Deck 24: Magnetic Fields and Forces

Question 1

A charged particle of mass 0.0040 kg is subjected to a 4.0 T magnetic field that acts at a right angle to its motion.If the particle moves in a circle of radius 0.10 m at a speed of 2.0 m/s,what is the magnitude of the charge on the particle?

A)0.020 C
B)50 C
C)0.00040 C
D)2500 C

Answer 1

0.020 C

Question 2

A solenoid is wound with 470 turns on a form 4 cm in diameter and 50 cm long.The windings carry a current in the sense that is shown.The current produces a magnetic field,of magnitude 4.1 mT,at the center of the solenoid.In the figure,the current in the solenoid windings is closest to:

A)3.5 A
B)3.0 A
C)2.6 A
D)4.3 A
E)3.9 A

Answer 2

3.5 A

Question 3

A rigid rectangular loop,which measures 0.30 m by 0.40 m,carries a current of 9.9 A,as shown.A uniform external magnetic field of magnitude 1.8 T in the negative x-direction is present.Segment CD is in the x-z plane and forms a 19° angle with the z-axis,as shown.The y-component of the magnetic force on segment AB is closest to:

A)+5.1 N
B)-5.1 N
C)+1.7 N
D)-1.7 N
E)zero

Answer 3

+5.1 N

Question 4

Alpha particles (charge = +2e,mass = 6.68 × 10^-27 kg)are accelerated in a cyclotron to a final orbit radius of 0.30 m.The magnetic field in the cyclotron is 0.80 T.The period of circular motion of the alpha particles is closest to:

A)0.16 μs
B)0.25 μs
C)0.33 μs
D)0.40 μs
E)0.49 μs

Question 5

A solenoid of length 18 cm consists of a wire wrapped tightly around a wooden core.The magnetic field strength is 2.0 T inside the solenoid.If the solenoid is stretched to 29 cm by applying a force to it,what does the magnetic field become?

A)1.2 T
B)3.2 T
C)15 T
D)2.0 T

Question 6

A wire carrying a current is shaped in the form of a circular loop of radius 4.0 mm.If the magnetic field strength at its center is 1.1 mT with no external magnetic fields contributing to it,what is the magnitude of the current that flows through the wire?

A)7.0 A
B)28 A
C)14 A
D)17 A

Question 7

An electron in a magnetic field has a cyclotron frequency of 8.0 × 10¹² Hz.What is the magnetic field strength? (The mass of an electron is 9.1 × 10^-31 kg,and the charge of an electron is 1.6 × 10^-19 C. )

A)290 T
B)4.5 T
C)0.22 T
D)0.0035 T

Question 8

A very short solenoid contains N = 20 circular loops of wire.The radius of the loop is R = 0.5 meters,and the length of the solenoid is d = 0.02 meters.Approximately what is the magnetic field in the center of the solenoid?

A)µ_oN I/d
B)µ_oN I/(2R)
C)µ_oN I/(2dR)
D)µ_oN I/(2pR)

Question 9

A circular coil of wire of 200 turns and diameter 6 cm carries a current of 7 A.It is placed in a magnetic field of 0.90 T with the plane of the coil making an angle of 30° with the magnetic field.What is the torque on the coil?

A)3.1 N ∙ m
B)1.8 N ∙ m
C)5.9 N ∙ m
D)8.2 N ∙ m
E)1.5 N ∙ m

Question 10

Two parallel,straight wires are 7.0 cm apart and each carries a 18.0 A current in the same direction.One wire is securely anchored,and the other is attached in the center to a movable cart.If the force needed to move the wire when it is not attached to the cart is negligible,with what force does the wire pull on the cart? Both wires are 50.0 m long.

A)4600 dynes
B)370 dynes
C)660 dynes
D)9300 dynes

Question 11

How far from the center of a wire carrying a 2.0 A current does the magnetic field that results from the current have a magnitude of 4.0 × 10^-5 T?

A)1.0 mm
B)0.5 mm
C)0.16 mm
D)13 mm

Question 12

Alpha particles (charge = +2e,mass = 6.68 × 10^-27 kg)are accelerated in a cyclotron to a final orbit radius of 0.50 m.The magnetic field in the cyclotron is 0.50 T.The kinetic energy of an alpha particle in the final orbit is closest to:

A)3.0 MeV
B)2.6 MeV
C)3.4 MeV
D)3.9 MeV
E)4.3 MeV

Question 13

A rigid rectangular loop,which measures 0.30 m by 0.40 m,carries a current of 5.5 A,as shown.A uniform external magnetic field of magnitude 2.9 T in the negative x-direction is present.Segment CD is in the x-z plane and forms a 35° angle with the z-axis,as shown.An external torque applied to the loop keeps it in static equilibrium.The magnitude of the external torque is closest to:

A)1.1 N ∙ m
B)0.73 N ∙ m
C)1.3 N ∙ m
D)1.4 N ∙ m
E)1.6 N ∙ m

Question 14

A very long wire generates a magnetic field of 0.0050 × 10^-4 T at a distance of 3.0 mm.What is the magnitude of the current?

A)7.5 mA
B)1.7 mA
C)3300 mA
D)24,000 mA

Question 15

The magnitude of a magnetic field a distance 3.0 µm from a wire is 20.0 × 10^-4 T.How much current is flowing through the wire? Assume the wire is the only contributor to the magnetic field.

A)30 mA
B)188 mA
C)19 mA
D)377 mA

Question 16

A uniform magnetic field of magnitude 0.80 T in the negative z-direction is present in a region of space.A uniform electric field is also present.In the figure,the electric field is set at 20,800 V/m in the positive y-direction.An electron is projected with an initial velocity vo = 2.6 × 10⁴ m/s.in the positive x-direction.The y-component of the initial force on the electron is closest to:

A)- 7 × 10^-15 N
B)+ 7 × 10^-15 N
C)- 3 × 10^-15 N
D)+ 3 × 10^-15 N
E)zero

Question 17

An electron enters a magnetic field of 0.75 T with a velocity perpendicular to the direction of the field.At what frequency does the electron traverse a circular path? (The mass of an electron is 9.1 × 10^-31 kg,and the charge of an electron is 1.6 × 10^-19 C. )

A)2.1 × 10¹⁰ Hz
B)4.8 × 10^-7 Hz
C)2.1 × 10¹⁴ Hz
D)4.8 × 10^-11 Hz

Question 18

A square wire 2.0 cm on each side is oriented so that a 5.00 mT magnetic field makes a 30.0º angle with the normal to the plane of the square.If a 1.0 mA current causes a 32.0 N · m torque on the wire,how many loops does the wire make?

A)3.2 × 10¹⁰
B)3.2 × 10⁶
C)1.6 × 10¹⁰
D)4.5 × 10¹⁰

Question 19

A mass spectrograph is operated with deuterons, which have a charge of +e and a mass of
3.34 × 10^-27 kg. Deuterons emerge from the source, which is grounded with negligible velocity. The velocity of the deuterons as they pass through the accelerator grid is 8.0 × 10⁵ m/s. A uniform magnetic field of magnitude B = 0.20 T, directed out of the plane, is present at the right of the grid.
In the figure,the deuterons are in circular orbit in the magnetic field.The radius of the orbit and the initial sense of deflection are closest to:

A)60 mm,upward
B)70 mm,upward
C)60 mm,downward
D)70 mm,downward
E)80 mm,downward

Question 20

A 5 m length of wire carrying a current of 5 A lies on a horizontal table with a rectangular top of dimensions 0.300 m × 0.400 m.The ends of the wire are attached to opposite ends of a diagonal of the rectangle.A vertical magnetic field of 0.30 T is present.What magnetic force acts on this segment of wire?

A)0.75 N
B)1.1 N
C)7.5 N
D)zero
E)The force cannot be determined without knowing the shape of the length of wire.

Question 21

A horseshoe magnet has straight sides as shown.An electron travelling directly at the north pole is deflected.Why?

Question 22

A negatively charged particle is moving to the right,directly above a wire having a current flowing to the right,as shown below.In which direction is the magnetic force exerted on the particle?

A) into the page
B) out of the page
C) downward
D) upward
E) The magnetic force is zero since the velocity is parallel to the current.

Question 23

A metal wire is bent into a square and carries a uniform current throughout it.The net magnetic field at the center of this square is zero.

Question 24

Two long,parallel bars carry equal currents in opposite directions.The magnetic fields due to these two currents cancel each other midway between the two bars.

Question 25

Two particles with equal mass,one carrying a charge q and the other -2q,each enter at right angles to a uniform magnetic field with the same velocity.Which is deflected into a circular path with the larger radius?

Question 26

If the magnetic field strength is 20 μT at a distance of 3.0 cm from a long,straight,very thin current-carrying wire,the field will be 5 μT 12 cm away from the wire.

Question 27

If a magnetic compass is placed between the plates of a fully charged capacitor,the compass will align itself along the electric field lines.

Question 28

If a charged particle moves at right angles to a magnetic field,it experiences a force.How can the particle have an acceleration if the kinetic energy remains constant?

Question 29

Two long,current-carrying wires run parallel to each other.If these wires tend to push away from each other,the currents in them must be in opposite directions.

Question 30

The net magnetic field inside a conductor must be zero.

Question 31

A ring with a clockwise current (as seen from above the ring)is situated with its center directly above another ring,which has a counterclockwise current,as shown in the figure.In what direction is the net magnetic force exerted on the top ring?

A)upward
B)downward
C)The net force is zero.
D)Impossible to determine

Question 32

Inside of an ideal solenoid,the closer you get to the metal windings,the stronger the magnetic field gets.

Question 33

A positive charge in the figure is moving to the right and experiences a vertical (upward)magnetic force.In which direction is the magnetic field?

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

Question 34

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.

Question 35

A particle with a charge of 1 C experiences a force FBin a magnetic field.A second particle follows it at the same velocity.If the second particle has a charge of -2C,what force does it experience?

Question 36

Which of the following is an accurate statement?

A)Magnetic field lines have as their sources north and south poles.
B)A magnetic field line is,by definition,tangent to the direction of the magnetic force on a moving charge at a given point in space.
C)The magnetic force on a moving charge does not change its energy.
D)The magnetic force on a current-carrying wire is greatest when the wire is parallel to the magnetic field.
E)A current-carrying loop of wire tends to line up with its plane parallel to an external magnetic field in which it is positioned.

Question 37

A small plastic ball has an excess negative charge on it.If a magnet is placed close to this ball,it will either attract or repel the ball,depending upon which pole is closer to the ball.

Question 38

A solenoid 3.0 cm long consists of 8942 loops of wire.If the magnetic field inside the solenoid is 1.0 T,what is the magnitude of the current that flows through it?

A)2.7 A
B)0.37 A
C)34 A
D)3.0 A

Question 39

A compass is placed close to a stationary charge.If the charge remains stationary,the compass will not deflect,but if the charge moves,the compass will deflect.

Question 40

A charge moving in a straight line enters a uniform electric or magnetic field.If the motion of this charge is not affected,this field could only have been a magnetic field,not an electric field.

Question 41

Consider a solenoid of length L,N windings,and radius b (L is much longer than b).A current I is flowing through the wire.If the radius of the solenoid were doubled (becoming 2b),and all other quantities remained the same,the magnetic field

A)would remain the same.
B)would become twice as strong.
C)would become one half as strong.

Question 42

A charged particle moving within a static magnetic field

A)will always experience a magnetic force,regardless of its direction of motion.
B)may experience a magnetic force that will cause its speed to change.
C)may experience a magnetic force,but its speed will not change.
D)None of the above statements are true.

Question 43

A long straight wire has a constant current flowing to the right.A square ring is situated above the wire,and also has a constant current flowing through it (as shown).Which of the following statements is true?

A)The net magnetic force on the ring is upward,and there is also a net torque on the ring.
B)The net magnetic force on the ring is zero,and the net torque is zero.
C)The net magnetic force on the ring is downward,and there is also a net torque on the ring.
D)The net magnetic force on the ring is zero,but there is a net torque on the ring.
E)The net magnetic force on the ring is downward,and the net torque is zero.

Question 44

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 of:

A)16d
B)8d
C)4d
D)2d
E)d

Question 45

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)only contain a north pole.
B)still contain a north pole on the right and a new south pole on the left.
C)only contain a south pole.

Question 46

A metallic weight is suspended from a metal spring.If now a current is passed through the spring,

A)the spring will contract,raising the weight.
B)the spring will extend,lowering the weight.
C)the weight will not move.
D)whether or not the weight moves up or down depends on what the weight is made of (i.e. ,whether or not it is magnetizable).
E)None of these are true.

Question 47

Consider a solenoid of length L,N windings,and radius b (L is much longer than b).A current I is flowing through the wire.If the length of the solenoid became twice as long (2L),and all other quantities remained the same,the magnetic field inside the solenoid would

A)stay the same.
B)become twice as strong as initially.
C)become half as strong as initially.