Power and Resistance
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Basics of Algebra
Algebra – A generalization of arithmetic in
which letters representing numbers are
combined according to the rules of arithmetic.
Formula – A general fact, rule, or principle
expressed usually in mathematic symbols.
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d=v×t
Equation
A formal statement of the equality or
equivalence of mathematical or logical
expressions.
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Transposition
The transfer of any term of an equation from
one side over to the other side of an equation
with a corresponding change of sign.
8 + a = 12
a = 12 – 8
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Proportional
Proportional – The relation of one part to another or to the
whole with respect to magnitude, quantity, or degree.
Directly proportional – The relation of one part to one or more
other parts in which a change in one causes a similar change in
the other.
Inversely proportional – The relation of one part to one or more
parts in which a change in one causes an opposite change in the
other.
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Substitution
The replacement of one mathematical entity by
another of equal value.
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Resistance
Symbolized by R and measured in ohms ( ), it
is the opposition to current flow with the
dissipation of energy in the form of heat.
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The relationship between
resistance and current.
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The relationship between
resistance and current.
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Resistance is inversely
proportional to current
R
1
α
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I
Resistor – A device constructed of a material
that opposes the flow of current by having
some value of resistance.
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Ohm
Ohm – Unit of resistance, symbolized by the Greek capital
letter omega ( ).
Ohm’s Law – The current flow in a circuit is directly
proportional to the source voltage applied to the circuit and
inversely proportional to the resistance of the circuit.
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Current =
I=
Voltage
Re sis tan ce
V
R
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1V
1mA =
1KΩ
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Ohm’s law triangle
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Conductors and their resistance
Conductor’s Cross-Sectional Area
mil – one thousandth of an inch (.001 inch)
Circular Mil (cmil)
A unit of area equal to the area of a circle whose diameter is 1
mil, calculated by
cmil = (diameter in mils)2
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Resistance of a conductor
R=
p× l
a
ρ = resistivity of the conducting material
l = the length of the conductor in feet
a = the area of the conductor in circular mils
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Resistivity –
Measure of a
material’s
resistance to
current flow.
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Energy and Work
Energy – The capacity to do work.
Work – Work is done anytime energy is
transformed from one type to another.
Joule – Unit of work and energy.
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Power
Power – The amount of energy converted by a
component or circuit in a unit of time.
Watt (W) – Unit of electric power required to
do work at the rate of 1 joule/second.
1 horsepower (hp) = 746 watts
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Measuring power
P = 1V × 1mA = 1mW
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Kilowatt-hour
Kilowatt-hour – 1000 watts used for 1 hour
Kilowatt-hour meter – A meter used to
measure electric power
in kilowatt-hours.
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Body resistance
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This ends
Power and Resistance
Online Resource for ETCH 213
Faculty: B. Allen