# High school magnetic flux physics 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.

Thanks

• What do you mean by: "there is no change in flux". Through what area are you checking the flux? – npojo Jan 19 '18 at 21:15
• This situation may be easier to understand in terms of "motional EMF"; is that something that you've seen in your class? – Michael Seifert Jan 19 '18 at 21:45
• @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? – user341191 Jan 20 '18 at 1:11
• @MichaelSeifert no i dont think we have. – user341191 Jan 20 '18 at 1:11
• @user341191 - yes. – 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.