# Combining two ideal voltage sources with an impedance between them

Solving a problem, I came across the following source transformation:

My question is:

I know that I can do the following:

But why can I do the first image transformation, even with an impedance between the two voltage sources?

• @ViniciusACP Here's a major hint for you. A simple low-pass filter can be made with RC and RL. With RC the R goes to source and C goes to ground. With RL the L goes to source and R goes to ground. Both filters can have the same transfer function. $\frac{1}{1+s\frac{L}{R}}$ vs $\frac{1}{1+sRC}$. This is a hint. Notice how the shunt capacitor turned into a series inductor. not a series capacitor. – Harry Svensson Oct 8 '18 at 15:11