Transcript Energy&Power

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Energy and Power
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Objectives
• Define energy and power
• Calculate power in a circuit
• Properly select resistors based on power
considerations
• Explain energy conversion and voltage drop
• Discuss power supplies and their
characteristics
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DC Electric Power
• The electric power in watts associated with a
complete electric circuit or a circuit component
represents the rate at which energy is converted from
the electrical energy of the moving charges to some
other form, e.g., heat, mechanical energy, or energy
stored in electric fields or magnetic fields.
• For a resistor in a DC circuit the power is given by the
product of applied voltage and the electric current:
Power = Voltage x Current
P = VI
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Power
Power is the rate energy is “used” (actually converted
to heat or another form). Power is measured in watts (or
kilowatts). Notice that rate always involves time.
One watt = one joule/second
Three equations for power in circuits that are
collectively known as Watt’s law are:
P  IV
PI R
2
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V
P
R
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Power in an Electric Circuit
There is always a certain amount of power
in an electric circuit, and it is dependant on
the amount of resistance and the amount of
current, expressed as:
P=
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IR
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Equivalent Expressions for
Power
• Using Ohm’s law, and substituting V for
IR:
P = VI
• Using Ohm’s law, and substituting I for
V/R:
P = V2/R
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Watt’s Law
• The power relationships are known as
Watt’s law.
• Current (I) must be in amperes,
• Voltage (V) must be in volts,
• Resistance (R) must be in ohms.
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Power Dissipated in Resistor
Convenient expressions for the power
dissipated in a resistor can be obtained by
the use of Ohm's Law.
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Power
What power is dissipated in a 27 W resistor is the
current is 0.135 A?
Given that you know the resistance and current,
substitute the values into P =I 2R.
PI R
2
 (0.135 A)  27 W 
2
 0.49 W
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Power
What power is dissipated by a heater that draws 12 A
of current from a 120 V supply?
The most direct solution is to substitute into P = IV.
P  IV
 12 A 120 V 
 1440 W
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Power
What power is dissipated in a 100 W resistor with 5 V
across it?
V2
The most direct solution is to substitute into P 
.
R
V2
P
R
5 V


2
100 W
 0.25 W
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Small resistors operating in low
voltage systems need to be sized
for the anticipated power.
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Energy
1n
Energy, W, is the ability to do work and is
measured in joules. One joule is the work
done when a force of one Newton is
applied through a distance of one meter.
The symbol for energy, W, represents
work, but should not be confused with the
unit for power, the watt, W.
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1m
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Kilowatt-hour
• The kilowatt-hour (kWh) is frequently used
as a unit of energy. One kWh is used when
one thousand watts is used for one hour.
• Power utilized over a period of time
represents energy consumption.
W=Pt
• Energy can also be expressed as wattseconds (Ws), watt-hour (Wh).
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Energy
The kilowatt-hour (kWh) is a much larger unit of
energy than the joule. There are 3.6 x 106 J in a kWh.
The kWh is convenient for electrical appliances.
What is the energy used in operating a
1200 W heater for 20 minutes?
1200 W = 1.2 kW
20 min = 1/3 h
1.2 kW X 1/3 h =0.4 kWh
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Energy and Power
Energy is the ability to do work; and power
is the rate at which energy is used.
Power = energy/time
P=W/t
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Units of Energy and Power
• Energy is measured in joules (J)
• Power is measured in watts (W)
• By definition:
One watt is the amount of power when one
joule of energy used in one second.
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Amounts of Power
• Amounts of power less than one watt are
expressed as milliwatts (mW), microwatts
(W), and even picowatts (pW).
• Electrical utilities and transmitting stations
may use large amounts of power, expressed
as kilowatts (kW), and megawatts (MW).
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Heat produced by Current
• When there is current through resistance, the
collisions of the electrons produce heat, as a result
of the conversion of electrical energy.
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Resistor Power Rating
Resistor power rating is not related to ohmic
value but rather is determined by the
physical composition , size and shape of the
resistor.
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Resistor Power Rating
• Power rating of a resistor is the maximum amount
of power that a resistor can dissipate without being
damaged by excessive heat buildup.
• Power rating is directly related to surface area.
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Metal-film Resistors
Metal-film resistors
have standard power
ratings of 1/8 W, 1/4
W, 1/2 W, and 1 W.
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Selecting the Proper Power
Rating
• A resistor used in a circuit must have a
power rating in excess of what it will have
to handle.
• Ideally, a rating that is approximately twice
the actual power should be used when
possible.
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Resistor Failures
• When excessive power is applied to a resistor, the
resistor will overheat.
• The resistor will burn open, or its resistance value
will be greatly altered.
• Overheated resistors may be charred (burnt), or
the surface color may change.
• Resistors suspected of being damaged should be
removed from the circuit and checked with an
ohmmeter.
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Energy Conversion and Voltage
Drop in Resistance
• As electrons flow through resistors, some of
their energy is given up as heat.
• The same number of electrons entering a
resistor will exit it, only their energy will be
less, so the voltage exiting a resistor is less
than the voltage entering the resistor.
• There is a voltage drop across a resistor.
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Power Supplies
A power supply is a device that provides
power to a load.
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Power Supplies
• A battery is a dc power supply that converts
chemical energy into electrical energy.
• Electronic power supplies generally convert
110 VAC (alternating current) from a wall
outlet into a regulated dc (direct current) at
a level suitable for electronic components.
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Ampere-hour Ratings of Batteries
• Because of their limited source of chemical
energy, batteries have a certain capacity that limits
the amount of time over which they can produce a
given power level.
• The ampere-hour rating determines the length of
time that a battery can deliver a certain amount of
current to a load at the rated voltage.
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Power Supply Efficiency
An important characteristic of electronic
power supplies is efficiency, which is the
ratio of output power to input power.
Efficiency = Pout/Pin
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Power Loss
The output power of an electronic power
supply is always less that the input power,
because some of the input power is used to
operate the power supply circuitry.
POUT = PIN - PLOSS
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Efficiency of Power Supplies
High efficiency means that little power is
lost and there is a higher proportion of
output power for a given input power.
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Summary
• The power rating in watts of a resistor determines
the maximum power that it can handle safely.
• A resistor should have a power rating higher than
the maximum power that it is expected to handle in
the circuit.
• Power rating is not related to resistance value.
• A resistor normally opens when it burns out.
• Energy is the ability to do work and is equal to
power multiplied by time.
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Summary
• The kilowatt-hour equals one thousand watts used
for one hour or any other combination of watts
and hours that has a product of one.
• A power supply is an energy source used to
operate electrical and electronic devices.
• A battery is one type of power supply that
converts chemical energy into electrical energy.
• An electronic power supply converts commercial
energy into regulated dc at various voltages levels.
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Summary
• The output power of a supply is the output voltage
times the load current.
• A load is a device that draws current from the
power supply.
• The capacity of a battery is measured in amperehours (Ah).
• One ampere-hour equals one ampere used for one
hour, or any combinations of amperes and hours
that has a product of one.
• A circuit with high efficiency wastes less power
than one with a lower efficiency.
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