# To practice Tactics Box 25.1Using Lenz’s law. Lenz’s law is a useful rule for determining…

To practice Tactics Box 25.1 Using Lenz’s law. Lenz’s law is a useful rule for determining the direction of the induced current in a loop. Specifically, it says that there is an induced current in a closed conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field opposes the change in the flux. The following Tactics Box summarizes the essential steps in using Lenz’s law.

TACTICS BOX 25.1 Using Lenz’s law

1. Determine the direction of the applied magnetic field. The field must pass through the loop.

2. Determine how the flux is changing. Is it increasing, decreasing, or staying the same?

3. Determine the direction of an induced magnetic field that will oppose the change in the flux:

• Increasing flux: The induced magnetic field points opposite the applied magnetic field.

• Decreasing flux: The induced magnetic field points in the same direction as the applied magnetic field.

• Steady flux: There is no induced magnetic field.

• Determine the direction of the induced current. Use the right-hand rule to determine the current direction in the loop that generates the induced magnetic field you found in step 3.

• Follow the steps above to solve the problem: A wire carries a current in the direction indicated in the figure. A loop is located next to the wire. If the current in the wire increases, is there a clockwise current around the loop, a counterclockwise current, or no current? (Figure 1)

Part A) Determine the direction of the magnetic field of the current-carrying wire.

Part B) As the current in the wire increases, the magnetic field around the wire increases in magnitude without changing its direction. Does this affect the flux through the loop, and if so, how?

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