Magnetic flux (20-1)

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Question

Magnetic flux through a surface

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Review

It turns out that a simple way to describe the emf in any of these situations is in terms of the change in magnetic flux through the loop in Figures 20-2 and 20-3. We define this in the same way that we defined electric flux in Section 16-6: It’s the area $A$ of the surface outlined by the loop, multiplied by $B \cos{\theta}$ , the component of the magnetic field that’s perpendicular to that surface (see part (a) of Figure 20-4). In equation form, the magnetic flux $\Phi_B$ (“phi-sub-B”) through the loop is

(The subscript $B$ reminds us that this is the flux of the magnetic field $\vec{B}$ .) As parts (b) and (c) of Figure 20-4 show, the flux $\Phi_B$ can be positive or negative.