# Thevenins theorem

#### sage sky

Joined May 15, 2010
7
hi i have a question on thevenins theorem i have done the calulations but my problem is the thevenins voltage for the circuit i have attached my question and answer have a caculated the Vab correctly?

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#### thatoneguy

Joined Feb 19, 2009
6,359
Please post attachments as .PNG or .PDF Files.

#### sage sky

Joined May 15, 2010
7

#### thatoneguy

Joined Feb 19, 2009
6,359
What is the floating 100 Ohm resistor?

is it supposed to be connected to anything, or is it the internal resistance of the meter?

--ETA: You should keep your numbers with more significant digits.

I have for the first part, 0.129V, currents of 71.43mA and 90.90mA, for example. Use the ENG function of calculator.

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#### sage sky

Joined May 15, 2010
7
the 100 ohms is the internal resitance of the meter. i recalculated and i also have 0.132 volts is this the voltage i use in the thevenins circuit?

#### The Electrician

Joined Oct 9, 2007
2,903
When you calculate Vab, you are subtracting nearly equal quantities and you lose digits due to cancellation.

Follow thatoneguy's advice and carry more digits in your calculations. You must be using a modern calculator; there shouldn't be any problem to carry at least 8 digits.

#### sage sky

Joined May 15, 2010
7
oh yea so am ok use more significant numbers gotcha now is the Vab i have calculated the voltage i use for the thevenins circuit?

#### The Electrician

Joined Oct 9, 2007
2,903
oh yea so am ok use more significant numbers gotcha now is the Vab i have calculated the voltage i use for the thevenins circuit?
Which Vab have you calculated? The value of .129 volts or .132?

#### sage sky

Joined May 15, 2010
7
this is my updated version is this correct method of getting vab?

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#### The Electrician

Joined Oct 9, 2007
2,903
This is almost correct , but you still aren't carrying enough digits, and you need to calculate Va and Vb with respect to the ground shown in the problem schematic diagram.

You have: Vab = 7.272 - 7.142 = 131 mA

You are being careless, and this will cost you points on an exam. Where did the 1 digit (in cyan) come from? You haven't done the subtraction correctly.

Why do you have the units as mA (in red)? This is a voltage, not a current.

Calculate Va as the voltage across R3, not R1, and calculate Vb as the voltage across R4, not R2. This is because the ground reference is at the bottom of your circuit.

Va = Ia x R3 = 71.42857mA x 40Ω = 2.8571428V
Vb = Ib x R4 = 90.90909mA x 30Ω = 2.7272727V

Vab = 2.8571428V - 2.7272727V = .1298701V