if you connect a12v circuit connected to 24v PSU what will happen? the voltages 12v and 24v could be different, I just want to know what happens? According to ohms law more voltage fewer amps, but same resistance, only watts would be the same, it seems like it would work out?

 

You kill the circuit, and the circuit failing may kill the 24V PSU.
What happens when you put too much air into a balloon?

What fails exactly in the circuit will depend on what the circuit actually is ... it will never be a good idea.

 

No.

Ohms law is:

V = I * R

For the same resistance, double the voltage and you double the current, and, since:

P = V * I

You quadruple the power.

Edited to add: But most circuits, including a simple incandescent lamp, are not simple resistances. For anything other than a resistor, the relationship is more complicated.