How to Find Thevenin Resistance after Shorting Batteries?

  • Thread starter Thread starter hussain bani
  • Start date Start date
Click For Summary

Discussion Overview

The discussion revolves around finding the Thevenin resistance (Rth) after shorting batteries in a circuit. Participants explore various methods and considerations for calculating Rth, including theoretical approaches and practical implications.

Discussion Character

  • Technical explanation
  • Homework-related
  • Debate/contested

Main Points Raised

  • Some participants suggest using an ohmmeter across the relevant resistors to determine Rth, emphasizing the importance of recognizing parallel and series resistor combinations.
  • One participant warns against physically shorting real batteries, noting that it can be dangerous and may not yield accurate results due to changes in the battery's chemical reactions.
  • Another participant proposes that instead of shorting the batteries, one can remove them and use a jumper wire to bypass them, although this may not apply to circuits with active or non-linear components.
  • A participant shares their approach to simplifying complex circuits, advocating for the use of familiar shapes and combinations of resistors to find equivalent resistance.
  • Some participants agree that mastering resistor combinations is essential for efficiently finding Rth, suggesting that it should not take long if one is competent in the concept.
  • One participant mentions that homework problems are often designed to yield specific numerical answers, which may require estimation in some cases.

Areas of Agreement / Disagreement

While there are multiple methods proposed for finding Rth, there is no consensus on the best approach, particularly regarding the treatment of real batteries versus theoretical models. Some participants emphasize the importance of understanding resistor combinations, while others highlight the potential dangers of shorting batteries.

Contextual Notes

Participants express varying opinions on the appropriateness of shorting batteries and the implications for circuit analysis. There are also mentions of the complexity of circuits and the need for careful simplification, indicating that assumptions about circuit behavior may vary.

Who May Find This Useful

This discussion may be useful for students and practitioners in electrical engineering or physics who are learning about Thevenin equivalents and circuit analysis techniques.

hussain bani
Messages
9
Reaction score
0
Thevenine, how to find Rth

I short the batteries for Rth, but after doing short i don't understand how to find Rth, can sum 1 help me finding out ?
 

Attachments

  • Fullscreen capture 4142013 115412 PM.bmp.jpg
    Fullscreen capture 4142013 115412 PM.bmp.jpg
    19.7 KB · Views: 599
Physics news on Phys.org
You are simply putting an ohmeter across Rab...what does the ohmeter see?

I would put the 4 ohm and the 16 ohm in parrallel...then add the 2 ohm in series. Put that result in parallel with the 15 ohm resistor...then add the 2 ohm resistor to that result in series. Then put in parallel with the 8, then parallel with the other 8...then add that result to the 5 ohm in series.

If you are having a hard time seeing which resistors are in parallel...parallel implies "voltage across is the same. If you look at the 16 ohm resistor and put a voltage meter across it...then slid it down so it is across the 4 ohm resistor...the voltage would be the same for both. A.K.A parallel.
 
I short the batteries for Rth, but after doing short i don't understand how to find Rth, can someone help me finding out ?
 
Last edited by a moderator:
hussain bani said:
I short the batteries for Rth, but after doing short i don't understand how to find Rth, can someone help me finding out ?
Hi hussain bani. You short the batteries only on paper. You don't do this to real batteries, shorting them can be dangerous or damaging, and won't give you the right answer, anyway, because the chemical reaction inside changes drastically when a battery is abused like this.

So, on paper, you short the battery and determine the current that flows through that short circuit wire. Together with the no-load voltage, you can then calculate Rth = Vo/c ÷ Is/c
 
Last edited by a moderator:
In a circuit with other components, you can just take the batteries out and put a jumper wire where the batteries were.

Mathematically they just want you to bypass the batteries so they no longer have a effect on the circuit. Of course this is only for finding Rthevenin.
 
smashbrohamme said:
In a circuit with other components, you can just take the batteries out and put a jumper wire where the batteries were.
In general, no. That won't work where the circuit includes active or non-linear devices (e.g., semiconductors).
 
My procedure when it came to doing thevenin equivalents, especially when you get nasty diagrams like that, was to start by simplifying the circuit (I have a thing against diagonal lines in electric diagrams).

Reading your question, what you want to do is find the equivalent resistance. Use all the rules and tricks you have learned up to this point and attempt to turn that mess of resistors into a nice and simple equivalent resistor. You will be doing quite a bit of parallel resistor combinations from what I have seen.

Something else to note, homework problems for circuit analysis are usually carefully rigged so that the answer is a number, but that is not always the case. (In my analysis of it, I had to guess on a couple numbers, but my number was near 9, but as you will know, simply putting the answer on your homework is NOT satisfactory for a full grade.)
 
All the methods stated above are correct. If you are shying away from simplifying resistors, perhaps you need to step back and master that concept first. Re-drawing the circuit in a more familar shape is always preferred as mentioned above.

If you are competent in combining resistors, it should take you no longer than a minute or two to find the Rth in my opinion.

And my point about "what would a ohmeter read" is a legitimate statement. If you wire those resistors in real life and put an ohmeter across Rab...it will be the same as the calculated Rth...or the combined resistance.

Thevenin and norton will actually help you solve more complicated circuits once you learn it.
 
Thevenine

Thanks everyone for their kind explanation, i got it...:)
 

Similar threads

Thevenin Theorum
  • · Replies 1 ·
Replies
1
Views
2K
Thevenin Resistance and the Time Constant
  • · Replies 12 ·
Replies
12
Views
4K
Thevenin Equivalent : Wrong value for Rth
  • · Replies 7 ·
Replies
7
Views
3K
Engineering Create a circuit in simulation to prove my work (2 batteries and 1 lamp bulb)
  • · Replies 5 ·
Replies
5
Views
2K
Engineering How do I find the Thevenin Equivalent of this circuit?
  • · Replies 10 ·
Replies
10
Views
3K
Engineering Homework: Rx=? to get maximum power transfer at point M
  • · Replies 4 ·
Replies
4
Views
2K
Engineering Find the Thevenin equivalent circuit
  • · Replies 42 ·
2
Replies
42
Views
7K
Find thevenin equivalent resistance with diode.
  • · Replies 15 ·
Replies
15
Views
11K
Help finding Thevenim Equivalent - Circuit Analysis
  • · Replies 8 ·
Replies
8
Views
2K
How can I determine Rth in Thevenin with a transconductance source?
  • · Replies 13 ·
Replies
13
Views
4K