26. Let Rbe the radius of a capacitor plate and rbe the distance from axis of the capacitor. For points
with r≤R, the magnitude of the magnetic field is given by
B=µ0ε0r
2
dE
dt ,
and for r≥R,itis
B=µ0ε0R2
2r
dE
dt .
The maximum magnetic field occurs at points for which r=R, and its value is given by either of the
formulas above:
Bmax =µ0ε0R
2
dE
dt .
There are two values of rfor which B=Bmax/2: one less than Rand one greater. To find the one that
is less than R,wesolve µ0ε0r
2
dE
dt =µ0ε0R
4
dE
dt
for r. The result is r=R/2=(55.0mm)/2=27.5 mm. To find the one that is greater than R,wesolve
µ0ε0R2
2r
dE
dt =µ0ε0R
4
dE
dt
for r. The result is r=2R=2(55.0 mm) = 110 mm.
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