How Does Inductance Affect a DC Circuit?

[smell5]

An inductor of negligible resistance and an inductance of 0.2 H is
connected in series with a 330 Ω resistor to a 12V d.c. supply.
Determine:
(a) the time constant of the circuit
(b) the voltage drop across the inductor after two time constants
(c) the voltage drop across the resistor after three time constants
(d) the resistance of a 0.2 H coil used to replace the inductor if the
circuit’s time constant falls to 0.55 ms.

This is the full question, I have calculated A, B, and C, however I am struggling with D, any help would be great and clarification of my current answer would be even better

(a) 0.606ms
(b) 1.62V
(c) 11.4V
(d) :'(

 

[ehild]

The coil is equivalent with an ideal inductor and a resistor connected in series with it. The time constant is related to the inductance and the total resistance in the loop. You know the total resistance from the new time constant...

ehild

 

[smell5]

so, resistance = inductance / time constant
R = 36 ohms?

 

[NascentOxygen]

so, resistance = inductance / time constant
R = 36 ohms?

Close. Try calculating it again.

 

[smell5]

Hmmm, I still get the same answer. Am I along the right track with the equation used?

 

[NascentOxygen]

Show your calculations.

 

[smell5]

I actually don't know where i got that answer from, my calculations are now something way different.

I am completely off track here, I am all confused.

 

[smell5]

is the answer 33 ohms!

 

[vela]

You're probably not going to get much help if it's only going to be a guessing game as to what you've actually done. Show your calculations.

 

[CWatters]

You don't normally get many marks for the answer, most of the marks are for showing how you got there.

 

[NascentOxygen]

is the answer 33 ohms!

That looks right.