1
Reminders and Announcements
•
HW 16 is due today; HW 17 is due next
Tuesday; HW 18 is due next Friday
• No quiz this week but some of recitation
No quiz this week, but some of recitation
time will be used for course evaluation.
•
We will be grading the exams early next
week.
•
Help Room
Hours (Physics 117):
– M 4-8 pm, T 3-6:30, W 3-6:30, R 4:30-8, F 2-6
Resistors in an AC Circuit
•
Consider a circuit
consisting of an AC source
and a resistor
•
The AC source is
symbolized by
•
Δ
v
=
Δ
v
R
=
Δ
v
max
sin
ω
t
•
Δ
v
R
is the instantaneous
voltage across the resistor
Resistors in an AC Circuit, 2
•
The instantaneous current in the resistor is
sin
sin
max
max
I
R
R
v
V
i
ω
t
ω
t
R
R
Δ
Δ
=
=
=
•
The instantaneous voltage across the
resistor is also given as
Δ
v
R
=
I
max
R
sin
ω
t
Phasor Representation
rms Current and Voltage
•
The average current in one cycle is zero
• The
rms current
is the average of importance
in an AC circuit
•
rms stands for
root mean square
•
Alternating voltages can also be discussed in
terms of rms values
0707
2
max
max
I
I
.
I
rms
=
=
0707
2
max
max
.
rms
V
V
V
Δ
Δ
=
=
Δ
Power
•
The rate at which electrical energy is dissipated in
the circuit is given by
P
=
i
2
R,
where
i
is the
instantaneous current
•
The average power delivered to a resistor that
carries an alternating current is
2
I
av
rms
P
R
=

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2
Notes About rms Values
•
rms values are used when discussing
alternating currents and voltages because
– AC ammeters and voltmeters are designed to
AC ammeters and voltmeters are designed to
read rms values
– Many of the equations that will be used have
the same form as their DC counterparts
Which of the following statements might be
true for a resistor connected to a sinusoidal
AC source?
25%
25%
25%
25%
1.
2.
1
2
3
4
3.
4.
0 of 126
Inductors in an AC Circuit
•
Kirchhoff’s loop rule
can be applied and
gives:
0 or
0
max
,
sin
L
v
v
di
v
L
dt
di
v
L
V
ω
t
dt
Δ
+ Δ
=
Δ
−
=
Δ
=
= Δ
Current in an Inductor
•
The equation obtained from Kirchhoff's loop rule can be
solved for the current
max
sin
max
cos
V
V
i
ω
t dt
ω
t
Δ
Δ
=
= −
∫
•

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- Spring '09
- Alternating Current, Inductor, Electrical resistance