How Do Voltage Polarity and Thevenin's Theorem Affect Circuit Measurements?

[jamie3009]

I would really appreciate if someone could clear this up for me.

Basically, with the first one (link below), we are told to set the source (V_s) to 6v and then measure the voltage across each resistor. It however says in the question to make sure the polarities of the measured quantities are correct. My issue is, because the arrows above the resistors are going in the opposite direction to the arrow for the supply, will the values of V₁ and v₂ be negative or still positive?

http://www.flickr.com/photos/53512951@N06/8122890952/in/photostream

Also, with another circuit, we are dealing with Thevenins Theorem and have been given a circuit containing a set of resistors and a 12v supply which has not yet been simplified. We are then told to put a load resistor (R_l) between A and B and measure both the voltage and current across the load resistor. We are told to do this for a number of different resistor values. Should the voltage across R_l be the same as the supply voltage for each of the different load resistors? it then says to remove the load resistor and measure the open circuit voltage across A - B. Should this also be the same as the supply voltage and the voltage seen across each of the load resistors which were placed between A and B?

Cheers.

 

[berkeman]

I would really appreciate if someone could clear this up for me.

Basically, with the first one (link below), we are told to set the source (V_s) to 6v and then measure the voltage across each resistor. It however says in the question to make sure the polarities of the measured quantities are correct. My issue is, because the arrows above the resistors are going in the opposite direction to the arrow for the supply, will the values of V₁ and v₂ be negative or still positive?

http://www.flickr.com/photos/53512951@N06/8122890952/in/photostream

Also, with another circuit, we are dealing with Thevenins Theorem and have been given a circuit containing a set of resistors and a 12v supply which has not yet been simplified. We are then told to put a load resistor (R_l) between A and B and measure both the voltage and current across the load resistor. We are told to do this for a number of different resistor values. Should the voltage across R_l be the same as the supply voltage for each of the different load resistors? it then says to remove the load resistor and measure the open circuit voltage across A - B. Should this also be the same as the supply voltage and the voltage seen across each of the load resistors which were placed between A and B?

Cheers.

You may need to ask your instructor or lab TA what they want for the resistor voltage polarities. The usual convention is to quote the voltage increase going through the power supply as positive, and the voltage *drops* across the resistors as positive.

On the 2nd question, have you learned how to model a real power supply? Does a real power supply have zero output resistance?

 

[aralbrec]

Arrow notation for voltage has the head pointing at the highest potential. If you find voltages that are opposite in sense to what is on the diagram, you must record them as negative.

The Vs arrow has correct sense, for example. The battery symbol uses the long edge to indicate the + side (the higher voltage side) so the arrow across the battery needs to point at the long edge. Vs would be recorded as +ve as it agrees with the sense of the arrow.

For the Thevenin question, Thevenin says all the stuff inside the circuit connected to A/B is equivalent to a voltage source in series with an impedance like so:


FIG 1

+-----Rth------+ A
|
Vth
|
+--------------+ B


(Rth = thevenin resistance, Vth = thevenin voltage)

No matter how complicated the circuit may be, the element attached to A/B sees that equivalent circuit.

Suppose that is true, will the voltage change across A/B if you attach different resistance values (assuming Rth is not zero)?

In the following complete circuit:


+----R----+------+ A
|...|
V...R
|...|
+---------+-------+ B


(ascii art isn't working too well; the dots are empty space)

lf you measure the voltage across A/B will it be equal to V?I think that should answer your questions. You might want to ask how you might come up with the thevenin values in the equivalent circuit of figure 1 because that is what this lab is leading up to. What voltage do you measure in fig 1 with A/B open? What current do you measure with A/B short circuited?