Question 1

A long straight cylindrical shell carries current i uniformly distributed over its cross section. The magnitude of the magnetic field is greatest:

Accepted Answer

A)  at the inner surface of the shell 
B)  at the outer surface of the shell 
C)  inside the shell near the middle 
D)  in hollow region near the inner surface 
E)  near the center of the hollow region 
A)  at the inner surface of the shell 
B)  at the outer surface of the shell 
C)  inside the shell near the middle 
D)  in hollow region near the inner surface 
E)  near the center of the hollow region

Question 2

In the figure, there are three wires carrying currents in the indicated directions, and an Amperian loop. The sign of the contribution of the current in each wire to the path integral in Ampere's Law is:

Accepted Answer

A)  i1 +, i2 -, i3 0 
B)  i1 -, i2 +, i3 0 
C)  i1 +, i2 +, i3 - 
D)  i1 -, i2 -, i3 + 
E)  i1 +, i2 -, i3 - 
A)  i1 +, i2 -, i3 0 
B)  i1 -, i2 +, i3 0 
C)  i1 +, i2 +, i3 - 
D)  i1 -, i2 -, i3 + 
E)  i1 +, i2 -, i3 -

Question 3

Lines of the magnetic field produced by a long straight wire carrying a current are:

Accepted Answer

A)  in the direction of the current 
B)  opposite to the direction of the current 
C)  leave the wire radially 
D)  circles concentric with the wire 
E)  lines similar to those produced by a bar magnet 
A)  in the direction of the current 
B)  opposite to the direction of the current 
C)  leave the wire radially 
D)  circles concentric with the wire 
E)  lines similar to those produced by a bar magnet

Question 4

Two long straight wires enter a room through a window. One carries a current of 3.0 A into the room while the other carries a current of 5.0 A out. The magnitude of the path integral around the window frame is:

Accepted Answer

A)  2.5 *10-6 T.m 
B)  3.8 * 10-6 T.m 
C)  6.3*10-6 T.m 
D)  1.0 *10-5 T.m 
E)  none of these 
A)  2.5 *10-6 T.m 
B)  3.8 * 10-6 T.m 
C)  6.3*10-6 T.m 
D)  1.0 *10-5 T.m 
E)  none of these

Question 5

Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is:

Accepted Answer

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

Question 6

Two long straight wires are parallel and carry current in the same direction. The currents are 8.0 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 7

Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is:

Accepted Answer

A)  zero 
B)  non-zero and along a line connecting the wires 
C)  non-zero and parallel to the wires 
D)  non-zero and perpendicular to the plane of the two wires 
E)  none of the above 
A)  zero 
B)  non-zero and along a line connecting the wires 
C)  non-zero and parallel to the wires 
D)  non-zero and perpendicular to the plane of the two wires 
E)  none of the above

Question 8

The diagrams show three circuits consisting of concentric circular arcs (either half or quarter circles of radii r, 2r, and 3r) and radial lengths. The circuits carry the same current. Rank them according to the magnitudes of the magnetic fields they produce at C, least to greatest.

Accepted Answer

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

Question 9

Two parallel long wires carry the same current and repel each other with a force F per unit length. If both these currents are doubled and the wire separation tripled, the force per unit length becomes:

Accepted Answer

A)  2F/9 
B)  4F/9 
C)  2F/3 
D)  4F/3 
E)  6F 
A)  2F/9 
B)  4F/9 
C)  2F/3 
D)  4F/3 
E)  6F

Question 10

The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:

Accepted Answer

A)  R 
B)  1/R 
C)  1/R2 
D)  1/R3 
E)  1/R3/2 
A)  R 
B)  1/R 
C)  1/R2 
D)  1/R3 
E)  1/R3/2

Question 11

In the figure, if the current element has a length of 1.0 mm, carries a current of 2.5 A, and is a distance of 4.8 cm from the point P, what is the magnitude of the magnetic field at point P?

Accepted Answer

A)  0 T 
B)  2.6 x 10-9 T 
C)  5.4 x 10-8 T 
D)  9.4 x 10-8 T 
E)  1.1 x 10-7 T 
A)  0 T 
B)  2.6 x 10-9 T 
C)  5.4 x 10-8 T 
D)  9.4 x 10-8 T 
E)  1.1 x 10-7 T

Question 12

The magnetic field inside a long ideal solenoid is independent of:

Accepted Answer

A)  the current 
B)  the number of turns of wire 
C)  the spacing of the windings 
D)  the cross-sectional area 
E)  the direction of the current 
A)  the current 
B)  the number of turns of wire 
C)  the spacing of the windings 
D)  the cross-sectional area 
E)  the direction of the current

Question 13

A hollow cylindrical conductor (inner radius = a, outer radius = b) carries a current i uniformly spread over its cross section. Which graph below correctly gives B as a function of the distance r from the center of the cylinder?

Accepted Answer

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

Question 14

Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in the same direction. The force per unit length of one wire on the other is:

Accepted Answer

A)  1 * 10-3 N/m, repulsive 
B)  1 * 10-3 N/m, attractive 
C)  4 * 10-5 N/m, repulsive 
D)  4 * 10-5 N/m, attractive 
E)  none of these 
A)  1 * 10-3 N/m, repulsive 
B)  1 * 10-3 N/m, attractive 
C)  4 * 10-5 N/m, repulsive 
D)  4 * 10-5 N/m, attractive 
E)  none of these

Question 15

In Ampere's law, , the integration must be over any:

Accepted Answer

A)  surface 
B)  closed surface 
C)  path 
D)  closed path 
E)  closed path that surrounds all the current producing 
A)  surface 
B)  closed surface 
C)  path 
D)  closed path 
E)  closed path that surrounds all the current producing

Question 16

Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is:

Accepted Answer

A)  north 
B)  east 
C)  south 
D)  west 
E)  zero 
A)  north 
B)  east 
C)  south 
D)  west 
E)  zero

Question 17

The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank the wires according to the magnitudes of the magnetic forces on them, from least to greatest.

Accepted Answer

A)  1, 2, 3 
B)  2, then 1 and 3 tie 
C)  2 and 3 tie, then 1 
D)  1 and 3 tie, then 2 
E)  3, 2, 1 
A)  1, 2, 3 
B)  2, then 1 and 3 tie 
C)  2 and 3 tie, then 1 
D)  1 and 3 tie, then 2 
E)  3, 2, 1

Question 18

Two long straight current-carrying parallel wires cross the x axis and carry currents I and 3I in the same direction, as shown. At what value of x is the net magnetic field zero?

Accepted Answer

A)  0 
B)  1 
C)  3 
D)  5 
E)  7 
A)  0 
B)  1 
C)  3 
D)  5 
E)  7

Question 19

In Ampere's law, the direction of the integration around the path:

Accepted Answer

A)  must be clockwise 
B)  must be counterclockwise 
C)  must be such as to follow the magnetic field lines 
D)  must be along the wire in the direction of the current 
E)  none of the above 
A)  must be clockwise 
B)  must be counterclockwise 
C)  must be such as to follow the magnetic field lines 
D)  must be along the wire in the direction of the current 
E)  none of the above

Question 20

A constant current is sent through a helical coil made of flexible wire. The coil:

Accepted Answer

A)  tends to get shorter 
B)  tends to get longer 
C)  tends to rotate about its axis 
D)  produces zero magnetic field at its center 
E)  none of the above 
A)  tends to get shorter 
B)  tends to get longer 
C)  tends to rotate about its axis 
D)  produces zero magnetic field at its center 
E)  none of the above

Exam 29: Magnetic Fields Due to Currents

A long straight cylindrical shell carries current i uniformly distributed over its cross section. The magnitude of the magnetic field is greatest:

In the figure, there are three wires carrying currents in the indicated directions, and an Amperian loop. The sign of the contribution of the current in each wire to the path integral in Ampere's Law is:

Lines of the magnetic field produced by a long straight wire carrying a current are:

Two long straight wires enter a room through a window. One carries a current of 3.0 A into the room while the other carries a current of 5.0 A out. The magnitude of the path integral around the window frame is:

Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is:

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

Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is:

The diagrams show three circuits consisting of concentric circular arcs (either half or quarter circles of radii r, 2r, and 3r) and radial lengths. The circuits carry the same current. Rank them according to the magnitudes of the magnetic fields they produce at C, least to greatest.

Two parallel long wires carry the same current and repel each other with a force F per unit length. If both these currents are doubled and the wire separation tripled, the force per unit length becomes:

The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:

In the figure, if the current element has a length of 1.0 mm, carries a current of 2.5 A, and is a distance of 4.8 cm from the point P, what is the magnitude of the magnetic field at point P?

The magnetic field inside a long ideal solenoid is independent of:

A hollow cylindrical conductor (inner radius = a, outer radius = b) carries a current i uniformly spread over its cross section. Which graph below correctly gives B as a function of the distance r from the center of the cylinder?

Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in the same direction. The force per unit length of one wire on the other is:

In Ampere's law, , the integration must be over any:

Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is:

The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank the wires according to the magnitudes of the magnetic forces on them, from least to greatest.

Two long straight current-carrying parallel wires cross the x axis and carry currents I and 3I in the same direction, as shown. At what value of x is the net magnetic field zero?

In Ampere's law, the direction of the integration around the path:

A constant current is sent through a helical coil made of flexible wire. The coil:

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