# Homework Help: Determining the constants R and L of a coil

1. Jun 8, 2010

We determined the constants R and L of a coil by placing it in series with a standard resistor of 10 ohms and reading the voltages on the terminals of Rs, the coil and the series circuit complete. What are the values of R and L for the following voltage readings at 60 Hz:
Vrs = 20; Vcoil = 22,4; Vt = 36

R = 5 ohms
L = 26,5mH

I find the current
Vrs = I . Zr
20 = I . 10 I = 2A
Now I do not know how to find R and L

2. Jun 9, 2010

### cartonn30gel

Re: Impedance

Is this an experiment you've done? Do you have a circuit diagram for this? I'm curious because those voltages don't agree with each other. (if they are measured at the same time, and the circuit is what I think it is)

A diagram is almost a necessity for any circuit question since a small change might significantly alter how current and voltage behave.

3. Jun 9, 2010

### cartonn30gel

Re: Impedance

Is this an experiment you've done? Do you have a circuit diagram for this? I'm curious because those voltages don't agree with each other. (if they are measured at the same time, and the circuit is what I think it is)

A diagram is almost a necessity for any circuit question since a small change might significantly alter how current and voltage behave.

4. Jun 9, 2010

### ehild

Re: Impedance

The current is OK. You know that in Ac circuits, the voltage is impedance times current.
Usually the ohmic resistance of a coil can not be ignored, so you need to treat the coil as an ideal inductor with impedance wL and resistor R in series.

This resistance of the coil adds to the resistance of the total circuit,
too.

Do you know how to calculate the resultant impedance of an inductor L and a resistor R connected in series?

ehild

5. Jun 9, 2010

### cartonn30gel

Re: Impedance

All right, I get it now. The question should mention that those are the AC peak voltages. You should use phasor magnitudes in your calculations. So they aren't measured at the same time.

6. Jun 9, 2010

### ehild

Re: Impedance

Or rather RMS voltages. The AC voltmeters read the RMS voltage usually.

ehild