ohm’s law, power, and energy
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8/13/2019 Ohm’s Law, Power, and Energy
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Ohm€s Law, Power, and Energy
EEE 3
A. Nerves
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OBJECTIVES
€ Understand the importance of Ohm•s law and howto apply it to a variety of situations.
€ Be able to plot Ohm•s law and understand how to‚readƒ a graphical plot of voltage versus current.
€ Become aware of the differences between powerand energy levels and how to solve for each.
€ Understand the power and energy flow of asystem, including how the flow affects theefficiency of operation.
€ Become aware of the operation of a variety offuses and circuit breakers and where each isemployed.
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OHM€S LAW
€ The first equation to be described iswithout question one of the most importantto 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 periodof time or for instantaneous responses.
€ The equation can be derived directly from
the following basic equation for all physicalsystems:
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OHM€S LAW
€ Every conversion of energyfrom one form to anothercan be related to thisequation.
€ In electric circuits, theeffect we are trying to
establish is the flow ofcharge, or current.
€ The potential difference, orvoltage, between twopoints is the cause(‚pressureƒ), and the
opposition is the resistanceencountered.
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OHM€S LAW
€ Georg Simon Ohm, a German physicist,
identified several cause-and-effect
relationships between circuit current,
voltage, and resistance.
€ Ohm stated that current is directly
proportional to voltage and inversely
proportional to resistance.
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OHM€S LAW
€ Ohm•s law can be statedmathematically as
where
I = the circuit current
E = the applied voltage
R = the circuit resistance
R
E
I €
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OHM€S LAW
Two statements we can derive from Ohm•s law are:
1. The current through a fixed resistance:
a. increases when the applied voltage increases.b. decreases when the applied voltage
decreases.
2. The current generated by a fixed voltage:a. decreases when the resistance increases.
b. increases when the resistance decreases. 8
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OHM€S LAW
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OHM€S LAW
FIG. 4.2 Basic circuit.
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OHM€S LAW
FIG. 4.3 Defining polarities.
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OHM€S LAW
FIG. 4.4 Example 4.3.
FIG. 4.5 Example 4.4.
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The Relationship Between
Current and Voltage
•If voltage increases, current will increase if
resistance remains constant.
•The greater the voltage, the higher the current
for any fixed resistance.
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Using Ohm€s Law to Calculate
Current
€ Ohm•s law can be used to calculate current in
this circuit as
AV
R
E I 5
2
10€
‚€€
15
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Using Ohm€s Law to Calculate
Current
€ The DC power supply has a maximum output of 200 V.
Will the fuse blow?
€ We solve for current as AV
R
E I 06.6
33
200€
‚€€
€ The fuse will blow.16
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Using Ohm€s Law to Calculate
Voltage
€ Ohm•s law can be used to calculate voltage in
this circuit as
V mA R I E 15100150 €‚ƒ€ƒ€
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Using Ohm€s Law to Calculate
Voltage
€ Two 6 „ speakers draw 2.08 A each from a DC
power supply. Solve for the source voltage.
€ The source voltage is found as
V A R I E 5.12608.2 €‚ƒ€ƒ€
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Using Ohm€s Law to Calculate
Resistance
€ Ohm•s law can be used to calculate voltage in
this circuit as‚€€€ 200
25
5
mA
V
I
E R
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Using Ohm€s Law to Calculate
Resistance
€ Solve for the minimum allowable rheostat
setting that will not cause the fuse to blow.
… The minimum resistance is found as
‚€€€ 602
120(min)
A
V
I
E R
20
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Current & Voltage
€ Current is directly proportional to voltage. … If voltage triples:
AV
R
E I 18
20
360€
‚€€
Current triples if voltage triples.It is directly proportional to voltage.
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Current & Resistance
€ Current is inversely proportional to resistance.€ If resistance triples
AV
R
E I 4
60
240€
‚€€
Tripling the resistance caused current to drop
to 1/3 of its original value.22
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Ohm€s Law Wheel
€ Cover the value of interest to see the correct
equation.
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Troubleshooting with Ohm€s
Law
€ Troubleshooting is the process of locating
circuit faults.
€ If a circuit is not operating as it should,Ohm•s law can help find the problem.
€ The first step is to determine the type of
circuit failure.
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Troubleshooting with Ohm€s Law:
Current Too Low
€ Ohm•s law tells us that there are tworeasons why current may be too low.
€ The applied voltage is too low.
… Current is directly proportional to voltage. … If the applied voltage is too low then circuitcurrent will be too low.
€ The circuit resistance is too high.
… Current is inversely proportional to resistance.
… If circuit resistance is too high, circuit currentwill be too low.
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Troubleshooting with Ohm€s Law:
Current Too High
€ Ohm•s law tells us that there are tworeasons why current may be too high.
€ The applied voltage is too high.
… Current is directly proportional to voltage.
… If the applied voltage is too high then circuitcurrent will be too high.
€ The circuit resistance is too low.
… Current is inversely proportional to resistance.
… If circuit resistance is too low, circuit currentwill be too high.
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Summary of Ohm€s Law
Relationships
€ This table summarizes the Ohm•s law
relationships between voltage, current and
resistance.
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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.
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PLOTTING OHM€S LAW
FIG. 4.6 Plotting Ohm€s law.
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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).
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PLOTTING OHM€S LAW
FIG. 4.9 Example 4.5.
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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 amountof time; that is, power is a rate of
doing work.
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POWER
€ Since energy is measured in joules (J) andtime in seconds (s), power is measured in
joules/second (J/s).
€ The electrical unit of measurement for
power is the watt (W), defined by:
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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 itsmaximum current rating.
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Calculating Power
€ The ampere is defined as
€1 A = 1 coulomb per second
€ The volt is defined as
€1 V = 1 joule per coulomb
€ If we multiply amperes times volts, the results is
in joules per second, the unit of measure for
power.
ond jouleCoulomb Joule
ond Coulomb sec/1
11
sec11 €ƒ
40
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Watt€s Law
€ One form of Watt•s law can be stated as
€For the circuit shown,
.EIP ƒ€
kW AV E I P 2.210220 €ƒ€ƒ€
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Watt€s Law
€ Another form of Watt•s law is P = IƒE = IƒIR or
€ P = I2R
€For the circuit shown,
W mA R I P 125500)500(
22€‚ƒ€ƒ€
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Watt€s Law
€ The third form of Watt•s law is P = IƒE = (E/R)ƒE or
€ P = E2/R
€For the circuit shown,
W V R E P 144
100120
2
€‚
€€
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POWER
FIG. 4.14 The nonlinear I-V characteristics of a 75 W lightbulb (Example 4.8).
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Power & Heat
€ Energy can not be created or destroyed .
€ It can only be converted into another form.
… A toaster converts electrical energy into heat. … A light bulb converts electrical energy into
light.
… A motor converts electrical energy into
mechanical energy (movement).€ A device that converts energy from one
form to another is called a transducer . 45
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Power Conversion
€ Power, like energy, can be converted from
one form to another.
… A DC power supply converts AC power to DC
power and then the DC load may convert theDC power into another form.
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Resistor Power Ratings
€ Resistors dissipate power as heat.
… The larger the resistor, the more heat it can
dissipate.
… Standard resistor power ratings are
1 W, † W,
‡ W, and
1/8 W.
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Basic Circuit Calculations
€ There are six basic forms and Ohm•s Law and
Watt•s Law:
R I E ƒ€
R
E
P
2
€
R
E
I €
. E I P ƒ€ R I ƒ€
2
I
E
R €
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Finding the Right Equation
€ In some cases none of the six basic equations
can be used to solve a circuit analysis problem.
… When this is the case, you may have to rearrange one
of the basic equations.
… For example, if voltage and power are known, and youwish to solve for resistance, find an equation with all
the variables, and then rearrange it to solve for the
unknown value.
.22 E Rtotransposed is E P €€
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The Simplest Circuit
€ The simplest electrical circuit contains a source,
a load, and the necessary conductors to connect
them together.
… The source supplies the power to the load.
… The load dissipates the power as heat, light,
mechanical energy, or some other form.
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Sources and Loads
€ A source can be as simple as a battery, or
very complex.
… A surround-sound receiver could be
considered a source for the system speakers.€ A load can also be simple or complex.
… A laptop computer could be considered a load
for its battery.
€ Whenever a load draws the maximumcurrent from its source, this is considered
to be a full load . 51
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Circuit Faults: The Open Circuit
€ An open circuit is a physical break in the
conduction path.
… Circuit resistance is close to infinite.
… Circuit current drops to zero.
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The Open Circuit
€ The applied voltage is measured across
the open component.
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Circuit Faults: The Short Circuit
€ A short circuit is the opposite of an open.
€ A short circuit is an unintentional low-
resistance connection between two points.
… This results in higher than normal current andusually results in a
fuse opening or
circuit breaker
tripping.
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ENERGY
€ For power, which is the rate of doing work,to produce an energy conversion of anyform, 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
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ENERGY
€ Note that the energy in kilowatthours issimply the energy in watthours divided by1000.
€ To develop some sense for the
kilowatthour energy level, consider that 1kWh is the energy dissipated by a 100 Wbulb in 10 h.
€ The kilowatthour meter is an instrumentfor 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.) 58
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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.)59
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ENERGY
TABLE 4.1 Typical wattage ratings of some common household
items. 60
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Efficiency
€ The efficiency of a circuit is the ratio of input
power to output power, usually expressed as a
ratio.
€ It is defined mathematically as
%100ƒ€
IN
OUT
P
P €
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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 mustalways be less than the applied energy due
to losses and storage within the system.
€ The best one can hope for is that W out and
W in 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. 63
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EFFICIENCY
FIG. 4.19 Basic components of a generating system. 64
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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 industrialplant, heavy equipment, simple circuit inthe home, or meters used in the laboratorymust be limited to ensure that the currentthrough the lines is not above the rated
value.€ To limit the current level, fuses or circuit
breakers are installed where the powerenters the installation, such as in the panelin the basement of most homes at the point
where the outside feeder lines enter thedwelling.
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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.
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
€ In homes built in recent years, fuseshave been replaced by circuitbreakers.
€ When the current exceeds ratedconditions, an electromagnet in thedevice will have sufficient strength todraw the connecting metallic link inthe breaker out of the circuit and open
the current path.
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
FIG. 4.22 Circuit
breakers.
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CIRCUIT BREAKERS, GFCIs, AND
FUSES
€ The most recentNational ElectricalCode (NEC)/PhilippineElectrical Code (PEC)requires that outlets in
the bathroom andother sensitive areasbe of the ground faultcircuit interrupt (GFCI)variety; GFCIs aredesigned to trip more
quickly than thestandard circuitbreaker.
FIG. 4.23 Ground fault circuitinterrupter (GFCI): 125 V ac, 60 Hz, 15
A outlet.
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Applications: Electric Lighting
IncandescentLamp
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Applications: Electric Lighting
Compact Fluorescent Lamp (CFL)
bulbs.com sciencetablets.blogspot.com
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Applications: Electric Lighting
Light Emitting Diode (LED) Lamp
ecnmag.com
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rootsbd.com
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Applications: Electric Lighting
flexflireleds.com
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Applications: Electric Lighting
spie.org76
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Applications: Electric Lighting
micro.rohm.com 77
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APPLICATIONSMicrowave Oven
FIG. 4.26 Microwave
oven.
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APPLICATIONSHousehold Wiring
€ The one specification that defines the overallsystem is the maximum current that can be drawnfrom the power lines since the voltage is fixed at120 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 areopting for the 200 A service even if they do nothave electric heat.
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APPLICATIONSHousehold Wiring
FIG. 4.27 200 A service conductors: (a) 4/0 aluminum and 2/0 copper; (b)
three-wire 4/0 aluminum service.
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