How Does Inductance Affect a DC Circuit?

AI Thread Summary
An inductor with negligible resistance and an inductance of 0.2 H is connected in series with a 330 Ω resistor to a 12V DC supply. The time constant of the circuit is calculated as 0.6 ms using the formula t = L/R. The user is uncertain about calculating the voltage drops across the inductor and resistor after specified time constants but is working on it. For the new inductor's resistance, the user initially calculated it as 363.63 Ω but was corrected to consider the existing 330 Ω resistor, leading to a final resistance of 33 Ω for the coil. The discussion clarifies the importance of accounting for total circuit resistance in calculations.
debreets
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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.

I have answer a) using formula t = L/R L is 0.2 and R being 330, t giving 0.0006 seconds ( or 0.6 ms )
Im unsure of b) and c) but will spend time on those shortly.
Part d) looked simular to a) I thought maybe use the same formula (t=L/R) but make R the subject, that would be R=t/L.
I plugged in 0.00055 for t and 0.2 for L, giving an answer of 363.63? time was given as 0.55 ms so converted to seconds giving 0.00055.
Am I on the right track or have I gone wrong?
 
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debreets said:
I know this is an old thread but I am struggling also, I have answer a) using formula t = L/R L is 0.2 and R being 330, t giving 0.0006 seconds ( or 0.6 ms )
Im unsure of b) and c) but will spend time on those shortly.
Part d) looked simular to a) I thought maybe use the same formula (t=L/R) but make R the subject, that would be R=t/L.
I plugged in 0.00055 for t and 0.2 for L, giving an answer of 363.63? time was given as 0.55 ms so converted to seconds giving 0.00055.
Am I on the right track or have I gone wrong?

They are asking for the resistance of a new inductor if it has the same inductance and it is put in series with the 330 Ohm resistor. So, you have a number that is on the way to getting the right answer.
 
Thanks for your reply,
I'm confused as I believe iv worked out the resistance of the inductor at 363.63 ohms, so would the resistance of the coil not be the same?
 
debreets said:
Thanks for your reply,
I'm confused as I believe iv worked out the resistance of the inductor at 363.63 ohms, so would the resistance of the coil not be the same?

No, because this new circuit is still factoring in the 330 Ohm resistor that is connected when you are calculating the total resistance.
 
Ok, so, 363 (reistance of ciruit) - 330 (resistance of resistor in circuit) = 33 ohm (resistance of coil)
Think iv got it now,
Thanks for your help
 
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