## PHYS208 Fundamentals of Physics II

### Hint for Ch. 31, 63E

a) Use Eqn.31-46 of *HRW* and solve for *i* = 0.80 *i*_{final}.

b) Evaluate the same equation for *Rt/L* = 1.

### Hint for Ch. 31, 67P

Take the time derivative of both sides of Eq. 31-46, the current as a function of time
in an energizing inductor.

### Hint for Ch. 31, 68P

a) Approximate the number of turns by dividing the circumference of the inner diameter
by the diameter of the wire, which is 628. Use formula 31-37 for the inductance of a toroid.

### Hint for Ch. 31, 69P

a) Immediately after closing switch, the current in the inductor is zero.

b) After a long time, the inductor is fully energized; with the current no longer
changing, the emf across the inductor is zero. In effect the inductor can be replaced by
a zero-ohm wire for the purpose of circuit analysis for *a long time after switch closure*.
Solve the resulting multiloop circuit for the currents.

c) *i*_{1} = 0 immediately on the switch opening
(there is no inductance indicated in that branch).
The rightmost branch with the inductor is going
to keep the same value of current that it had just before the switch was opened.

d) The power source has been removed and there is dissipation in the circuit.

"http://www.physics.udel.edu/~watson/phys208/exercises/hints/0420.html"

Last updated April 20, 1998.

Copyright George Watson, Univ. of Delaware, 1998.