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Chapter 4
Ohm’s Law, Power,
and Energy
Introductory Circuit Analysis, 12/e
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OBJECTIVES
• Understand the importance of Ohm’s law and how
to apply it to a variety of situations.
• Be able to plot Ohm’s law and understand how to
“read” a graphical plot of voltage versus current.
• Become aware of the differences between power
and energy levels and how to solve for each.
• Understand the power and energy flow of a
system, including how the flow affects the
efficiency of operation.
• Become aware of the operation of a variety of
fuses and circuit breakers and where each is
employed.
Introductory Circuit Analysis, 12/e
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OHM’S LAW
• The first equation to be described is
without question one of the most important
to be learned in this field.
• It is applicable to dc circuits, ac circuits,
digital and microwave circuits, and, in fact,
any type of applied signal.
• In addition, it can be applied over a period
of time or for instantaneous responses.
• The equation can be derived directly from
the following basic equation for all physical
systems:
Introductory Circuit Analysis, 12/e
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OHM’S LAW
• Every conversion of energy
from one form to another
can be related to this
equation.
• In electric circuits, the
effect we are trying to
establish is the flow of
charge, or current.
• The potential difference, or
voltage, between two
points is the cause
(“pressure”), and the
opposition is the resistance
encountered.
Introductory Circuit Analysis, 12/e
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OHM’S LAW
• In summary, therefore, the absence of an applied
“pressure” such as voltage in an electric circuit will
result in no reaction in the system and no current
in the electric circuit.
• Current is a reaction to the applied voltage and not
the factor that gets the system in motion.
• To continue with the analogy, the greater the
pressure at the spigot, the greater is the rate of
water flow through the hose, just as applying a
higher voltage to the same circuit results in a
higher current.
Introductory Circuit Analysis, 12/e
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OHM’S LAW
Introductory Circuit Analysis, 12/e
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OHM’S LAW
FIG. 4.2 Basic circuit.
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OHM’S LAW
FIG. 4.3 Defining polarities.
Introductory Circuit Analysis, 12/e
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OHM’S LAW
FIG. 4.4 Example
4.3.
FIG. 4.5 Example
4.4.
Introductory Circuit Analysis, 12/e
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PLOTTING OHM’S LAW
• Graphs, characteristics, plots, and the like
play an important role in every technical
field as modes through which the broad
picture of the behavior or response of a
system can be conveniently displayed.
• It is therefore critical to develop the skills
necessary both to read data and to plot
them in such a manner that they can be
interpreted easily.
Introductory Circuit Analysis, 12/e
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PLOTTING OHM’S LAW
FIG. 4.6 Plotting Ohm’s law.
Introductory Circuit Analysis, 12/e
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PLOTTING OHM’S LAW
FIG. 4.7 Demonstrating on an I-V
plot that the lower the resistance, the
steeper is the slope.
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PLOTTING OHM’S LAW
FIG. 4.8 Applying Eq.
(4.7).
Introductory Circuit Analysis, 12/e
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PLOTTING OHM’S LAW
FIG. 4.9 Example 4.5.
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PLOTTING OHM’S LAW
• This diode ideally acts as a lowresistance path to current in one
direction and a high-resistance path
to current in the reverse direction,
much like a switch that passes
current in only one direction.
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PLOTTING OHM’S LAW
FIG. 4.10 Semiconductor diode
characteristics.
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POWER
• In general, the term power is applied
to provide an indication of how much
work (energy conversion) can be
accomplished in a specified amount
of time; that is, power is a rate of
doing work.
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POWER
• Since energy is measured in joules (J) and
time in seconds (s), power is measured in
joules/second (J/s).
• The electrical unit of measurement for
power is the watt (W), defined by:
Introductory Circuit Analysis, 12/e
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POWER
• In equation form, power is determined by:
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POWER
FIG. 4.12 Defining the power to a
resistive element.
FIG. 4.13 Example
4.6.
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POWER
• The power associated with any
supply is not simply a function of the
supply voltage. It is determined by the
product of the supply voltage and its
maximum current rating.
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POWER
FIG. 4.14 The nonlinear I-V characteristics of a 75 W light
bulb (Example 4.8).
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ENERGY
• For power, which is the rate of doing work,
to produce an energy conversion of any
form, it must be used over a period of time.
• The energy (W) lost or gained by any
system is therefore determined by:
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ENERGY
Introductory Circuit Analysis, 12/e
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ENERGY
• Note that the energy in kilowatthours is
simply the energy in watthours divided by
1000.
• To develop some sense for the
kilowatthour energy level, consider that 1
kWh is the energy dissipated by a 100 W
bulb in 10 h.
• The kilowatthour meter is an instrument
for measuring the energy supplied to the
residential or commercial user of electricity.
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ENERGY
FIG. 4.16 Kilowatthour meters: (a) analog; (b) digital. (Courtesy of ABB Electric
Metering Systems.)
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ENERGY
FIG. 4.17 Cost per kWh and average kWh per customer
versus time. (Based on data from Edison Electric Institute.)
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ENERGY
TABLE 4.1 Typical wattage ratings of some common
household items.
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EFFICIENCY
• A flowchart for the energy levels associated
with any system that converts energy from
one form to another is provided in Fig.
4.18.
• Note that the output energy level must
always be less than the applied energy due
to losses and storage within the system.
• The best one can hope for is that Wout and
Win are relatively close in magnitude.
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EFFICIENCY
• Conservation of energy requires that
– Energy input = energy output + energy lost
or stored by the system
FIG. 4.18 Energy flow through a system.
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EFFICIENCY
FIG. 4.19 Basic components of a generating system.
Introductory Circuit Analysis, 12/e
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EFFICIENCY
FIG. 4.20 Cascaded system.
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
• The incoming power to any large industrial
plant, heavy equipment, simple circuit in
the home, or meters used in the laboratory
must be limited to ensure that the current
through the lines is not above the rated
value.
• To limit the current level, fuses or circuit
breakers are installed where the power
enters the installation, such as in the panel
in the basement of most homes at the point
where the outside feeder lines enter the
dwelling.
Introductory Circuit Analysis, 12/e
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
FIG. 4.21 Fuses: (a) CC-TRON® (0–10 A); (b) Semitron (0–600 A); (c)
subminiature surface-mount chip fuses.
Introductory Circuit Analysis, 12/e
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
• In homes built in recent years, fuses
have been replaced by circuit
breakers.
• When the current exceeds rated
conditions, an electromagnet in the
device will have sufficient strength to
draw the connecting metallic link in
the breaker out of the circuit and open
the current path.
Introductory Circuit Analysis, 12/e
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
FIG. 4.22 Circuit
breakers.
Introductory Circuit Analysis, 12/e
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
• The most recent
National Electrical
Code requires that
outlets in the
bathroom and other
sensitive areas be of
the ground fault circuit
interrupt (GFCI)
variety; GFCIs are
designed to trip more
quickly than the
standard circuit
breaker.
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FIG. 4.23 Ground fault circuit
interrupter (GFCI): 125 V ac, 60 Hz, 15
A outlet.
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APPLICATIONS
Fluorescent versus Incandescent
FIG. 4.24 A 23 W, 380 mA compact
fluorescent lamp (CFL).
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APPLICATIONS
Fluorescent versus Incandescent
TABLE 4.2 Comparison of the lumens generated by
incandescent and fluorescent bulbs.
Introductory Circuit Analysis, 12/e
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APPLICATIONS
Fluorescent versus Incandescent
FIGURE 4.25 Internal construction of the CFL of
Fig. 4.24.
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APPLICATIONS
Microwave Oven
FIG. 4.26 Microwave
oven.
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APPLICATIONS
Household Wiring
• The one specification that defines the overall
system is the maximum current that can be drawn
from the power lines since the voltage is fixed at
120 V or 240 V (sometimes 208 V).
• For most older homes with a heating system other
than electric, a 100 A service is the norm.
• Today, with all the electronic systems becoming
commonplace in the home, many people are
opting for the 200 A service even if they do not
have electric heat.
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APPLICATIONS
Household Wiring
FIG. 4.27 200 A service conductors: (a) 4/0 aluminum and 2/0 copper; (b)
three-wire 4/0 aluminum service.
Introductory Circuit Analysis, 12/e
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COMPUTER ANALYSIS
FIG. 4.28 Circuit to be analyzed using
PSpice and Multisim.
Introductory Circuit Analysis, 12/e
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COMPUTER ANALYSIS
PSpice
FIG. 4.29 Using PSpice to determine the
voltage, current, and power levels for the circuit
in Fig. 4.28.
Introductory Circuit Analysis, 12/e
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COMPUTER ANALYSIS
Multisim
FIG. 4.30 Using Multisim to determine the voltage
and current levels for the circuit of Fig. 4.28.
Introductory Circuit Analysis, 12/e
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