# Help.. Finding voltage and power

Discussion in 'Homework Help' started by Vortec, Aug 12, 2007.

1. ### Vortec Thread Starter New Member

Aug 12, 2007
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Need help on what should do for this problem.

I understand that V=IR, so I've tried finding the total resistance in this circuit, but I'm stucked with the current part. What should i do to calculate the current?

Thanks.

Question: Find the voltage Vg and the power dissipated in the 20ohm resistor.

View attachment qn.bmp

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
This is a straightforward problem to test your knowledge of how to simplify a complex resistor network. The solution lies in applying the rules for combining resistors in parallel and series.

hgmjr

3. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Take a look at this AAC tutorial on parallel and series circuits.

Take a shot at applying the techniques it describes.

hgmjr

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Upon reflection, this problem appears to be about the application of Ohm's law as well as parallel and series resistors.

The exercise is to start with the one known current (1A) flowing through the 9 ohm resistor and work the problem backward to determine what voltage (Vg) that would have to be applied to produce that 1 Amp current.

HINT: Ohms Law permits you to calculate that the voltage across the 9 ohm resistor is I time R. Once you have calculated the voltage across the 9 ohm resistor you then have the voltage across the series combination of the 1 ohm and the 2 ohm resistor. Ohm's Law again allows you to calculate the current that is flowing through these two series resistors. You will need to apply Kirchoff's Current Law to determine how the currents are flowing in and out of the various nodes. You will need to apply Kirchoff's Voltage Law to determine the various node voltages.

It might be less confusing if you redraw the circuit diagram so that it has a more conventional layout.

hgmjr

5. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,401
1,218
You might want to rethink that drawing phatstyles21.

6. ### Dave Retired Moderator

Nov 17, 2003
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Joe,

Zoom the page to 150%+ and its pretty clear.

Dave

7. ### phatstyles21 New Member

Aug 11, 2007
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I did a typo on resistors 7,8, and 9, and swapped 10 and 11.

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8. ### hgmjr Moderator

Jan 28, 2005
9,030
214
You may want to go ahead and withdraw your earlier flawed attachment to avoid any unintentional confusion.

hgmjr

9. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,401
1,218
Good job phatstyles21.

Dave, saw the error then printed it to confirm ... not that I expected any changes from screen to print

10. ### spar59 Active Member

Aug 4, 2007
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I'm sure that many others may have already put their brains & calculators to work to check they are both still working so I don't claim to be the only only able to solve this. However I do have the answers and though posting them here now would absolve you of having to work it out yourself I am happy to confirm any answer you come up with.

Steve