- #1

fawk3s

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## Homework Statement

There are two circuits I don't entirely understand. I am also new to calculations with complex numbers and when to and when not to use them, so be easy on me. Anyways, the point is to find out what the different meters are showing, when you are given the initial voltage and the frequency of the current.

Here's the first one:

http://desmond.imageshack.us/Himg141/scaled.php?server=141&filename=21362017.png&res=landing

So basically, its a circuit consisting of 2 resistors and an inductor. Whats interesting, but also confusing to me, is what the wattmeter is showing. The voltmeter U2, which is connected to basically the same point as the wattmeter, is showing 138 V. The ampermeter I1 is showing 1.24 A, which is measuring the same current as the wattmeter. That gives us the power P=1.24*138=171.12 W.

By doing some calculations I found out that 171.12 W is the power on the inductor and the resistor R2 together, on their impedance. But 92 W is the power on the resistor R2 alone.

So I guess my question here would be why is it so? How does the wattmeter work? I always thought that the wattmeter would measure the voltage and current, then somehow multiply them, and give us the result. But clearly, the multiplication of what the voltmeter and the ampermeter are showing us gives us a different result.

I do understand that the inductor preserves evergy as a magnetic field and then releases it, but I am still confused here. The voltmeter registers the voltage on the inductor, why doesn't the wattmeter?

The second situation I don't understand:

http://desmond.imageshack.us/Himg16/scaled.php?server=16&filename=ac2h.png&res=landing

In this case, I don't really understand much. I am guessing you can add the capacities together, because they are in parallel. So that would give us a single 31 μF capacitor. But where to go from there? I am guessing we need the total impedance to find the current in I1. From there on I am guessing we could find U0 and U2. But how to find the impedance in this case? I tried a few ways but I guess they were wrong because the numers didnt match the ones in the picture. And again, why is the wattmeter showing the sum of the powers on resistors R2 and R3?

EDIT: for the ones who don't see the images for some reason:

1. http://desmond.imageshack.us/Himg141/scaled.php?server=141&filename=21362017.png&res=landing

2. http://desmond.imageshack.us/Himg16/scaled.php?server=16&filename=ac2h.png&res=landing

Thanks in advance,

fawk3s

## Homework Equations

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