Theravin equivalent resistance/current little help

[asdf12312]

theravin equivalent resistance/current little help!

Homework Statement

Homework Equations

The Attempt at a Solution

found theravin voltage (=-3.75V) but i have a question about the current. the answer is supposed to be -1mA and i found the only way to get this answer was to ignore the 3mA source and the 5k resistor altogether. is there an explanation for this?? just need slight clarification bcause I'm not very smart :(

also, i have a problem when finding theravin resistance method by turning off all independent sources and solving. is there a way to find it using that method? the resistance is supposed to be V(th)/I(sc)=-3.75V/-0.001A=3.75k ohms. but when i use that method i end up putting 10k and 15k in parallel, to get 6k ohms, and put that in parallel with 5k to get R(th)=2.73k ohm. i know i am doing it wrong, is there a way to get the right value though??

 

[phyzguy]

By the way, it's Thevenin, not Theravin. You calculate the Thevenin resistance by shorting out the voltage sources, and opening up the current sources, then calculating the equivalent resistance between a and b. When you do this, the 30V voltage source shorts out the 10K resistor, and you're left with just the 5K resistor in parallel with the 15K resistor. The parallel combination of these two gives 3.75K, as you said.

 

[asdf12312]

hmm...ok. any reason why the 10k resistor gets 'shorted' out? other than its on the far end and not really connected to the other resistors? my teacher never mentioned this so i never knew :P

so if there are two resistors on the left side of a source, and 1 to the right of it, if you short the source all 3 resistors are also??

 

[gneill]

hmm...ok. any reason why the 10k resistor gets 'shorted' out? other than its on the far end and not really connected to the other resistors? my teacher never mentioned this so i never knew :P

so if there are two resistors on the left side of a source, and 1 to the right of it, if you short the source all 3 resistors are also??

Yes, of course. If there's a zero resistance path in parallel with a resistance, the net resistance is zero.

 

[phyzguy]

hmm...ok. any reason why the 10k resistor gets 'shorted' out? other than its on the far end and not really connected to the other resistors? my teacher never mentioned this so i never knew :P

so if there are two resistors on the left side of a source, and 1 to the right of it, if you short the source all 3 resistors are also??

The methodology for calculating the Thevenin resistance is that you replace all voltage sources with short circuits and all current sources with open circuits and then calculate the equivalent resistance. After you do this, the 10K resistor on the left has a short (i.e. a zero resistance resistor) in parallel with it. 10K in parallel with zero is zero, i.e., a short.