A cylindrical pot magnet is shown in Fig. b$.$ The coil, which has

$1000$ turns and a resistance of $20,$ is connected to a constant$-$voltage

source of $60$ V DC$.$ The plunger passes the point at which the airgap is

$10$ mm travelling upwards with a uniform velocity of $2\mathrm{.}0$ m$/$s$.$ Make an

estimate of the value of the current at this instant, assuming that the

initial airgap length was $15$ mm$.$ Apportion the instantaneous input

power into heat, magnetic$-$energy storage$-$rate and mechanical

components. Hence estimate the magnetic force on the plunger.