# Schaums Basic Elec Eng



## jdd18vm (Jul 22, 2007)

Can someone help me to understand problem 3.6? Sorry its cut off..its 60Hz and the word "the" the reactive power.

For (a) I don't understand how to solve for L whereby getting 45.94mH. Also the if Z=100/5= 20 when do i use that

in (B) i dont see why R=1 and Z=2?

John

sorry not sure why it posted twice, and the smiley was supposed to be the letter b


----------



## Flyer_PE (Jul 22, 2007)

John,

Hope this helps:

The are directly giving you the value of R at 10 ohms

The value of Z is indirectly given from 100V/5 amps as 20 ohms.

Since Z = r + jx the value of x is the square root of z^2 - r^2 = 17.32 ohms reactive

Since this is a 60hz system ohmega = 2*pi*f = 377

x = ohmega * L

Solving for L: L = 17.32/377 = 0.04594 H = 45.94mH

The power factor of the circuit is the cosine of the angle between the voltage and current. Since we know all sides of the triangle, the cosine is R/Z = 10/20 = 1/2

Let me know if this is of any help. I tend to mess these things up when I type them in.

Jim


----------



## jdd18vm (Jul 22, 2007)

IFR_Pilot said:


> John,
> Hope this helps:
> 
> The are directly giving you the value of R at 10 ohms
> ...



Jim, Thats great yes it helps, that I understand. Is the value of X always square root Z^2-R^2? Either I hadn't come across or didn't recognize it.

John


----------



## Flyer_PE (Jul 22, 2007)

jdd18vm said:


> Jim, Thats great yes it helps, that I understand. Is the value of X always square root Z^2-R^2? Either I hadn't come across or didn't recognize it.
> John



Yep. It's a simple right triangle. Z^2 = R^2 + X^2

Given any two, the third is simple algebra.

Jim


----------



## jdd18vm (Jul 22, 2007)

IFR_Pilot said:


> Yep. It's a simple right triangle. Z^2 = R^2 + X^2
> Given any two, the third is simple algebra.
> 
> Jim


When I sat back down at the kitchen table i slapped my forehead realizing what i just asked.

thanks again


----------



## Flyer_PE (Jul 22, 2007)

jdd18vm said:


> When I sat back down at the kitchen table i slapped my forehead realizing what i just asked.
> thanks again



Welcome to the wonderful world of studying things you haven't had to think about since graduation day. I spent a lot of time bashing head on desk studying for that damn test. I just wish I had found this forum earlier than I did.

Jim


----------



## jdd18vm (Jul 22, 2007)

IFR_Pilot said:


> Welcome to the wonderful world of studying things you haven't had to think about since graduation day. I spent a lot of time bashing head on desk studying for that damn test. I just wish I had found this forum earlier than I did.
> Jim


Its been 24 years, and I got a BS in Architecture, never had real Engineering courses. I think I'm screwed but i will keep plugging.

I found a student on craigslist, looking to tutor in guitar, is a Senior here at NC State in EE. I'm going to have him help me out with the fundamentals, as well as brush up on guitar.

I'll be posting a lot more I'm sure.

John


----------

