# Why is this voltage negative?

1. Oct 6, 2008

Why is this voltage negative??

Using superposition theorem
With voltage source only,
Vth1 = (3)(5000) / (2000+5000) = 2.14V
With current source only
Current through 5kΩ resistor
I = (0.007)(2000) / (2000+5000) = 2mA
The voltage across it will be
Vth2 = - (0.002)(5000) = -10V why is this voltage negative??
The resultant voltage is
Vth = Vth1 + Vth2
Vth = 2.14 - 10 = - 7.86V

Last edited by a moderator: May 3, 2017
2. Oct 8, 2008

### MATLABdude

Re: Why is this voltage negative??

The direction of current flow through the resistor (due to the current source) is the opposite of that due to the voltage source. As the voltage measurement is (implicitly) with the "-" terminal on the shared node on the bottom, this makes the voltage due to the current source negative.

EDIT: Not implicit measurement, I missed the ground symbol on the bottom node. The more intuitive way of realizing that there is a negative voltage is that current flows from higher potential to lower. And then realize that if the higher potential is zero (ground) then the lower potential must be negative.

Last edited by a moderator: May 3, 2017