Thevenin equivalent resistance problem

  • Thread starter glover261
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Homework Statement



XUot0kO.png

Homework Equations


V=IR

The Attempt at a Solution


[/B]
I found that the current around the circuit was 5mA using kirchoffs law ( i think)
If this is true then the T.E.V should be the voltage across the 120ohm resistor which is .6V (120*5e-3). Now i'm stuck and not sure how to find the T.E.R, thankyou
 

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  • #2
SammyS
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Homework Statement



[ IMG]http://i.imgur.com/XUot0kO.png[/PLAIN]

Homework Equations


V=IR

The Attempt at a Solution



I found that the current around the circuit was 5mA using kirchoffs law ( i think)
If this is true then the T.E.V should be the voltage across the 120ohm resistor which is .6V (120*5e-3). Now i'm stuck and not sure how to find the T.E.R, thankyou[/B]
Hello glover261. Welcome to PF !

So far, so good.

What have you been taught regarding how to find the Thevenin equivalent resistance ?
 
  • #3
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Hello glover261. Welcome to PF !

So far, so good.

What have you been taught regarding how to find the Thevenin equivalent resistance ?
Hi thanks, I thought you were supposed to take out all components except resistors in the circuit, current sources by open circuit and voltage sources through short circuit. Then just add up the total resistance of all the resistors, but that method does not seem to work on this problem, as none of the answers are the total resistance.
 
  • #4
SammyS
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Hi thanks, I thought you were supposed to take out all components except resistors in the circuit, current sources by open circuit and voltage sources through short circuit. Then just add up the total resistance of all the resistors, but that method does not seem to work on this problem, as none of the answers are the total resistance.
That's not quite right. You don't simply add all the resistances. You find the equivalent resistance across the terminals.
 
  • #5
phinds
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Hi thanks, I thought you were supposed to take out all components except resistors in the circuit, current sources by open circuit and voltage sources through short circuit. Then just add up the total resistance of all the resistors, but that method does not seem to work on this problem, as none of the answers are the total resistance.
Is "adding up the total resistance of all the resistors" really what happens in a circuit? Do parallel resistors add?

EDIT: I see sammy beat me to it.
 
  • #6
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Aren't they all in series though? or am i missing something
 
  • #7
SammyS
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Aren't they all in series though? or am i missing something
You must be missing something.

Draw what you have when the voltage source is shorted.

upload_2016-4-17_19-51-1.png
 
  • #8
phinds
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Aren't they all in series though? or am i missing something
Probably be a good idea to go back to basics and study what parallel and series really look like. This is a very common difficulty in beginning study of circuits so it's best to get really solid with it in the beginning.

EDIT: dammit, sammy, you're too fast for me :smile:
 
  • #10
phinds
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Sorry I still can't see it
Probably be a good idea to go back to basics and study what parallel and series really look like. This is a very common difficulty in beginning study of circuits so it's best to get really solid with it in the beginning.
 
  • #11
SammyS
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Sorry I still can't see it
See the terminals?

(phinds beat me this time !)
 
  • #12
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See the terminals?

(phinds beat me this time !)
So, the 180 and 100 resistors are in parallel. So Rt = 1/((1/180)+(1/100)) + 120 = 184 ohms? Is that right because I thought for a pair of components to be in parallel, both sides had to be connected to the same node.
 
  • #13
SammyS
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So, the 180 and 100 resistors are in parallel. No.

So Rt = 1/((1/180)+(1/100)) + 120 = 184 ohms?

Is that right because

I thought for a pair of components to be in parallel, both sides had to be connected to the same node. This is correct.
See the terminals?
 
  • #15
SammyS
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What do they mean?
Find the equivalent resistance across the terminals.

Look at the nodes again.

upload_2016-4-17_20-24-18.png
 
  • #16
SammyS
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There is an alternate way to find the Thevenin resistance.

Take the original circuit, and find the current which flows through a wire which is connected to the two terminals. (This is the short circuit current.)

This is the same current which will flow through the Thevenin equivalent circuit if its terminals are shorted.

In other words, this current times the Thenenin resistance is equal to the Thevenin voltage.
 

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