Thevenin's Theorem?

Question

Hi guys, could use some help on this one.
Here is a screenshot:
http://img232.imageshack.us/img232/8095/62339971np9.png

Answer 1

Easy:

500//800 +450 =480.769 ohms seen by the 100 V source. (Rth)

( 450 ohms is the 400 + 50 ohms)
note: // = in parallel with.

Now calculate the total current:

100V/480.769ohms = 0.208 amps

now do the current division between the two legs. you can neglect the 400 ohms and 50 ohms because they don't effect the two parallel legs and have the full current going through them anyway. so you are now looking at the parallel combination of 800 ohms and 500 ohms (500 from 400 ohms + 100 ohms).

800/(800+400 +100) = 0.138 A in outer leg (points a and b).

Now just multiply that current onn the load resistor:

0.138amps X 500 ohms = 69 Volts. (Vab)

Answer 2

You need to begin by combining the resistors. Here are some steps to help you get started with YOUR homework.

1) Combine R2 and the Load resistors that are in series.

2) Combine the result in # 1 above with R3 using the formula for paralleling resistors.

3) Combine the result of # 2 with R1 and R5 where all three are in series.

4) Divide the resulting resistor from # 3 into 100 V to determine the current flowing out of the source.

5) The current calculated in # 4 is same current flows through R1 and R5. This allows you to compute the voltage across R3.
100 V - (R1 * i) - (R5 * i) = voltage across R3.

6) The voltage across R3 is the same voltage across R2 and the Load. Label this voltage is as the Thévenin voltage source (Vth). Label R2 as Rth.

7) Note that Vth is not 100 V, you have to go through steps 1-6 to get the proper Vth.