Physics flux question

Hi can some one help me understand why the answer to this question is C. Also why is a current or emf created in this situation when you move a conductor in a B field? I thought that there is no change in flux, its simply moving in a fairly uniform magnetic field.


  • 1
    $\begingroup$ What do you mean by: "there is no change in flux". Through what area are you checking the flux? $\endgroup$ – npojo Jan 19 '18 at 21:15
  • $\begingroup$ This situation may be easier to understand in terms of "motional EMF"; is that something that you've seen in your class? $\endgroup$ – Michael Seifert Jan 19 '18 at 21:45
  • $\begingroup$ @npojo wait is the wire experiencing a change in flux because part of the loop is being lifted so the area of the wire loop is increasing - .hence change in flux? $\endgroup$ – user341191 Jan 20 '18 at 1:11
  • $\begingroup$ @MichaelSeifert no i dont think we have. $\endgroup$ – user341191 Jan 20 '18 at 1:11
  • $\begingroup$ @user341191 - yes. $\endgroup$ – npojo Jan 20 '18 at 6:10

The flux is the constant $\vec{B}$ field times the effective area of the loop, where by "effective" I mean the projection of the area in the (signed) plane normal to $\vec{B}$--and that changes.

Step 1 is establish the direction of $\vec{B}$ in drawing: It is south-to-north, which would be "into" the drawing.

The next step is to figure out when the effective area is changing the fastest. This occurs at "A" and "C". At "B" ("D") the area is maximal (minimal)-- which is where it's stationary, i.e., changing the least.

Step 3 is figuring out the sign of the effect so that current is flowing from "P" to "Q". You can use the formula, or appeal to Lenz's Law.

Lenz's Law states the current will flow to maintain the flux. Since the projected area is decreasing at "C", you need a current that adds flux "into" the drawing. A quick check of the right-handed rule should convince you that this occurs when current flows from "P" to "Q"--which is the desired result. Hence C: "C" is correct.


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