Chapter 29 Alternating Currents

1
Chapter 29Alternating Currents
2
Topics

• Generators
• Alternating Currents
• In resistors
• In inductors
• In capacitors
• Reactance

3
Generators
Magnetic flux
Induced emf
4
Generators
Power delivered to resistor
Average power
5
Generators
Average power
where the root mean square (rms) voltage
and current are given by
6
Generators
0
Average power
7
Alternating Currents - Resistors
Assume potential difference across resistor is
given by
8
Alternating Currents - Resistors
Since
then
The voltage across the resistor is in phase with
the current
9
Alternating Currents - Resistors
then
The voltage across the resistor is in phase with
the current
10
Alternating Currents - Inductors
Since
then
The voltage across the inductor leads the
current by 90o
11
Alternating Currents - Inductors
At t 0, the voltage is at its peak, while the
current is zero and becoming positive
12
Alternating Currents - Capacitors
Since
then
The voltage lags the current by 90o
13
Alternating Currents - Capacitors
At t 0, the voltage is at its peak, while the
current is zero and becoming negative
14
Reactance
The reactance X of a device is defined by
Therefore,
resistor
inductor
capacitor
Reactance is a general measure of resistance to
current
15
Summary

• Average power delivered by a generator
• P Irms Vrms
• Reactance Phase
• X R (V in phase)
• X wL (V leads current)
• X 1/wC (V lags current)