# Homework Help: Coil resistance question

1. Sep 16, 2014

### ilovescience85

Good afternoon all,

I recently submitted a assignment and my lecturer has questions a figure i have put as an answer.

Question was; 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: 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 used R=L/t = 0.2/0.00055 = 363.64ohms
circuit original resistance is 330Ω so an additional 63.64Ω would be apostioned to the new coil??? He has asked where the 63.64ohms has come from??

Am i right in what i've siad or have i totally misunderstood?

Any help would be great, i still passed the assignment but just want to be sure.

Thanks

2. Sep 16, 2014

### Staff: Mentor

Your method new total resistance look fine. But pay attention to the old resistance: was it 300 or 330 Ohms?

Last edited: Sep 16, 2014
3. Sep 16, 2014

### ilovescience85

4. Sep 16, 2014

### Staff: Mentor

Ta da!

5. Sep 16, 2014

### ilovescience85

Hi sorry the 33.64ohms was the answer I had put above was just a typo. Why is he questioning this figures origin?

6. Sep 16, 2014

### Staff: Mentor

If your answer was 33.64 Ω then I don't know why he would question it, unless you didn't show your work.

The alternative is that he misunderstood the question and assumed that the 330 Ω resistor was replaced as well. Then the new coil would have to have the full resistance value of about 364 Ω.