Technological Impacts

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DRILL
April 29, 2009
Ohms Law states that voltage equals current
times resistance, VIR. Watts Law states that
power equals voltage times current, PVI. Watts
Law is most useful when the voltage and
current are given, but sometimes they are not
known. Derive 2 more equations for Power. One
equation should involve only voltage and
resistance. The second equation should involve
only current and resistance.
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Derivation 1 voltage and resistance only
1. Start with Ohms Law VIR
2. Divide both sides by R V/RI
3. State Watts Law PVI
4. Substitute V/R for I PV(V/R)
5. Combine like terms PV2/R
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Derivation 2 current and resistance only
1. Start with Ohms Law VIR
2. State Watts Law PVI
3. Substitute IR for V P(IR)I
4. Combine like terms PI2R
Lets check our two derived equations by
re-solving yesterdays drill questions.
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Solve the following problems. Draw a
schematic diagram and label all electrical
components. Show all equations, substitutions,
and box in your answers with units. 1. What is
the power when 12 amperes flow through a 15 ohm
resistor?
PI2R
12 A
15 W
P (12 A)2(15W)
P (144 A2)(15W)
P 2160 Watts
This is the same answer we got yesterday by
another method!
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Solve the following problems. Draw a
schematic diagram and label all electrical
components. Show all equations, substitutions,
and box in your answers with units. 2. What is
the power when a 9 Volt battery is connected to a
0.3 ohm resistor?
PV2/R
P (9 V)2/(0.3W)
0.3 W
9 V
P (81 V2)/(0.3W)
P 270 Watts
This is the same answer we got yesterday by
another method!
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There are six ways to generate (create)
electricity 1. Friction - Rubbing two objects
together can remove electron(s) from some
neutral atoms, and deposit the electron(s)
on other neutral atoms. The atoms that
lose electrons become positively charged,
while the atoms that gain electrons become
negatively charged. This type of electricity
is called 'static' electricity, because it is
not moving, but is stationary.
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One example of static electricity in nature
is the accumulation of charges on clouds,
which eventually results in lightning as the
large static charge discharges into the
ground. Friction is not a practical method
of creating electricity for use in modern
electrical devices such as TV, radio,
refrigerators, etc.
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2. Chemical - Electricity can be produced by
placing two dissimilar metals in a liquid
called an electrolyte. For example, if zinc
(Zn) and copper (Cu) are placed in salt water,
electricity can be produced. Some examples
of chemical cells car batteries, AA batteries,
AAA batteries, hearing aid batteries,
flashlight batteries, etc.
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3. Light - Electricity can be produced by
using special devices called photocells,
which convert light into electrical energy.
Photocells are found in devices like 'solar'
calculators, which use light to create
electricity. Solar panels are becoming very
popular as a way of producing large amounts
of electricity. Photocells should not be
confused with photoresistors.
Photoresistors do not create electricity, they
merely change resistance when exposed to
light.
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4. Heat - Electricity can be produced by
using special devices called thermocouples,
which convert heat into electrical energy.
The thermocouple is made of two dissimilar
metals, joined at a junction. Thermocouples
are often used in control systems. For
example, the thermocouple can be used to turn a
device on or off depending on the temperature.
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5. Pressure - Electricity can be produced by
using special devices called piezoelectric
cells, which convert pressure into electrical
energy. Piezoelectric cells are often found
in tiny microphones and earphones. In a
microphone, a piezoelectric cell converts sound
pressure into electrical current. In an
earphone, a piezoelectric cell converts
electrical current into sound. What very
common modern device utilizes a tiny microphone
and earphone?
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6. Magnetism - Electricity can be produced by
moving a magnet past a wire (or vice versa),
which converts motion (kinetic energy) into
electrical energy. This method is the most
practical way of generating electricity in
large amounts. In order to make the magnet move
past the wire (or vice versa), we can utilize
a turbine to turn the generator. In our IOT
unit on Power and Energy, we saw
turbine-generator combinations used in
hydroelectric plants, wind turbine power
plants, nuclear power plants, coal-fired steam
cycle power plants.
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All metal Wires have resistance. The resistance
of a wire is based on 4 factors 1. Material 2.
Length 3. Cross-sectional area 4. Temperature
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1. Material Some materials conduct electricity
better than other materials. For example, gold
conducts electricity better than copper. However,
gold is extremely expensive compared to copper,
so it is only used in very special cases.
Material technology (one of the 9 Core
technologies) is very important in Electricity
and Electronics.
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2. Length A long wire will have more resistance
than a shorter wire of the same material,
cross-sectional area, and temperature. Imagine
that a long wire is simply two shorter wires
connected end-to-end in series. When two or
more resistors are connected in series, their
combined resistance is greater than any of the
individual resistors. Therefore, a long wire has
a resistance which is the sum of the resistances
of the shorter pieces. One of the reasons for
the development of modern micro-circuits is to
reduce the resistance, thus causing the power
requirements to be minimized.
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3. Cross-sectional Area A thick wire will have
less resistance than a thin wire of the
same material, length, and temperature. Imagine
that a thick wire is simply two or more wires
bundled together side-by-side in parallel. When
two or more resistors are connected in parallel,
their combined resistance is less than any of the
individual resistors. Therefore, a thick wire
has a resistance which is less than the
resistances of the thinner pieces. You have
probably noticed that the wire in a light bulb is
very thin, thus giving it a high resistance which
causes it to get hot and glow. The same thing
occurs in an electric toaster, where the
wire becomes red hot.
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4. Temperature In general, as the temperature of
a wire increases, the resistance increases. The
opposite is true as well. As the temperature of a
wire decreases, the resistance decreases. The
changes of resistance with temperature can be
explained by looking at the Chemistry involved.
When a metal is heated, the molecules move
farther apart and move faster. Therefore, for
electrons to jump from one atom to another
(electrical current) in a heated metal, they must
move farther, and they must hit a 'moving target'
(the next atom). This makes it more difficult
for electrons to move, thus increasing the
resistance.
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Conversely, when a metal is cooled, the molecules
move closer together, and become less active.
Therefore, for electrons to jump from one atom to
another (electrical current) in a cooled
metal, they do not have to move as far, and they
can easily hit a more stationary target (the next
atom). This makes it easier for electrons to
move, thus lowering the resistance. Modern
engineers and scientists are developing
low-temperature superconductors so that
electrical power requirements can be minimized.