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Electric Circuits
•
Chapter 20
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Section 20.05: Alternating Current
Section 20.09: Internal Resistance
Section 20.10: Kirchhoff’s Rules
Section 20.11: Measuring Voltages and Currents
Section 20.12: Capacitors in Series and Parallel
Section 20.13: RC Circuits
Alternating Currents (AC)
The voltage and current change
polarity and magnitude
periodically
V = V0Sin2
π
ft;
Ι =
Ι0
Sin2
ft
Peak voltage
frequency
Peak current
Power (P) =
VI = I0V0Sin22
ft
Average power =
I0V0/2
Root meansquare (rms) values
Irms = I0/
√ ;
2
=
/√
Vrms
V0
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View Full Document 20.9 Internal Resistance
So far we’ve just considered batteries and generators as contributing their emf to a circuit.
In reality, they too have some resistance.
This is called
internal resistance,
r
.
In batteries it’s due to the chemicals, and in
generators it’s due to wire resistance.
So, if a battery is connected to a load resistor,
R
, then the internal resistance,
r
,
is in series with the load:
Notice:
R
and
r
are in series!
Thus, the voltage across the
battery (known as the
terminal voltage,
VT
) is less
than the full voltage
V
, since
some is lost across
r
.
VT
Thus,
r
T
V
V
V

=
Terminal voltage
Battery voltage
Voltage drop across the
internal resistance,
r
In many circuits, applying the series or parallel methods is not sufficient to analyze them.
There are two other rules we can use called Kirchhoff’s Rules
:
1.
Junction Rule
– Current into a junction has to equal current out.
It is based on conservation of charge.
I
flows into junction, and
I
1 and
I
2 flow out, thus:
I
I2
I1
2
1
I
I
I
+
=
2.
Loop Rule
– Around any closed circuit loop, the sum of the potential
(voltage) drops has to equal the sum of the potential rises.
It is based on conservation of energy.
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This note was uploaded on 08/04/2009 for the course PHYS 2002 taught by Professor Blackmon during the Spring '08 term at LSU.
 Spring '08
 BLACKMON
 Physics, Current, Resistance

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