Problem 2

An ideal DC source of emf

\mathcal E

is connected in series with a resistor

R>0

and ideal inductor

L>0

. Before the switch closes the inductor current is zero; at

t=0

the switch closes, with positive current chosen along the source. Neglect internal resistance and take all elements as constant. (a) Use Kirchhoff's law to find

I(t)

for

t\ge0

. (b) Determine the steady current

I_\infty

. (c) Find when the current reaches

0.90I_\infty

and check the initial and long-time limits.

problems.proof.stepOf

problems.proof.prevStep problems.proof.nextStep

\mathcal E=RI+L\frac{dI}{dt},\quad I(0)=0

problems.proof.analysis

Kirchhoff's law equates source emf to the resistor and inductor voltage drops. The condition $I(0)=0$ fixes the transient solution.

problems.proof.pitfall. Do not omit the back emf, which opposes changes in current.