# Homework Help: Formula for ohm resistance

1. Jul 20, 2008

### swampland

1. The problem statement, all variables and given/known data

What is the time constant of a 2mh inductor and a 200 ohm resistor connected in series?

2. Relevant equations

3. The attempt at a solution

2. Jul 20, 2008

### LowlyPion

Would you like to say attempt anything to do with the problem, or would you just like the answer?

3. Jul 20, 2008

### swampland

Hello LowlyPion, Could you please explain how i can solve the problem?

4. Jul 21, 2008

### alphysicist

Hi swampland,

I would start with figuring out section 2 for your original post. What equation do you think is relevant to finding the time constant of a resistor/inductor series circuit?

Once you find that equation I think you'll see why filling out all parts of the posting template can be helpful to you, since finding the equation is the hardest part of this problem. What do you get?

5. Jul 21, 2008

### swampland

Hello, I'am sorry but i am totally lost on this math, could you please do it and then explain it so i can get a idea of how you come up with the answer Thank You

6. Jul 21, 2008

### alphysicist

No, this forum has clear rules, and one rule is that no one is allowed to do your homework for you.

Try to answer the question in my last post: what equation is used to find the time constant in a resistor/inductor circuit? Once you find that equation in your textbook, I think you'll find the math is not very hard at all.

7. Jul 22, 2008

### swampland

8. Jul 22, 2008

### swampland

Hello, Would the formula be TC= L over R ?

9. Jul 22, 2008

### alphysicist

That looks right to me. What did you get for the answer?

10. Jul 23, 2008

### swampland

I came up with 1msec, That seem right?

11. Jul 23, 2008

### alphysicist

That's doesn't look right to me. You said the time constant was:

$$\tau = \frac{L}{R}$$

where L=2 mH and and R =200 ohms. Are you getting 1 msec=0.001 second from this?

If so, are you converting the inductance correctly? What number are you putting in your calculator for L?