Transcript 4. Temperature

DRILL
April 29, 2009
Ohm’s Law states that voltage equals current times resistance,
V=IR.
Watt’s Law states that power equals voltage times current,
P=VI.
Watt’s 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 Ohm’s Law:
2. Divide both sides by R:
V=IR
V/R=I
3. State Watt’s Law:
P=VI
4. Substitute V/R for I:
P=V(V/R)
5. Combine like terms:
P=V2/R
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Derivation #2 – current and resistance only
1. Start with Ohm’s Law:
V=IR
2. State Watt’s Law:
P=VI
3. Substitute IR for V:
P=(IR)I
4. Combine like terms:
P=I2R
Let’s check our two derived equations by re-solving
yesterday’s 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?
12 A
P=I2R
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?
P=V2/R
0.3 W
9V
P= (9 V)2/(0.3W)
P= (81 V2)/(0.3W)
P= 270 Watts
This is the same answer we got yesterday
by another method!
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POLY ENGINEERING
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
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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.
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