A single loop of wire is connected to an ammeter. The ammeter reports a positive

Question

A single loop of wire is connected to an ammeter. The ammeter reports a positive number when the current runs through the loop in the clockwise direction (as viewed from the top). When a bar magnet of unknown polarity is initially placed above the loop as pictured, and then rotated 180 degrees clockwise when viewed from the side, the ammeter reports a peak current of -2.4 MICRO Amperes.

a. Which end of the magnet is its north pole? Give a thorough explanation for how this is determined.

b. If the magnet is rotated at the same speed, but counter-clockwise (when viewed from the side) instead, will the ammeter report a positive current, negative current, or zero current? Explain your answer.

c. If the magnet is moved toward the loop instead of rotated, will the ammeter report a positive current, negative current, or zero current? Explain your answer.

d. If the magnet is moved away from the loop instead of rotated, will the ammeter report a positive current, negative current, or zero current? Explain your answer.

Explanation / Answer

To make an analysis of how the magnetic flux through the loop is, we could consider to use the right hand rule. And we must consider the Lenz's Law to explain in which direction the induced current is.

Recall the lenz's Law established that: "The induced current in a loop is in the direction that creates a magnetic field that opposes the change in magnetic flux through the area enclosed by the loop"

Then,

a) Since the current is negative when magnet rotates 180 degrees clockwise, the current is counterclockwise because a clockwise current is defined positive in the problem.

by the right hand rule, This current creates a magnetic field which is "outward of the plane" (viewed from the top) or upward (viewed from the side).

Since the Lenz's law establishes that this magnetic field must be opposite to the magnetic field of the magnet, the north pole of the magnet must be on the top of it.

b) If the magnet is rotated counterclockwise the direction of the inducce current changes and it is clockwise. therefore, the current is positive. (this is the opposite case of the part a)

c) Since the north pole is on the top of the magnet, when the magnet is moved toward the loop its magnetic lines are upward through the loop, in this case the induced current must create a magnetic field opposite to the change of external magnetic flux. Therefore, the magnetield of the loop is downward and by the right hand rule the current is clockwise.

d) in this case ocurrs the opposite to the previous part.

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