Question 1

The magnetic field a distance 2 cm from a long straight current-carrying wire is 2 * 10-5 T. The current in the wire is:

Accepted Answer

A)  0.16 A 
B)  1.0 A 
C)  2.0 A 
D)  4.0 A 
E)  25 A 
A)  0.16 A 
B)  1.0 A 
C)  2.0 A 
D)  4.0 A 
E)  25 A C

Question 2

In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is:

Accepted Answer

A)  east 
B)  up 
C)  north 
D)  down 
E)  west 
A)  east 
B)  up 
C)  north 
D)  down 
E)  west E

Question 3

A toroid has a square cross section with the length of an edge equal to the radius of the inner surface. The ratio of the magnitude of the magnetic field at the inner surface to the magnitude of the field at the outer surface is:

Accepted Answer

A)  1/4 
B)  1/2 
C)  1 
D)  2 
E)  4 
A)  1/4 
B)  1/2 
C)  1 
D)  2 
E)  4 D

Question 4

A square loop of current-carrying wire with edge length a is in the xy plane, the origin being at its center. Along which of the following lines can a charge move without experiencing a magnetic force?

Accepted Answer

A)  x = 0, y = a/2 
B)  x = a/2, y = a/2 
C)  x = a/2, y = 0 
D)  x = 0, y = 0 
E)  x = 0, z = 0 
A)  x = 0, y = a/2 
B)  x = a/2, y = a/2 
C)  x = a/2, y = 0 
D)  x = 0, y = 0 
E)  x = 0, z = 0

Question 5

Two parallel wires carrying equal currents of 10 A attract each other with a force of 1 mN. If both currents are doubled, the force of attraction will be:

Accepted Answer

A)  0.25 mN 
B)  0.5 mN 
C)  1 mN 
D)  2 mN 
E)  4 mN 
A)  0.25 mN 
B)  0.5 mN 
C)  1 mN 
D)  2 mN 
E)  4 mN

Question 6

In the figure, the current element , the point P, and the three vectors (1, 2, 3) are all in the plane of the page. The direction of , due to this current element, at the point P is:

Accepted Answer

A)  in the direction marked "1" 
B)  in the direction marked "2" 
C)  in the direction marked "3" 
D)  out of the page 
E)  into the page 
A)  in the direction marked "1" 
B)  in the direction marked "2" 
C)  in the direction marked "3" 
D)  out of the page 
E)  into the page

Question 7

A 45-m long wire is coiled so that it makes a coil containing 100 circular loops, one on top of the other. If the wire carries a current of 13 A, what is the magnetic dipole moment of the coil?

Accepted Answer

A)  21 A·m2 
B)  6.7 A·m2 
C)  3.3 A·m2 
D)  2.6 A·m2 
E)  1.2 A·m2 
A)  21 A·m2 
B)  6.7 A·m2 
C)  3.3 A·m2 
D)  2.6 A·m2 
E)  1.2 A·m2

Question 8

The magnetic field at any point in the xy plane is given by , where is the position vector of the point, A is a constant, and is a unit vector in the +z direction. The net current through a circle of radius R, in the xy plane and centered at the origin is given by:

Accepted Answer

A)   $\pi$AR2/$\mu$0 
B)  2 $\pi$AR/$\mu$0 
C)  4 $\pi$AR3/3$\mu$0 
D)  2 $\pi$AR2/$\mu$0 
E)   $\pi$AR2/2$\mu$0 
A)   $\pi$AR2/$\mu$0 
B)  2 $\pi$AR/$\mu$0 
C)  4 $\pi$AR3/3$\mu$0 
D)  2 $\pi$AR2/$\mu$0 
E)   $\pi$AR2/2$\mu$0

Question 9

A long straight cylindrical shell has an inner radius Ri and an outer radius Ro. It carries a current i, uniformly distributed over its cross section. A wire is parallel to the cylinder axis, in the hollow region (r i). The magnetic field is zero everywhere outside the shell (r > Ro). We conclude that the wire:

Accepted Answer

A)  is on the cylinder axis and carries current i in the same direction as the current in the shell 
B)  may be anywhere in the hollow region but must be carrying current i in the direction opposite to that of the current in the shell 
C)  may be anywhere in the hollow region but must be carrying current i in the same direction as the current in the shell 
D)  is on the cylinder axis and carries current i in the direction opposite to that of the current in the shell 
E)  does not carry any current 
A)  is on the cylinder axis and carries current i in the same direction as the current in the shell 
B)  may be anywhere in the hollow region but must be carrying current i in the direction opposite to that of the current in the shell 
C)  may be anywhere in the hollow region but must be carrying current i in the same direction as the current in the shell 
D)  is on the cylinder axis and carries current i in the direction opposite to that of the current in the shell 
E)  does not carry any current

Question 10

An ordinary magnetic compass is placed flat on the ground. A wire carrying a large current i from east to west is placed directly above the compass. The end of the compass needle marked "N" will point:

Accepted Answer

A)  north 
B)  south 
C)  east 
D)  west 
E)  the compass will act as an electric motor, hence the needle will keep rotating 
A)  north 
B)  south 
C)  east 
D)  west 
E)  the compass will act as an electric motor, hence the needle will keep rotating

Question 11

If the magnetic field is uniform over the area bounded by a circle with a radius R, the net current through the circle is:

Accepted Answer

A)  0 
B)  2 $\pi$RB$\mu$0 
C)   $\pi$R2B/$\mu$0 
D)  RB/2$\mu$0 
E)  2RB/$\mu$0 
A)  0 
B)  2 $\pi$RB$\mu$0 
C)   $\pi$R2B/$\mu$0 
D)  RB/2$\mu$0 
E)  2RB/$\mu$0

Question 12

Two long straight wires are parallel and carry current in opposite directions. The currents are 8.0 A and 12 A and the wires are separated by 0.40 cm. The magnetic field at a point midway between the wires is:

Accepted Answer

A)  0 T 
B)  4.0 * 10-4 T 
C)  8.0* 10-4 T 
D)  12 * 10-4 T 
E)  20 * 10-4 T 
A)  0 T 
B)  4.0 * 10-4 T 
C)  8.0* 10-4 T 
D)  12 * 10-4 T 
E)  20 * 10-4 T

Question 13

A coulomb is:

Accepted Answer

A)  one ampere per second 
B)  the quantity of charge which will exert a force of 1 N on a similar charge at a distance of 1 m 
C)  the amount of current in each of two long parallel wires separated by 1 m, which produces a force of 2 * 10-7 N per meter 
D)  the amount of charge which flows past a point in one second when the current is 1 A 
E)  an abbreviation for a certain combination of kilogram, meter and second 
A)  one ampere per second 
B)  the quantity of charge which will exert a force of 1 N on a similar charge at a distance of 1 m 
C)  the amount of current in each of two long parallel wires separated by 1 m, which produces a force of 2 * 10-7 N per meter 
D)  the amount of charge which flows past a point in one second when the current is 1 A 
E)  an abbreviation for a certain combination of kilogram, meter and second

Question 14

A semi-infinite wire runs along the positive y axis, beginning at the origin (x = y = 0) and extending infinitely far in the +y direction. If the current in the wire is 12 A, what magnetic field does it create on the x axis at the point x = 3.5 cm?

Accepted Answer

A)  0 T 
B)  3.4 x 10-5 T 
C)  6.9 x 10-5 T 
D)  1.4 x 10-4 T 
E)  4.3 x 10-4 T 
A)  0 T 
B)  3.4 x 10-5 T 
C)  6.9 x 10-5 T 
D)  1.4 x 10-4 T 
E)  4.3 x 10-4 T

Question 15

The diagram shows three arrangements of circular loops, centered on vertical axes and carrying identical currents in the directions indicated. Rank the arrangements according to the magnitudes of the magnetic fields at the midpoints between the loops on the central axes, from least to greatest.

Accepted Answer

A)  1, 2, 3 
B)  2, 1, 3 
C)  2, 3, 1 
D)  3, 2, 1 
E)  3, 1, 2 
A)  1, 2, 3 
B)  2, 1, 3 
C)  2, 3, 1 
D)  3, 2, 1 
E)  3, 1, 2

Question 16

Magnetic field lines inside the solenoid shown are:

Accepted Answer

A)  clockwise circles as one looks down the axis from the top of the page 
B)  counterclockwise circles as one looks down the axis from the top of the page 
C)  toward the top of the page 
D)  toward the bottom of the page 
E)  in no direction since B = 0 
A)  clockwise circles as one looks down the axis from the top of the page 
B)  counterclockwise circles as one looks down the axis from the top of the page 
C)  toward the top of the page 
D)  toward the bottom of the page 
E)  in no direction since B = 0

Question 17

In Ampere's law, the symbol is:

Accepted Answer

A)  an infinitesimal piece of the wire that carries current i 
B)  in the direction of 
C)  perpendicular to 
D)  a vector whose magnitude is the length of the wire that carries current i 
E)  none of the above 
A)  an infinitesimal piece of the wire that carries current i 
B)  in the direction of 
C)  perpendicular to 
D)  a vector whose magnitude is the length of the wire that carries current i 
E)  none of the above

Question 18

Two long ideal solenoids (with radii 20 mm and 30 mm respectively) have the same number of turns of wire per unit length. The smaller solenoid is mounted inside the larger, along a common axis. It is observed that there is zero magnetic field within the inner solenoid. The current in the inner solenoid must be:

Accepted Answer

A)  two-thirds the current in the outer solenoid 
B)  one-third the current in the outer solenoid 
C)  twice the current in the outer solenoid 
D)  half the current in the outer solenoid 
E)  the same as the current in the outer solenoid 
A)  two-thirds the current in the outer solenoid 
B)  one-third the current in the outer solenoid 
C)  twice the current in the outer solenoid 
D)  half the current in the outer solenoid 
E)  the same as the current in the outer solenoid

Question 19

Which graph correctly gives the magnitude of the magnetic field outside an infinitely long, very thin, straight current-carrying wire as a function of the distance r from the wire?

Accepted Answer

A)  I 
B)  II 
C)  III 
D)  IV 
E)  V 
A)  I 
B)  II 
C)  III 
D)  IV 
E)  V

Question 20

A solenoid is 3.0 cm long and has a radius of 0.50 cm. It is wrapped with 500 turns of wire carrying a current of 2.0 A. The magnetic field at the center of the solenoid is:

Accepted Answer

A)  9.9 *10-8 T 
B)  1.3 *10-3 T 
C)  4.2 * 10-2 T 
D)  16 T 
E)  20 T 
A)  9.9 *10-8 T 
B)  1.3 *10-3 T 
C)  4.2 * 10-2 T 
D)  16 T 
E)  20 T

Exam 29: Magnetic Fields Due to Currents

The magnetic field a distance 2 cm from a long straight current-carrying wire is 2 * 10-5 T. The current in the wire is:

In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is:

A toroid has a square cross section with the length of an edge equal to the radius of the inner surface. The ratio of the magnitude of the magnetic field at the inner surface to the magnitude of the field at the outer surface is:

A square loop of current-carrying wire with edge length a is in the xy plane, the origin being at its center. Along which of the following lines can a charge move without experiencing a magnetic force?

Two parallel wires carrying equal currents of 10 A attract each other with a force of 1 mN. If both currents are doubled, the force of attraction will be:

In the figure, the current element , the point P, and the three vectors (1, 2, 3) are all in the plane of the page. The direction of , due to this current element, at the point P is:

A 45-m long wire is coiled so that it makes a coil containing 100 circular loops, one on top of the other. If the wire carries a current of 13 A, what is the magnetic dipole moment of the coil?

The magnetic field at any point in the xy plane is given by , where is the position vector of the point, A is a constant, and is a unit vector in the +z direction. The net current through a circle of radius R, in the xy plane and centered at the origin is given by:

An ordinary magnetic compass is placed flat on the ground. A wire carrying a large current i from east to west is placed directly above the compass. The end of the compass needle marked "N" will point:

If the magnetic field is uniform over the area bounded by a circle with a radius R, the net current through the circle is:

Two long straight wires are parallel and carry current in opposite directions. The currents are 8.0 A and 12 A and the wires are separated by 0.40 cm. The magnetic field at a point midway between the wires is:

A coulomb is:

A semi-infinite wire runs along the positive y axis, beginning at the origin (x = y = 0) and extending infinitely far in the +y direction. If the current in the wire is 12 A, what magnetic field does it create on the x axis at the point x = 3.5 cm?

The diagram shows three arrangements of circular loops, centered on vertical axes and carrying identical currents in the directions indicated. Rank the arrangements according to the magnitudes of the magnetic fields at the midpoints between the loops on the central axes, from least to greatest.

Magnetic field lines inside the solenoid shown are:

In Ampere's law, the symbol is:

Which graph correctly gives the magnitude of the magnetic field outside an infinitely long, very thin, straight current-carrying wire as a function of the distance r from the wire?

A solenoid is 3.0 cm long and has a radius of 0.50 cm. It is wrapped with 500 turns of wire carrying a current of 2.0 A. The magnetic field at the center of the solenoid is:

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The magnetic field a distance 2 cm from a long straight current-carrying wire is 2 * 10^-5 T. The current in the wire is: