**Unformatted text preview: **Next, after a Very longtime, the
“Clock?- is reset to. 0 and the switch.
is thrownrto position b;- (d) What is the time-constant: rdescribing the change in current through the inductor?
We have a new formula available for. time constants in LR circuit-s: r = UR. But the R” in the
fonnula. refers to the tore-Z resismnee "in series with the”; inductor. Redrawing yell}: circuit-will
help you to determine" this R! Essentially the inductor discharges through R1 and R1- in parallel. RR ﬁtm-
R1+RZ 6+4
L '15.>.<10'_3 The time constant is r = — = — = 6.25 ms
R 2.4 E The equivalent- resistance is RE (e) Sketch the time dEPﬂldeﬂce inf-the current. through the inductor”. The current. dies .a‘eiay exponentiallystalting from the Imax
computed. in part (1)) according to. I = 11m exp (—r -_/ r) and resembles the ﬁgures (I) Whatis the-energy stored injthe‘ inductor-:3ruse-Rafter the-switch gocs'to position 11.?”
First you must write down an equation fer the time-dependence of the current. Che ole-that
your formula is correCt: does-it produce Zthe-rightxanswer- atftime 02? What about. at. -.t. = oo? . LI2 L L» . - L12 2 '-
The. stored energy 1's U = 2 = 3.]: = Eﬂmﬁexth/ﬂf 2 £1“ exp{——t]
. - . - 1- 2x8ms where Um: is: value calculated in part (c). Thus U = 20.83 m] 6x196?) = 1.61 In]
.- . . ' - m3 ...

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- Fall '08
- RIEGER