http://www.allaboutcircuits.com/vol_1/chpt_10/8.html

how did they calculated the size of the voltage source

???

i cant do this using KVL

 

What voltage source are you referring to?

hgmjr

 

the voltage source of the minimised
Thevenin circuit

???

 

If you temporarily remove the 2 Ohm resistor from the circuit, you will end up with a total voltage across R1 and R3 of 21 volts (28V - 7V). The total resistance in the loop is 5 ohms. Ohm's Law permits you to calculate the current in the loop by dividing 21 by 5. The current is 4.2 Amps. Next, mulitply 4 Ohms times 4.2 Amps and you get 16.8 volts dropped across R1. Subtract 16.8 Volts from 28 Volts and you get 11.2 Volts. This is where the 11.2 volts comes from.

hgmjr

 

thanks i think igot think
ill try to apply it to other questions