FIGURE 22-6
-A single current-carrying circular loop of radius R is placed next to a long, straight wire, as shown in the sketch in Figure 22-6. The current in the wire points to the right and is of magnitude I. In which direction must current flow in the loop to produce zero magnetic field at its center?
A) counterclockwise
B) clockwise
C) It is indifferent if clockwise or counterclockwise.
D) The current in the loop should be zero A.

Question

FIGURE 22-6 
-A single current-carrying circular loop of radius R is placed next to a long, straight wire, as shown in the sketch in Figure 22-6. The current in the wire points to the right and is of magnitude I. In which direction must current flow in the loop to produce zero magnetic field at its center?
A) counterclockwise 
B) clockwise 
C) It is indifferent if clockwise or counterclockwise. 
D) The current in the loop should be zero A.

Quizplus · Accepted Answer

According to Ampere's Law, the magnetic field of the long straight wire is given by B = (μ0I)/(2πr), where r is the distance from the wire. At the center of the loop, the magnetic field due to the wire is purely radial and directed upward, perpendicular to the plane of the loop. To cancel this field, the loop must produce a magnetic field also directed upward at its center. By the right-hand rule, the direction of the magnetic field due to a counterclockwise current in the loop is upward, while the direction due to a clockwise current is downward. Therefore, the current in the loop must be clockwise to produce zero magnetic field at its center.

FIGURE 22-6 -A Single Current-Carrying Circular Loop of Radius R Is Placed