Deck 7: Magnetic Fields

Question 1

A long wire is known to have a radius greater than 4.0 mm and to carry a current uniformly distributed over its cross section.If the magnitude of the magnetic field is 0.285 mT at a point 4.0 mm from the axis of the wire and 0.200 mT at a point 10 mm from the axis, what is the radius of the wire?

A)4.6 mm
B)7.1 mm
C)5.3 mm
D)12 mm
E)10 mm

Answer 1

5.3 mm

Question 2

If a = 2.0 cm, b = 5.0 cm, and I = 20 A, what is the magnitude of the magnetic field at the point P?

A)4.5 $\mu$ T
B)7.5 $\mu$ T
C)9.0 $\mu$ T
D)6.0 $\mu$ T
E)3.6 $\mu$ T

Answer 2

6.0 $\mu$ T

Question 3

A long solenoid (diameter = 5.0 cm) is wound with 960 turns per metre of thin wire through which a current of 300 mA is maintained.A wire carrying 12 A is inserted along the axis of the solenoid.What is the magnitude of the magnetic field at a point 2.0 cm from the axis?

A)0.41 mT
B)0.48 mT
C)0.38 mT
D)0.56 mT
E)0.24 mT

Answer 3

0.38 mT

Question 4

Two long parallel wires carry unequal currents in the same direction.The ratio of the currents is 3 to 1.The magnitude of the magnetic field at a point in the plane of the wires and 10 cm from each wire is 4.0 $\mu$ T.What is the larger of the two currents?

A)5.3 A
B)3.8 A
C)4.5 A
D)3.0 A
E)0.5 A

Question 5

Each of two long straight parallel wires separated by a distance of 16 cm carries a current of 20 A in the same direction.What is the magnitude of the resulting magnetic field at a point that is 10 cm from each wire?

A)57 $\mu$ T
B)80 $\mu$ T
C)64 $\mu$ T
D)48 $\mu$ T
E)40 $\mu$ T

Question 6

A long straight wire carries a current of 40 A in a region where a uniform external magnetic field has a 30- $\mu$ T magnitude and is parallel to the current.What is the magnitude of the resultant magnetic field at a point that is 20 cm from the wire?

A)70 $\mu$ T
B)40 $\mu$ T
C)10 $\mu$ T
D)50 $\mu$ T
E)36 $\mu$ T

Question 7

A solenoid 4.0 cm in radius and 4.0 m in length has 8000 uniformly spaced turns and carries a current of 5.0 A.Consider a plane circular surface (radius = 2.0 cm) located at the centre of the solenoid with its axis coincident with the axis of the solenoid.What is the magnetic flux through this surface? (1 Wb = 1 T . m2)

A)63 $\mu$ Wb
B)16 $\mu$ Wb
C)0.25 mWb
D)10 $\mu$ Wb
E)5.0 $\mu$ Wb

Question 8

A long hollow cylindrical conductor (inner radius = 2.0 mm, outer radius = 4.0 mm) carries a current of 12 A distributed uniformly across its cross section.A long wire which is coaxial with the cylinder carries an equal current in the same direction.What is the magnitude of the magnetic field 3.0 mm from the axis?

A)1.1 mT
B)1.4 mT
C)1.7 mT
D)2.0 mT
E)0.2 mT

Question 9

A 2.0-cm length of wire centred on the origin carries a 20-A current directed in the positive y direction.Determine the magnetic field at the point x = 5.0 m on the x-axis.

A)1.6 nT in the negative z direction
B)1.6 nT in the positive z direction
C)2.4 nT in the negative z direction
D)None of the above

Question 10

Three long wires parallel to the x axis carry currents as shown.If I = 20 A, what is the magnitude of the magnetic field at the origin?

A)37 $\mu$ T
B)28 $\mu$ T
C)19 $\mu$ T
D)47 $\mu$ T
E)58 $\mu$ T

Question 11

A hollow cylindrical (inner radius = 1.0 mm, outer radius = 3.0 mm) conductor carries a current of 80 A parallel to its axis.This current is uniformly distributed over a cross section of the conductor.Determine the magnitude of the magnetic field at a point that is 2.0 mm from the axis of the conductor.

A)8.0 mT
B)3.0 mT
C)5.3 mT
D)16 mT
E)1.2 mT

Question 12

A hollow cylindrical (inner radius = 2.0 mm, outer radius = 4.0 mm) conductor carries a current of 24 A parallel to its axis.This current is uniformly distributed over a cross section of the conductor.Determine the magnitude of the magnetic field at a point that is 5.0 mm from the axis of the conductor.

A)0.96 mT
B)1.7 mT
C)0.55 mT
D)1.2 mT
E)0.40 mT

Question 13

One long wire carries a current of 30 A along the entire x axis.A second long wire carries a current of 40 A perpendicular to the xy plane and passes through the point (0, 4, 0) m.What is the magnitude of the resulting magnetic field at the point y = 2.0 m on the y axis?

A)4.0 $\mu$ T
B)5.0 $\mu$ T
C)3.0 $\mu$ T
D)7.0 F $\mu$ T
E)1.0 $\mu$ T

Question 14

A segment of wire of total length 2.0 m is formed into a circular loop having 5 turns.If the wire carries a 1.2-A current, determine the magnitude of the magnetic field at the centre of the loop.

A)79 $\mu$ T
B)69 $\mu$ T
C)59 $\mu$ T
D)89 $\mu$ T
E)9.4 $\mu$ T

Question 15

A straight wire (length = 8.0 m) is bent to form a square.If the wire carries a current of 20 A, what is the magnitude of the magnetic field at the centre of the square?

A)17 $\mu$ T
B)14 $\mu$ T
C)11 $\mu$ T
D)20 $\mu$ T
E)36 $\mu$ T

Question 16

In the figure, if a = 2.0 cm, b = 4.0 cm, and I = 2.0 A, what is the magnitude of the magnetic field at point P?

A)49 $\mu$ T
B)39 $\mu$ T
C)50 $\mu$ T
D)69 $\mu$ T
E)13 $\mu$ T

Question 17

What is the magnitude of the magnetic field at point P if a = R and b = 2R?

A)
B)
C)
D)
E)

Question 18

A current-carrying 2.0-cm long segment of wire is inside a long solenoid (radius = 4.0 cm, n = 800 turns/m, current = 50 mA).The wire segment is oriented perpendicularly to the axis of the solenoid.If the current in the wire segment is 12 A, what is the magnitude of the magnetic force on this segment?

A)22 $\mu$ N
B)16 $\mu$ N
C)18 $\mu$ N
D)12 $\mu$ N
E)0 $\mu$ N

Question 19

If a = 1.0 cm, b = 3.0 cm, and I = 30 A, what is the magnitude of the magnetic field at point P?

A)0.62 mT
B)0.59 mT
C)0.35 mT
D)0.31 mT
E)0.10 mT

Question 20

The segment of wire (total length including portions of incoming and outgoing wire = 6R) is formed into the shape shown and carries a current I.What is the magnitude of the resulting magnetic field at the point P?

A)
B)
C)
D)
E)

Question 21

A long wire lies in a tangle on the surface of a table, as shown below.When a current is run through the wire as shown, the largest component of the magnetic field at X points:

A)into the table.
B)out of the table.
C)parallel to the nearest segment of wire.
D)antiparallel to the nearest segment of wire.
E)along a circle which has its centre at the centre of the overall loop.

Question 22

At a point in space where the magnetic field is measured, the magnetic field produced by a current element:

A)points radially away in the direction from the current element to the point in space.
B)points radially in the direction from the point in space towards the current element.
C)points in a direction parallel to the current element.
D)points in a direction parallel to but opposite in direction to the current element.
E)points in a direction that is perpendicular to the current element and perpendicular to the radial direction.

Question 23

A solenoid consists of 100 circular turns of copper wire.Parts of 3 turns, A, B and C, are shown below. When a current flows through the coil,

A)both A and C are repelled by B.
B)A is attracted to B; C is repelled by B.
C)neither A nor C is attracted to or repelled by B.
D)A is repelled by B; C is attracted to B.
E)both A and C are attracted to B.

Question 24

An ideal solenoid of radius a has n turns per unit length and current I.The magnetic flux $\phi$ B through any area completely inside the solenoid, centred on the solenoid axis but at a 45 $\degree$ angle to the axis, so that it touches the inside of the solenoid, as shown below, is:

A)
B)
C)( $\mu$ 0 $\pi$ a2nI.)
D)2 $\mu$ 0 $\pi$ a2nI.
E)0.

Question 25

Which diagram correctly shows the magnetic field lines created by a circular current loop in which current flows in the direction shown?

A)
B)
C)
D)
E)

Question 26

A thin infinitely large current sheet lies in the y-z plane.Current of magnitude Js per unit length along the z axis travels in the y-axis direction, which is up out of the page.Which diagram below correctly represents the direction of the magnetic field on either side of the sheet?

A)
B)
C)
D)
E)

Question 27

By using a compass to measure the magnetic field direction at various points adjacent to a long straight wire, you can show that the wire's magnetic field lines are:

A)straight lines in space that go from one magnetic charge to another.
B)straight lines in space that are parallel to the wire.
C)straight lines in space that are perpendicular to the wire.
D)circles that have their centres on the wire and lie in planes perpendicular to the wire.
E)circles that have the wire lying along a diameter of the circle.

Question 28

When the number of turns in a solenoid and its length are both doubled, the ratio of the magnitude of the new magnetic field inside to the magnitude of the original magnetic field inside is:

A)0.25
B)0.50
C)1
D)2
E)4

Question 29

A conducting hollow cylinder (inner radius = a, outer radius = b) carries a current of 40 A that is uniformly distributed over the cross section of the conductor.If a = 3.0 mm and b = 6.0 mm, what is the magnitude of the (line) integral around a circular path (radius = 5.0 mm) centred on the axis of the cylinder and in a plane perpendicular to that axis?

A)50 $\mu$ T . m
B)30 $\mu$ T .m
C)22 $\mu$ T . m
D)37 $\mu$ T . m
E)47 $\mu$ T . m

Question 30

We find that N current loops are coplanar and coaxial.The first has radius a and current I.The second has radius 2a and current 2I, and the pattern is repeated up to the Nth, which has radius Na and current NI.The current in each loop is counter clockwise as seen from above.The magnitude of the magnetic field at the centre of the loops is:

A)
B)
C)
D)
E)

Question 31

On the average, in a ferromagnetic domain permanent atomic magnetic moments are aligned ____ to one another.

A)antiparallel
B)parallel
C)perpendicular
D)alternately parallel and antiparallel
E)randomly relative

Question 32

Equal currents of magnitude I travel out of the page in wires M and N.Eight directions are indicated by letters A through H. The direction of the magnetic field at point P is:

A)E.
B)F.
C)G.
D)H.
E)A.

Question 33

Equal currents of magnitude I travel into the page in wires M and N.Eight directions are indicated by letters A through H. The direction of the magnetic field at point P is:

A)B.
B)C.
C)D.
D)E.
E)F.

Question 34

The magnetic moment of an electron (charge = -e; mass = me) moving in a circular orbit of radius r with speed v about a nucleus of mass mN is proportional to:

A)r.
B)v.
C)vr.
D)evr.
E)mNvr.

Question 35

A long solenoid (n = 1200 turns/m, radius = 2.0 cm) has a current of a 0.30 A in its winding.A long wire carrying a current of 20 A is parallel to and 1.0 cm from the axis of the solenoid.What is the magnitude of the resulting magnetic field at a point on the axis of the solenoid?

A)0.60 mT
B)0.85 mT
C)52 $\mu$ T
D)0.40 mT
E)0.75 mT

Question 36

A toroid is made of 2000 turns of wire of radius 2.00 cm formed into a donut shape of inner radius 10.0 cm and outer radius 14.0 cm.When a 30.0-A current is present in the toroid, the magnetic field at a distance of 11.0 cm from the centre of the toroid is:

A)0.0857 T.
B)0.109 T.
C)0.120 T.
D)0.600 T.
E)0.685 T.

Question 37

An ideal solenoid of radius a has n turns per unit length and current I.The magnetic flux $\phi$ B through any circular area of radius a inside the solenoid, centred on and perpendicular to the solenoid axis is:

A)
B)
C)( $\mu$ 0 $\pi$ a2nI.)
D)2 $\mu$ 0 $\pi$ a2nI.
E)0.

Question 38

Gauss's Law states that the net electric flux, , through any closed surface is proportional to the charge enclosed: .The analogous formula for magnetic fields is:

A)
B)
C)
D)
E)

Question 39

A single circular (radius = R) loop of wire is located in the yz plane with its centre at the origin.The loop has a clockwise current as seen from the point (+R, 0, 0).The direction of the magnetic field at the point:

A)(0, 0, 0) is -i and at the point (+R, 0, 0) is -i.
B)(0, 0, 0) is -i and at the point (0, +2R, 0) is -i.
C)(0, 0, 0) is +i and at the point (+R, 0, 0) is +i.
D)(0, 0, 0) is +i and at the point (0, +2R, 0) is +i.
E)None of the above

Question 40

A 0.50-m long solenoid consists of 1000 turns of copper wire wound with a 4.0 cm radius.When the current in the solenoid is 18 A, the magnetic field at a point 1.0 cm from the central axis of the solenoid is:

A)0.090 mT.
B)0.36 mT.
C)23 mT.
D)36 mT.
E)45 mT.

Question 41

What current in a solenoid 15.0-cm long wound with 100 turns would produce a magnetic field equal to that of the Earth, 5.00 *10-5 T?

Question 42

When a ferromagnetic material that has been magnetised is brought to a temperature greater than the Curie temperature, what happens to its residual magnetism?

A)Nothing happens to the residual magnetism.
B)The residual magnetism disappears.
C)The residual magnetism reaches its highest value.
D)All the magnetic domains causing magnetism become a single domain.
E)The material of the magnet melts causing currents that are magnetic.

Question 43

A long solenoid (n = 80 turns/cm) carries a current of 70 mA.Determine the magnitude of the magnetic field inside the solenoid.

Question 44

A superconducting wire carries a current of 1.0 *104 A.Find the magnetic field at a distance of 1.0 m from the wire.

Question 45

The planetary model of the hydrogen atom consists of an electron in a circular orbit about a proton.The motion of the electron of charge 1.60 * 10-19 C creates an electric current.The radius of the electron orbit is 5.30 *10-11 m and the electron's velocity is 2.20 *106 m/s.What is the magnetic field strength at the location of the proton?