(a) What would have to be the self-inductance of a solenoid for it to store 10.0 $\mathrm{J}$ of energy when a $1.50-\mathrm{A}$ current runs throught it? $(\mathrm{b})$ 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 $30.11 . )$ Is this a realistic length for ordinary laboratory use?