In this passage, a “loop” refers to any closed segment of wire, such as the metallic wire rectangle in figure 1. If the plane of the loop is perpendicular to the magnetic field, and if the magnetic field is uniform over a discrete region of space (as in figure 1), then the magnetic flux through the loop is simply
F = BAin
where B is the magnetic field strength, and Ain is the area of the loop inside the magnetic field. For instance, if one third of the loop is “immersed” in the magnetic field, then Ain is one third the area of the whole rectangle.
Faraday's law of induction implies that, whenever the magnetic flux through a metal loop changes, current flows around the loop. No current flows when the magnetic flux stays constant.
In figure 1, the magnetic field points into the page with strength B0, inside the dashed lines. Outside the dashed lines, the field vanishes.
Starting from the position drawn in figure 1, the loop is pushed rightward at constant speed, all the way through the magnetic field.
FIGURE 1

1. | Which graph best shows the magnetic flux through the rectangle as a function of time?

A. | A

B. | B

C. | C

D. | D

2. | Current flows around the rectangle when the rectangle is: I. Entering the magnetic field II. Fully inside the magnetic field III. Leaving the magnetic field

A. | I only

B. | I and II

C. | I and III

D. | I, II, and III

3. | Which diagram best represents the magnetic field lines generated by one side of the rectangle, when current flows through it?

A. | A

B. | B

C. | C

D. | D

4. | Which of the following could NOT cause current to flow around this metal rectangle?

A. | Hold the rectangle motionless near a power line through which the electric current is decreasing.

B. | Move the rectangle towards a bar magnet.

C. | Move a bar magnet towards the rectangle.

D. | Place a motionless bar magnet inside the rectangle.

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