# Impedance

Discussion in 'Homework Help' started by Teszla, Sep 27, 2013.

1. ### Teszla Thread Starter Member

Jun 7, 2013
43
0
A voltage source has inner impedance of Z=Ri+jXi. To it is connected a load with impedance Z=R+jX. What value should the impedance of the load have if:

a) The voltage across the load is 90 degrees out of phase with the current?
b) The voltage across the load is in phase with the current?
c) The current is in phase with the open circuit voltage source?

Can someone give me some leads how to calculate this.

2. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
Can you write down the relationships between current and voltage for the 3 situations asked?

You do realise that the "inner impedance" is in series with the load impedance so it is asking you to combine these to fit the expressions you have written down, in each case.

Show some working for more.

3. ### Teszla Thread Starter Member

Jun 7, 2013
43
0
If the current is 90 degrees out of phase with the voltage then I guess it's either a capacitive or inductive load? And for b) and c) I would guess the load impedance is pure resistance. Actually I don't see what's the difference between b) and c).

4. ### WBahn Moderator

Mar 31, 2012
20,229
5,755
To see the difference between b) and c), develop the equation for the current as a function of load reactance and also for the open circuit voltage and the load voltage, also as a function of load reactance. Then find the expressions needed to satisfy the constraints asked in each question -- they are different constraints.

5. ### Teszla Thread Starter Member

Jun 7, 2013
43
0
Here's how I thought of the problem:

What's next?

Mar 31, 2012
20,229
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7. ### Teszla Thread Starter Member

Jun 7, 2013
43
0
Ok hope it works now

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8. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
So what happened to my list in terms of (R+jX) ?

9. ### WBahn Moderator

Mar 31, 2012
20,229
5,755
So what is the phase angle of the voltage across the load relative to the current through the load?

What is the phase angle of the open circuit voltage to relative to the current through the load?

Do you notice that studiot and I are basically asking you to do the same thing?