# Simple Circuits question that i cant do

Discussion in 'Homework Help' started by medontknowye, Nov 9, 2011.

Nov 9, 2011
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2. ### Georacer Moderator

Nov 25, 2009
5,154
1,281
Your thread has been moved to the Homework Help section, where it is more appropriate.

The Homework Help section asks that you post up the work you have done so far, so we can see it and find where you went wrong, or suggest better approaches. We will not do all your work for you.

Thank you.

3. ### medontknowye Thread Starter New Member

Nov 9, 2011
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please someone help me im new to this.. and i got a test tommorow .. and i need to figure this out asap..please someone help me.

4. ### medontknowye Thread Starter New Member

Nov 9, 2011
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hi. can u help me jst once..i can figure out the current for the first question which is 8 i think but im not sure how to calculate the urrent on the 12 ohms...i also know how to calculate the last question which is P=IV or P=I^2R or P=V^2/R please jst help me with second question

5. ### Adjuster Late Member

Dec 26, 2010
2,147
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You could use the current division rule (Google it), otherwise you know the total current, the current in the 10ohms is obvious, the remainder goes into the 6ohms AB.

You then have X amps flowing into a parallel combination of two resistors...

6. ### medontknowye Thread Starter New Member

Nov 9, 2011
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so the current on the 10 ohms would be 3 and on the 6 it will be 5 right? on the first segment

then i used I=(12/6+12)*5 to get 3.33 for the other 6 ohms so that means that 12 ohms is 1.67 ohms is this wrong or right?.. if not can u correct please

7. ### thatoneguy Expert

Feb 19, 2009
6,349
732
It might help to re-draw it.

start with the40V Source as a battery, put 10Ω across it. Then a 6Ω resistor which leads to both a 6Ω and 12Ω in parallel.

Remove battery, find total resistance (combining series and parallel resistances to a single resistor across power supply. (Thevenin Equivalent Circuit)

Divide power supply voltage by that resistance and you have your current drawn from source.

Voltage across the first resistor will be battery voltage, it is independent of the rest, divide voltage by resistance for current through that resistor.

Subtract the current through the resistor in parallel with voltage source from the total current in the Thev Equivalent circuit and use in the followng equations:

Use the current through the series resistor to find the voltage it drops, which will give you the voltage across the last two which are in parallel.

Subtract voltage dropped across that series resistor from source voltage to find voltage across parallel pair.

Divide that voltage by the resistance and you have solved for all voltages and currents.

8. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
512

That answer is wrong.

You need to combine to one equivalent resistor and get the source current.

To combine any two parallel resistor to a single resistor use this:

REQ = (R1 x R2) / (R1 + R2)

Series resistors simply add.

To find the current divided between two parallel resistors, it is inversely proportional to the current in the branch. In other words: if one parallel side has three times as much resistance, it will have 1/3 of the total current.

You really need to learn the basics or switch to literature. Everything you will need to learn has to stand on the basic laws like Ohm's Law.

9. ### testing12 Member

Jan 30, 2011
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I suppose this is late since you have the test today buy it may be of benefit in future to learn nodal and mesh analysis.

10. ### EL7819 New Member

Apr 15, 2011
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Last edited: Dec 7, 2011