Thevenin equivalent resistance

1. Dec 8, 2013

bigu01

i am confused at the 20v voltage since it becomes short circuit.i had started reading the circuit from the left side 1M ohm and 2k ohm are in parallel and in series with the 3k.knowing that current wants less resistant ways it will go to the killed voltage source...after that im wasnt able to decide in what way the current will go

2. Dec 8, 2013

3. Dec 8, 2013

bigu01

Doing as you suggested i am getting Rt(thevenin equivalent resistance) as 4.2k ohms while book solution is 4.0k and i havent done any calculation error.at least I think I haven't.

4. Dec 8, 2013

Simon Bridge

i.e. which resistors are left and what is the circuit?

5. Dec 8, 2013

bigu01

Is the eq res. of 1M and 2k parallel to 3k? If yes can you please tell me why?because calculating it as parallel i got the result right

6. Dec 8, 2013

Simon Bridge

The 1M, 2k and 3k are, indeed, parallel.
But they are also disconnected from the rest of the circuit by the short.
Even if you did include it, you would not get exactly 4.0k. Be careful about rounding off too soon.

Actually physically draw the circuit (without the load) with the short in place of the 20V source.
If you put an ohmmeter between a and b, what would it read?

7. Dec 8, 2013

bigu01

Well if i calculate only the right side of the circuit I'm getting 4.4 considering the two 6k's are in series then in parallel with 3k then in series with 2k.

8. Dec 8, 2013

Simon Bridge

One of the 6k resistors is in parallel with a short circuit though.

9. Dec 8, 2013

bigu01

So i should ignore every resistance affiliated with the short circuit,draw the circuit with only three resistances(6k in parallel with 3k then in series with 2k) and get 4.0k

10. Dec 8, 2013

Simon Bridge

You technically use all the resistances - but you have to include the effect of the short circuit.
The effect of the short is to give a large chunk of the circuit an equivalent resistance of zero.

That leaves only the three by the terminals having any effect on the resistance seen at the terminals.

Sometimes it helps to see which resistors have any effect if you imagine you replace the load with a small current source and trace where the current would go. All the resistors the current pass through are the ones that you use.

11. Dec 9, 2013