1 \begin{problem*}{23.22}
2 A rectangular coil with resistance $R$ has $N$ turns, each of length
3 $l$ and width $w$ as shown in Figure P23.22. The coil moves in a
4 uniform magnetic field \vect{B} with constant velocity $v$. What are
5 the magnitude and direction of the total magnetic force on the coild
8 \Part{b} moves within, and
11 \end{problem*} % problem 23.22
15 As in Problem 13, $d\Phi_B/dt = wBv$, so the induced current is
17 I = \frac{\varepsilon}{R} = \frac{-d\Phi_B/dt}{R} = \frac{-wvBN}{R} \;,
19 where the $-$ sign indicates it is counterclockwise (against the
20 changing flux direction). The force on the leading wires is
22 \vect{F} = I\vect{l}\times\vect{B} = -I\cdot Nw\cdot B\ihat
23 = \ans{\frac{-v(wBN)^2}{R}\ihat} \;.
27 Once the coil is inside the magnetic field, the flux becomes constant,
28 so there is no induced emf driving a current, and thus \ans{no net
32 The situation here is the inverse of that in \Part{a}, so the induced
33 emf is clockwise, but the current through the portion of loop in the
34 magnetic field is \emph{still up}, so the force is unchanged.
36 \vect{F} = \ans{\frac{-v(wBN)^2}{R}\ihat}