).
B) continue straight ahead (path b)
C) speed up.
D) slow down.
E) curve downward (path c)

Question 9

At a particular instant, an electron moves eastward in a uniform magnetic field that is directed straight downward. The magnetic force that acts on it is

A) directed to the South.
B) zero.
C) Westward.
D) directed upward.
E) Northward.

Question 10

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 maximum when the particle moves

A) straight downward.
B) horizontally.
C) straight upward.
D) upward at an angle of 45° above the horizontal.

Question 11

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 observing the following:
Electrons moving upward feel a magnetic force in the NW direction;
Electrons moving horizontally North are pushed down;
Electrons moving horizontally South-East are pushed upward.
He naturally concludes that the magnetic field at this landing site is in which direction?

A) E
B) NE
C) SW
D) SE
E) W

Question 12

At a particular instant, a proton moves Eastward in a uniform magnetic field that is directed straight downward. The magnetic force that acts on it is

A) Westward.
B) zero.
C) directed upward.
D) to the South.
E) Northward.

Question 13

A charged particle moves with its velocity perpendicular to a magnetic field. The field acts to change the particle's

A) velocity.
B) energy.
C) mass.
D) magnitude of momentum.
E) charge.

Question 14

In a pair of accelerating plates, such as found inside a CRT, the electrons are emitted

A) from the anode which is negative, toward the cathode which is positive.
B) from the anode which is positive, toward the cathode which is negative.
C) from the cathode which is negative, toward the anode which is positive.
D) from the cathode which is positive, toward the anode which is positive.

Question 15

When a voltage is applied for display on an oscilloscope CRT, it is usually shown on the screen plot

A) vertically.
B) diagonally.
C) horizontally.

Question 16

A mass spectrometer needs

A) perpendicular magnetic and electric fields.
B) colinear magnetic and electric fields.
C) only a magnetic field.
D) a perpendicular gravitational field.
E) only an electric field.

Question 17

A proton, moving in a uniform magnetic field, moves in a circle perpendicular to the field. If the proton's speed tripled, its time to go around its circular path would be

A) double.
B) triple.
C) one third.
D) one half.
E) the same.

Question 18

A charged particle is observed traveling in a circular path in a uniform magnetic field. If the particle had been traveling twice as fast, the radius of the circular path would be

A) 1/ times the original radius.
B) one-fourth the original radius.
C) one-half the original radius.
D) four times the original radius.
E) twice the original radius.

Question 19

What happens to the cyclotron frequency of a charged particle if its speed doubles?

A) It halves.
B) It triples.
C) It does not change.
D) It doubles.
E) It is times as big.

Question 20

The magnetic moment of a current loop does not depend upon

A) the current.
B) the area.
C) the magnetic field.

Question 21

The maximum torque is exerted on a current loop when the plane of the loop is at what angle to the magnetic field?

A) 0.°
B) 45.°
C) 90.°
D) 135.°
E) -45.°

Question 22

A vertical wire carries a current straight up in a region where the magnetic field vector points due North. What is the direction of the resulting force on this current?

A) downward
B) South
C) North
D) West
E) East

Question 23

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. Compared to the magnetic moment of the smaller loop, the magnetic moment of the larger loop is

A) 8 times stronger.
B) 4 times stronger.
C) 16 times stronger.
D) times stronger.
E) 2 times stronger.

Question 24

Two parallel straight wires carry currents in the same direction. The magnetic forces between the wires are such that

A) the wires repel each other.
B) the wires attract each other.
C) they are pulled in the direction of the wires.
D) the wires feel a clockwise torque.

Question 25

What device converts mechanical energy into electrical energy?

A) motor
B) spectrometer
C) transformer
D) solenoid
E) generator

Question 26

What in a DC motor reverses the polarity and current each half-cycle?

A) fuse
B) solid ring commutator
C) a split ring commutator
D) a CRT

Question 27

The torque on the rotating loop of wire in a DC motor ________ as the loop rotates.

A) is constant in magnitude
B) varies in magnitude
C) is inversely proportional to the current
D) is zero

Question 28

What fundamental fact underlies the operation of essentially all electric motors?

A) Alternating current and direct current are both capable of doing work.
B) A magnetic North pole carries a positive electric charge, and a magnetic South pole carries a negative electric charge.
C) Iron is the only element that is magnetic.
D) A current-carrying conductor placed perpendicular to a magnetic field will experience a force.
E) Opposite electric charges attract and like charges repel.

Question 29

The magnetic force between parallel wires is used by the National Bureau of Standards to define the

A) Coulomb.
B) Tesla.
C) Newton.
D) Ampere.
E) square-meter.

Question 30

A charged particle moving along a direction perpendicular to the plane of a loop of current-carrying wire, and which passes through the loop's center, will experience

A) a magnetic force perpendicular to its velocity.
B) a magnetic force parallel to the loop axis.
C) a magnetic torque.
D) no magnetic force.

Question 31

Consider electrons flowing Southward along a horizontal power line. The magnetic field generated directly below the wire by the current is pointing

A) Northward.
B) Southward.
C) Westward.
D) upward.
E) Eastward.

Question 32

At double the distance from a long current-carrying wire, the strength of the magnetic field produced by that wire decreases to

A) 1/ of its original value.
B) 1/2 of its original value.
C) 1/4 of its original value.
D) 1/8 of its original value.
E) /3 of its original value.

Question 33

A wire lying in the plane of the page carries a current toward the bottom of the page, as shown.

What is the direction of the magnetic force it produces on an electron that is moving perpendicularly toward the wire, also in the plane of the page, from your right?

A) zero
B) perpendicular to the page and away from you
C) toward the bottom of the page
D) perpendicular to the page and towards you
E) toward the top of the page

Question 34

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. Compared to the magnetic field at the center of the smaller loop, the magnetic field at the center of the larger loop is

A) 2 times stronger.
B) 8 times stronger.
C) neither 2, 4, or 8 times stronger.
D) 4 times stronger.

Question 35

A long solenoid produces a field at its center. An identical solenoid is placed end to end with the first (common axis). The magnetic field at the center of the 1st solenoid is now

A) half what it was.
B) zero.
C) double.
D) the same.
E) four times the original magnitude.

Question 36

The magnetic field near a strong permanent bar magnet might be 500

A) Gauss.
B) microGauss.
C) MT.
D) milliGauss.
E) Tesla.

Question 37

In a vacuum, the relative permeability is

A) 1.0.
B) 4 $\pi$ × 10^-7 T/A-m.
C) 8.99 × 10⁹.
D) infinite.
E) zero.

Question 38

The earth's magnetic field at the magnetic equator is approximately

A) 10⁴ Gauss.
B) 10⁴ T.
C) 10^-5 T.
D) 1/2 T.
E) 10^-3 T.

Question 39

The Earth reversed its magnetic field about how many years ago?

A) 70,000,000
B) 700
C) 7000
D) 11
E) 700,000

Question 40

The source of the earth's magnetic field is thought to be due to the motion of charged particles in the earth's

A) mantle.
B) inner core.
C) mantle & crust.
D) crust.
E) outer core.

Question 41

The Earth cannot contain a giant permanent magnet because

A) the Earth doesn't have enough iron.
B) the Earth isn't large enough.
C) the core would repel it.
D) ferromagnetism disappears at high temperature.
E) its magnetic field is too large.

Question 42

Captain Planet measures the magnetic forces on a proton which is vertically falling at 1996. m/s at the equator (take the Earth's field to be 0.40 Gauss horizontal and pointing Northward). What FORCE would she measure? (Give both amount of Newtons and direction.)

Question 43

A deuteron has the same charge as an electron but approximately twice a proton's mass. Suppose deuterons are accelerated in a cyclotron in an orbit of 750. mm at a frequency of 8.00 MHz. What magnetic field is needed?

Question 44

Consider a 1000. Gauss magnetic field along the z axis and a proton moving with an instantaneous velocity of
V= 3 î + 4 k km/s (note its speed is 5. km/s composed of 3. km/s along the x axis and 4. km/s along the z axis)
[proton: 1.67 × 10^-27 kg and 1.6 × 10^-19 Coulomb]

Question 45

Ten meters away from a long straight wire, the field is 10^-4 Tesla. What current flows through the wire?

Question 46

A very long straight wire carrying 2.0 A lies along the x-axis. A second very long parallel wire carrying 3.0 A in the same direction is positioned parallel to the x-axis and passes through the point (0, 1.0 m). What is the magnitude of the magnetic field at the point (0, 0, 1.0 m)?

Question 47

Two long parallel wires carry currents of 10. A in opposite directions. They are separated by 40. cm. What is the magnetic field in the plane of the wires at a point that is 20. cm from one wire and 60. cm from the other?

Question 48

A coil of 57. turns of wire has a diameter of 9.5 cm and carries 7.2 Amperes.
(a) How large is the magnetic field, in Tesla, at the center of the loop?
(b) How many Gauss is that?
(c) If the same length of wire made a coil with twice the original diameter, how large is the magnetic field now?

Question 49

In order to trap the starship Enterprise, the Klingons build a huge solenoid 10. light years long with a diameter of 2. million kilometers. Every kilometer length of the solenoid contains 100. turns of wire.
(a) What magnetic field is produced in the center of the solenoid with a current of 2000. Amperes? (Give your answer in MKS units)
(b) This is how many Gauss?

Question 50

A cyclotron operates at 10. MHz. What magnetic field is needed to accelerate protons?

A) 1.1 T
B) 0.12 T
C) 0.25 Gauss
D) 0.66 T
E) 0.25 T

Question 51

A doubly charged ion with velocity 6.0 × 10⁶ m/s moves in a path of radius 30. cm in a magnetic field of 0.80 T in a mass spectrometer. What is the mass of this ion?

A) 8.22 × 10^-27 kg
B) 12.8 × 10^-27 kg
C) 6.68 × 10^-27 kg
D) 16.4 × 10^-27 kg
E) 3.34 × 10^-27 kg

Question 52

A proton with velocity 2.0 × 10⁵ m/s enters a region where a uniform magnetic field of 0.30 T is present. The proton's velocity makes an angle of 60° with the magnetic field. What is the pitch of the helical path followed by the proton? (The "pitch" is the distance traveled parallel to the magnetic field while making one complete revolution around a magnetic field line.)

A) 0.250 m
B) 0.077 m
C) 0.022 m
D) 0.140 m
E) 0.055 m

Question 53

What is the force per meter on a straight wire carrying 5.0 A when it is placed in a magnetic field of 0.020 T? (The wire makes an angle of 27° with respect to the magnetic field lines.)

A) 0.26 N/m
B) 0.17 N/m
C) 0.022 N/m
D) 0.045 N/m
E) 0.071 N/m

Question 54

The magnetic field due to the current in a long, straight wire is 8.0 ?T at a distance of 4.0 cm from the center of the wire. What is the current in the wire?

A) 0.20 A
B) 0.80 A
C) 1.6 A
D) 3.2 A
E) 0.40 A

Question 55

Figure 19-4 shows two wires, in cross section, carrying 7.0 Amperes out of the page.

Determine the total magnetic field, due to both wires, at point X midway between the wires.

A) 0.15 Gauss
B) 0.59 Gauss
C) 0.0 Gauss
D) 0.29 Gauss
E) 0.79 Gauss

Question 56

Two long parallel wires carry currents of 20 A and 5.0 A in opposite directions. The wires are separated by 0.20 m. What is the magnetic field midway between the two wires?

A) 3.0 × 10^-5 T
B) 2.0 × 10^-5 T
C) 5.0 × 10^-5 T
D) 4.0 × 10^-5 T
E) 1.0 × 10^-5 T

Question 57

What is the magnetic field at the center of a circular loop of wire of radius 4.0 cm when a current of 2.0 A flows in the wire?

A) 0 T
B) 1.3 × 10^-6 T
C) 1.3 × 10^-5 T
D) 3.1 × 10^-5 T
E) 3.1 × 10^-6 T

Question 58

A solenoid 20. cm long is wound with 5000. turns of wire. What magnetic field is produced at the center of the solenoid when a current of 10. A flows?

A) 0.67 T
B) 1.4 T
C) 0.31 T
D) 0.84 T
E) 1.6 T