Discussion in 'Homework Help' started by Hawkeye87, Nov 4, 2008.

Oct 7, 2008
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2. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
In this case you can use use c^2=a^2 + b^2 for the magnitude of the resultant vector, then use any trig identity to solve for the angle.

theta = arctan(8/6)

3. ### Ratch New Member

Mar 20, 2007
1,068
3
Hawkeye87,

Although the reference you cited talks about vectors, voltage is not a vector quantity. It has magnitide but not direction. Alternating voltages can be represented by phasors, which have magnitude and an angular relationship between them. Phasors also have some vector like properties, but not all of them. To add voltage phasors, split up each into real and orthogonal parts. Add up the real parts and the orthogonal parts. Then convert back to polar form if desired, or leave it like it is. scubasteve_911 touched on how to convert to polar form.

Ratch

4. ### Hawkeye87 Thread Starter Active Member

Oct 7, 2008
52
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I figured out that question and i now have yet another....
If
V1 = 80sin(ωt + 25°)
V2 = 10sin(ωt - 15°)
find the sum Vt=V1+V2 where the angular velocity (ω) is 400.
Where do you start?

5. ### Ratch New Member

Mar 20, 2007
1,068
3
Hawkeye87,

Angular velocity is irrelevant, so long as it is the same for both voltages.

80/_25 = 72.50 +j33.81
10/_-15 = 9.66 +j-2.59

72.50+9.66 + j33.81-j2.59 = 82.16 + j31.22 = 87.90/_20.81

Ratch

6. ### Hawkeye87 Thread Starter Active Member

Oct 7, 2008
52
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now how would i calculate the frequency, period, peak to peak voltage of V1, the rms voltage of V2, the phase difference between V1 and V1 and the time difference between the two. Just set me up the equations and i'll have at it.

7. ### Ratch New Member

Mar 20, 2007
1,068
3
Hawkeye87,

2*Pi*f = w = 400

P = 1/f

You have to specify what the voltages given were.

25 degrees - 15 degrees = 10 degrees

(10/360)*P

I feel so guilty and ashamed for "helping" you so much. You will probably flunk out unless you can do problems like this yourself.

Ratch

8. ### Hawkeye87 Thread Starter Active Member

Oct 7, 2008
52
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don't feel guilty. your helping someone who has a test friday that knows maybe half this. So consider the fact that your aiding my learning process by filling in the gaps that the teacher leaves out.

9. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
Ratch, do you see anything wrong with the first paragraph on this page:

http://en.wikipedia.org/wiki/Phasor

Last edited by a moderator: Nov 5, 2008
10. ### Ratch New Member

Mar 20, 2007
1,068
3
The Electrician,

No, I don't. But that is not the page that Hawkeye87 referenced.

Ratch

11. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Back on topic, if you please, gentlemen. Let's help the O.P. with complex numbers, not argue about semantics.

Ratch: if you have a problem with the AAC website, make your comment in the appropriate forum - this one is for helping folk with homework.

12. ### Ratch New Member

Mar 20, 2007
1,068
3
thingmaker3,

I did help the OP, perhaps more that I should have. It is not a matter of semantics, it is about facts. Voltage is not a vector quantity.

I did not dwell on the mistake in the AAC, just a quick remark to get the OP on the right track. Which was helping him with his homework.

Ratch