Download PowerPoint Lecture Chapter 35

What you will learn:
•You will distinguish between parallel and
series circuits and series-parallel
combinations and solve problems dealing
with them.
•You will explain the function of fuses,
circuit breakers, and ground-fault
interrupters (GFI’s) and describe ammeters
and voltmeters.
Why it’s important:
•Electrical circuits are the basis of every
electrical device, from electric lights to
microwave ovens to computers.
Understanding circuits helps you to use
them, and to use them safely.
Chapter 23: Series and Parallel Circuits
I. A Battery and a Bulb (35.1)
A. Circuit- A complete pathway for electrons
to flow
1. Flow of electrons like flow of water in a
closed system of pipes
a. The battery would be analogous
to the pump
b. Wires analogous to the water
pipes
B. The water flows through the pump and the
electrons flow through the battery
II. Electric Circuits (35.2)
A. Electric Circuit- any path along which
electrons can flow
1. Must be a complete circuit with no
gaps
2. Gap usually provided by electric
switch
B. Most circuits have more than one device that
receives electrical energy. Devices can
be
connected in a circuit in one of two ways, series
or parallel.
III. Series Circuits (35.2)
A. Electric current has but a single pathway
through a series circuit. Current is the same
through each electrical device in the circuit
B. The total resistance to current in the circuit is the
sum of the individual resistances along the
circuit path
R = RA + RB + …..
C. The current in the circuit is equal to the voltage
supplied by the source divided by the total
resistance of the circuit (in accord with Ohm’s
law)
D. The total voltage impressed across a series
circuit divides among the electrical devices in the
circuit so that the sum of the “voltage drops”
across each device is equal to the total voltage
supplied by the source.
E. The voltage drop across each device is
proportional to its resistance. This follows from
the fact that more energy is wasted as heat when
a current passes through a high-resistance device
than when the same current passes through a
device offering little resistance.
QUESTION:
What happens to current in other lamps if one
lamp in a series circuit burns out?
ANSWER: The path to the current
will break and current will cease.
All lamps will go out
QUESTION:
What happens to the light intensity of each lamp in
a series circuit when more lamps are added to the
circuit?
ANSWER: This results in greater circuit resistance.
This decreases the current in the circuit and
therefore in each lamp, which causes dimming of
the lamps. Energy is divided among more lamps……
QUESTION:
What is the current through
this series circuit?
ANSWER: Use the equation for Ohm’s law and
solve. First calculate the total resistance.
R  RA  RB
V
I
R
R  9  6  15
30V
I
 2A
15
QUESTION:
In this simple series circuit, a
small electronic component is
connected before the LED light
bulb. What is this and why do
you think they use it?
ANSWER: The component is
a resistor. It produces a
voltage drop in accordance
to Ohm’s law
IV. Parallel Circuit (35.4)
A. Each device connects the same two points
A and B of the circuit. The voltage is therefore
the same across each device
B. The total current in the circuit divides among
the parallel branches. Because the voltage
across each branch is the same, the amount of
current in each branch is inversely proportional
to the resistance of the branch
Remember that the voltage is the
same across each of the branches
V
IA 
RA
C. The total current in the circuit equals the sum of
the currents in its parallel branches
I  I A  I B  IC
Where IA, IB, IC are currents through the branches
and I is the total current
D. As the number of parallel branches is
increased, the overall resistance of the circuit
is decreased (just as more check-out cashiers
at a supermarket decreases people-flow
resistance). With each added parallel path,the
overall circuit resistance is lowered. This means
the overall resistance of the circuit is less than
the resistance of any one of the branches.
1
1
1
1



R R A RB RC
QUESTION:
What happens to the current in other lamps if one
of the lamps in a parallel circuit burns out?
ANSWER: In one lamps burns out, the other lamps
will be unaffected. The current in any other single
branch is unchanged. The total current in the overall
circuit is decreased by an amount equal to the
current drawn by the lamp in question.
QUESTION:
What happens to the light
intensity of each lamp in a
parallel circuit when more
lamps are added in parallel
to the circuit
ANSWER: The light intensity for each lamp is
unchanged. Only the total resistance and the total
current in the total circuit changes. No changes in
any individual branch in the circuit occur
V. Schematic Diagrams (35.5)
A. Schematic diagram- simple diagrams to
represent electrical circuits.
1. Symbols used to represent
certain circuit elements
2. Circuit diagrams
(schematics) show electrical
connections, not the physical
layout
3. Common Symbols:
B. Examples of Series and Parallel circuits
VI. Combining Resistors in a Compound Circuit
(35.6)
A. Sometimes it is useful to know the equivalent
resistance of a circuit that has
several
resistors in its network
1. Equivalent resistance- value of the
single resistor that would comprise the
same load to the battery or power
source
2. Calculate using the rules for adding
resistors in series and parallel
R  RA  RB  RC  ....
1
1
1
1



R R A RB RC
B. Series circuits-
R  R A  RB
R  8  8  16
C. Parallel circuits-
1
1
1


 ....
R R A RB
1
2

R
8
2R  8
1 1 1 2
  
R 8 8 8
R  4
VII. Parallel Circuits and Overloading (35.7)
A. When add more devices (pathways) in
house, the combined resistance is lowered in
the circuit
1. Therefore, greater amount of current
occurs
2. Can overload circuit and may result in
heat (fire)
B. Connect fuses in series along supply line to
protect (fuse or circuit breaker prevents
overloading)
Ammeters and Voltmeters
Ammeter
• An ammeter measures current in any branch or
part of a circuit
• It does not change the current in the circuit
Voltmeter
• Used to measure voltage drop across some part
of a circuit
• Has very high resistance so that is causes the
smallest possible change in current or voltages in
the circuit
conventional current flow theory an older theory
stating that electric current flows from the more
positive source to the more negative source