# Homework Help: Inductance in DC network

1. Jan 31, 2013

### jimmy4

1. 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 calculated A, B, and C, however I am struggling with D.

(a) 0.606ms
(b) 1.62V
(c) 11.4V

2. Relevent Equations. resistance = inductance / time constant

2. Jan 31, 2013

### Staff: Mentor

Given the new time constant value, what is the total resistance? (Hint: use the formula you gave as a relevant equation).

If the external resistor comprises 330 Ω of that total resistance, how much should be ascribed to the new coil?

3. Jan 31, 2013

### jimmy4

resistance = inductance / time constant

R = 0.2H/0.55ms

R = 36 Ohms

Not sure much should much should be ascribed to the new coil, or how this is calculates into the equation

4. Jan 31, 2013

### Staff: Mentor

That resistance looks a bit too small. Check your units' powers of ten.

5. Jan 31, 2013

### jimmy4

R = 360 Ohms
Original Resister = 330 Ohms
Therefore resistance through the coil = 30 Ohms

6. Jan 31, 2013

### Staff: Mentor

That looks fine.

7. Jan 31, 2013

### jimmy4

Cheers gneill thanks a lot mate.

8. Feb 24, 2014

### hopkid

Hi guys, sorry to butt in on this post but I am currently completing this question and I'm struggling with b & c. I've worked through the lesson but cannot see any equations that I can use for the voltage drop across the resistor or the inductor. Would either of you be able to point me in the right direction? I've been stuck for a few days now so any guidance would be greatly appreciated. Thanks in advance.

9. Apr 9, 2014

### Ebies

The equation you are after is i=I[1-e^-Rt/L] and you will need to calculate I first before you can use the equation

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