CH09

Complete the following statement: The magnetic field around a current-carrying, circular loop is most like that of

• the Earth

• a current-carrying, rectangular loop

• a short bar magnet

• a long, straight, current-carrying wire

• two long, straight wires that carry currents in opposite directions

Two parallel wires have currents that have the same direction, but differing magnitude. The current in wire A is i; and the current in wire B is 2i. Which one of the following statements concerning this situation is true?

• Wire A attracts wire B with half the force that wire B attracts wire A.

• Wire A attracts wire B with twice the force that wire B attracts wire A.

• Both wires attract each other with the same amount of force.

• Wire A repels wire B with half the force that wire B attracts wire A.

• Wire A repels wire B with twice the force that wire B attracts wire A.

The drawing represents a device called Roget’s Spiral. A coil of wire hangs vertically and its windings are parallel to one another. One end of the coil is connected by a wire to a terminal of a battery. The other end of the coil is slightly submerged below the surface of a cup of mercury. Mercury is a liquid metal at room temperature. The bottom of the cup is also metallic and connected by a wire to a switch. A wire from the switch to the battery completes the circuit. What is the behavior of this circuit after the switch is closed?

• When current flows in the circuit, the coils of

  the wire move apart and the wire is extended

  further into the mercury.

• Nothing happens to the coil because there will

  not be a current in this circuit.

• A current passes through the circuit until all of the

  mercury is boiled away.

• When current flows in the circuit, the coils of the

  wire move together, causing the circuit to break at the surface of the mercury. The coil then

  extends and the process begins again when the circuit is once again complete.

Three very long, parallel wires (a small portion of each is shown in the drawing) are resting on a flat surface. The distance between wire B, which has a 15 mA current to the left, and its neighbors is 0.0015 m. Wire A carries a 10 mA current toward the right; and wire C carries a 5 mA current toward the right. Rank the wires in order of the magnitude of the net magnetic force on each, with the largest value first and the lowest value last.

• A > B > C

• B > A > C

• C > B > A

• A > C > B

• B > C > A

A copper cylinder has an outer radius 2R and an inner radius of R and carries a current i. Which one of the following statements concerning the magnetic field in the hollow region of the cylinder is true?

• The magnetic field within the hollow region may be represented as concentric circles with the direction of the field being the same as that outside the cylinder

• The magnetic field within the hollow region may be represented as concentric circles with the direction of the field being the opposite as that outside the cylinder.

• The magnetic field within the hollow region is parallel to the axis of the cylinder and is directed in the same direction as the current.

• The magnetic field within the hollow region is parallel to the axis of the cylinder

  and is directed in the opposite direction as the current.

• The magnetic field within the hollow region is equal to zero tesla.

The drawing shows two long, thin wires that carry currents in the positive z direction. Both wires are parallel to the z axis. The 50-A wire is in the x-z plane and is 5 m from the z axis. The 40-A wire is in the y-z plane and is 4 m from the z axis. What is the magnitude of the magnetic field at the origin?

• zero tesla

• 1 x 10^-6 T

• 3 x 10^-6 T

• 5 x 10^-6 T

• 7 x 10^-6 T

The drawing shows a rectangular wire loop that has one side passing through the center of a solenoid. Which one of the following statements describes the force, if any, that acts on the rectangular loop when a current is passing through the solenoid.

• The magnetic force causes the loop

  to move upward.

• The magnetic force causes the loop

  to move downward.

• The magnetic force causes the loop

  to move to the right.

• The magnetic force causes the loop to move to the left.

• The loop is not affected by the current passing through the solenoid or the magnetic field resulting from it.

An initially unmagnetized iron bar is placed next to a solenoid. Which one of the following statements describes the iron bar after the solenoid is connected to the battery?

• A magnetic force accelerates the bar to the right.

• Since the bar is unmagnetized, there will not be any affect on the bar.

• The magnetic field of the solenoid will cause a current to flow in a loop that extends from one end of the bar to the other and that continues until the battery is disconnected from the solenoid.

• The magnetic field of the solenoid induces magnetism in the bar with the bar’s north pole nearest to the solenoid.

• The magnetic field of the solenoid induces magnetism in the bar with the bar’s south pole nearest to the solenoid.

A single circular loop of wire with radius 0.020 m carries a current of 8.0 A is placed at the center of a solenoid that has length 0.65 m, radius 0.080 m, and 1400 turns. Determine the value of the current in the solenoid so that the magnetic field at the center of the loop is zero tesla.

• 1.4 × 10^-1 A

• 2.5 × 10^-4 A

• 4.4 × 10^-2 A

• 5.0 × 10^-3 A

• 9.3 × 10^-2 A

The drawing shows a toroid that has an inner radius of 0.040 m and an outer radius of 0.060 m. The toroid is composed of 120 turns and carries a constant current of 0.50 A. What is the magnetic field at the center of the toroid, where r = 0?

• zero tesla

• 1.2 × 10^-5 T

• 2.3 × 10^-3 T

• infinity

• This cannot be determined without more information.