(a) What would have to be the self-inductance of a solenoid for it to store 10.0 $\mathrm{J}$ of energy when a 2.00 -A current runs through it? (b) If this solenoid's cross-sectional diameter is $4.00 \mathrm{cm},$ and if you could wrap its coils to a density of 10 coils/mm, how long would the solenoid be? (See Exercise $30.15 . )$ Is this a realistic length for ordinary laboratory use?