# Reactive circuits, working out voltage of a voltage source

Discussion in 'Homework Help' started by cakfef, Jan 29, 2016.

1. ### cakfef Thread Starter New Member

Jan 29, 2016
5
0
How do I work out the Vs ? I have achieved an answer of 120V.

File size:
44.5 KB
Views:
20
2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
From voltage divider.
Vs = 80V * (1 + (R1||R2 + R4 )/R6) = 80V *(1 +30/20) = 80V*2.5 = 200V

3. ### Dodgydave Distinguished Member

Jun 22, 2012
4,986
745
You don't know the DC resistance of the inductor L2, so you can't find it.

If its a short circuit, it will form a voltage divider with the resistors R4,6 so it will be half of the supply voltage, which is 160V.

4. ### chuckey Well-Known Member

Jun 4, 2007
75
10
C2 isolates the inductor so the steady state current is zero around this bit.
Frank

5. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
MOD NOTE: Moved to Homework Help.

6. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
It's probably a reasonable assumption that the DC resistance of L2 is << R6, which is 20 kΩ.

7. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
C2 isolates L1 in DC steady state, but not L2.

8. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
So R1 and R2 don't factor into it at all?

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,392
497
Assuming Vs is DC voltage source.

1) All capacitors are fully charged and act as open circuits.
2) All inductors are fully charged and act as short circuits.

Redraw the circuit, replace all capacitors with opens circuits, replaced all inductors with a piece of wire (short circuit).

It looks like you will have 80 volts across R6. Reduce the circuit to two resistors, R6 and R whatever. The two resistors form voltage divider. Formula for voltage divider has 4 variables, you know 3 of those variables (R6, 80 volts across R6, R whatever), solve for Vs.

10. ### cakfef Thread Starter New Member

Jan 29, 2016
5
0
Thankyou good people Also on the same question how do I work out the power dissipation, Ive worked it out and got 512mV but don't know if its correct. Thanks

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,392
497
Power is measured in Watt. Why is your result in volts?

Power here is Voltage times Current.
P=VI
Since you don't know any of the currents, but you do know voltages and resistances, you can do a bit of manipulation:
V=IR
I=V/R
Therefore Power can be found by:
P=VI=V(V/R)=$\frac{V^{2}}{R}$

From the simplified circuit (the one with just two resistors, R6 and R whatever) work your way back to the full circuit with all the resistors. Find the voltages across each resistor and use the resistor values that were given to you in the problem description to find power dissipation of each resistor.

For example:
You already know that R6 is 20 kOhm, votage across R6 is 80 V.
So Power dissipated by R6 is then=80^2/20000=0.32 W

Last edited: Jan 30, 2016