# AC inductance problem issue.

Discussion in 'Homework Help' started by Neil Hatley, May 5, 2016.

1. ### Neil Hatley Thread Starter New Member

May 5, 2016
3
0
I am currently having issue with this Inductance problem. All that need to be solved is the resisatnce value but seems there is not enough info provided in the problem. The formula I found was no help for this problem.

I used multisim but I could not figure out how to input inductance to give me the correct 9 v on the right side of the problem. Also when it says k=0 i'm really lost on that.

I did 12*.47nH which gave me 5.64 V
I then subtracted 9-5.64v and got 3.36 V
I am lost at this point not sure if i need to do 3.36 x voltage remaining to find missing resistance.
Thanks

2. ### Veracohr Well-Known Member

Jan 3, 2011
559
77
First, you're looking for an inductance, right? The question asks for the value of L2. It's an inductor. Not a resistor.

Second, you're given the input voltage and the voltage across L2 which means you can calculate the voltage across L1. And since you're also given the current through the circuit you should also be able to calculate the impedance of L1 and the source frequency. And knowing the frequency, the current, and the voltage across L2, you should be able to calculate the value of L2.

I don't know from the info provided what 'k' is, but since the question involves two inductors I assume it means a coupling coefficient.

3. ### Neil Hatley Thread Starter New Member

May 5, 2016
3
0
Ok so I did some calculations and got
Voltage across L1 = 3 v
The resistance is 120 Ohms.

I just really am having problems with this problem. Not sure how to calculate the frequency without knowing the value of L2. Also without the frequency I can not calculate the impedance

4. ### Veracohr Well-Known Member

Jan 3, 2011
559
77
I don't know how you're arriving at any of the numbers you've calculated (other than L1=3V).

Do you understand the difference between impedance and resistance? Impedance is frequency dependent but resistance isn't. Since your problem doesn't state a resistance of either inductor you're probably supposed to assume it's 0. Therefore the impedance is entirely due to the inductance. If you've been given this problem to solve I assume you have learned how the impedance (or AC reactance) of an inductor depends on frequency.

If you know that you can use the information given. Start with L1. You have a voltage across it (3V as you said), and the given current value (100mA). This gives you an impedance for L1. You should have a formula that tells you the impedance of an inductor at a given inductance and frequency. You should be able to use that formula to calculate the frequency.

Once you know the frequency you should be able to calculate the value of L2 because you have the frequency, current through, and voltage across L2. You can easily calculate the impedance of L2, and you use the same formula as for L1 to calculate the inductance of L2 with the values you have calculated for frequency and impedance of L2.

5. ### crutschow Expert

Mar 14, 2008
13,496
3,373
You are getting hung up on looking for a cookbook formula to solve the problem without trying to understand the problem.

Using Multisim for this problem is way overkill.

Here's a hint: Since there are two inductors in series, the voltage division between them is independent of the frequency, since both their impedances change at the same ratio with frequency.

The current through them is also not needed to solve the problem unless you do want to solve for the signal frequency, which the problem did not ask for.
You stated you could not solve for frequency but you can.
You have the voltage across the inductor L1, and you know the current through it.
From that you can solve for its impedance, and thus the frequency that causes that inductive impedance for the known inductance.

6. ### MrAl Distinguished Member

Jun 17, 2014
2,554
515
Hi,

It sounds like you are approaching this in the wrong way.
You might note that it is a voltage divider, made out of inductors. That means you either have to know some method of circuit analysis or know how voltage dividers work. A hint is that L2 will be larger than L1 because there is a large percent of the voltage (12v) across that inductor (L2).

7. ### Neil Hatley Thread Starter New Member

May 5, 2016
3
0
So after spending a lot of time on this problem i am still lost not sure how this works and being in micro henreys makes it even harder.

8. ### DGElder Member

Apr 3, 2016
347
87

Can you solve this?

Vin = 24V
R1 = 6K ohms
Vout = 18V
R2 = ?
What is the current in the circuit?

9. ### crutschow Expert

Mar 14, 2008
13,496
3,373
It's just a voltage divider circuit, similar to the resistive divider show in post #8.
And why should the inductance being in micro henries make any difference?
The formula is the same, it just changes the exponent on the number.

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
511
Where did you find 0.47 nanoHenry?
The problem shows 470 microHenry, which is the same as 0.470 milliHenry.

The way you are doing the problem tells me that you don't have a clue on what you are doing.
For example:
Second inductor has 9 volts across it, 100 mA (0.1 A) through it.
Reactance of an inductor is wL
Apply Ohm's Law:
9=0.1*wL
wL=90 Ohm
Then, we know that voltage across other inductor is 12-9=3 volts. We know the inductance of other inductor, 470 uH, we know the current through other inductor, 100 mA.
Apply Ohm's Law:
3=0.1*w*0.000470
Now go back to second inductor;
wL=90 Ohm
63830*L=90
L=1.41 mH

This is first and last time I am doing it for you. Go read the freaking textbook, you probably paid 120 bucks for the freaking thing.

11. ### MrAl Distinguished Member

Jun 17, 2014
2,554
515
Hi,

As long as you have some algebra experience you can solve this.
Take a look at that resistive voltage divider in the post that follows yours above.
It is better to start with resistor circuits than to try to jump into inductors and capacitors.
If you can solve that resistor circuit then you can solve the inductor circuit. We can go into more theory too if you like about why and how it works.
You may also want to go over some basic theory like:
"The current in a series circuit is the same in every element in that circuit".
"The sum of voltage drops around a closed current path is equal to zero".