Review of AC

circuit
Keys to deal with R, L, and C in an AC

circuit
•
Across R, voltage and current are in

phase
•
Across L, the current lags behind voltage by 90
o
•
Across C, the current leads voltage by 90
o
•
When two elements are in series
the same current
passing through them
•
When two elements are in parallel
the same voltage
across them
Also,
Basics
I and V are in

phase
I lags V by 90 degree
I leads V by 90 degree
R
L
C
RLC in series
Key: I
R
, I
L
, and I
C
are all in phase
D
V
L
~ X
L
D
V
C
~ X
C
D
V
R
~ R
D
V
max
~ Z
What if RLC are not in series?
Which of the following is true
(a)
D
V
c
=
D
V
L
,
I
C
= I
L
(b)
D
V
c
=
D
V
L
,
I
R
= I
L
(c)
D
V
c
=
D
V
R
,
I
R
= I
L
(d)
D
V
L
=
D
V
R
,
I
R
= I
L
How do you draw the phasor diagram?
D
V
L
D
V
R
I
R
= I
L
I
C
(e)
D
V
c
=
D
V
L
+
D
V
R
,
I
C
= I
L
(f)
D
V
c
=
D
V
L
+
D
V
R
,
I
R
= I
L
I
max
D
V
c
=
D
V
R
+
D
V
L
Power dissipation
•
Only R dissipates power
P =
I
*
D
V, always
For L and C,
I
and
D
V are 90
o
out of phase
P
L
= 0, P
C
= 0
For R,
I
and
D
V are in

phase
P
R
=
I
D
V =
I
rms
2
R
Resonance in RLC circuit in series
For a fixed AC voltage source,
maximum power dissipation occurs
when X
L
= X
C
, ie. When Z = R
P
R
=
I
D
V =
I
rms
2
R
Where
I
rms
= V
rms
/Z
i.e.
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