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Science 9: Unit D – Electrical
Principles and Technologies
Topic 3: Ohm’s Law and Circuits
Resistance
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Resistance is a number value that tells us how difficult it
is for electrons to pass through an object. Almost all
objects, even conductors, have resistance. Resistance
depends on many factors including the thickness of the
object and the material of the object.
The thinner the wire the higher the resistance. The
longer the wire, the higher the resistance. The higher the
temperature of the wire, the higher the resistance.
Insulators have much a much higher resistance than
Conductors.
The job of a resistor is to slow down electrons long
enough so that the electrical energy in electrons is
transferred into another form of moving, heat, or light
energy.
Examples of Resistance
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A light bulb has a thin wire called a filament that
acts as a resistor. The wire, made of the metal
called tungsten (W) is a conductor which
becomes so hot with friction from the slowed
down electrons that it glows white hot.
The unit for resistance is the ohm which is given
the symbol Ω.
Resistance of a material is measured with an
ohmmeter.
Ohm’s Law
When dealing with any circuit an easy way
to calculate resistance is with the following
formula:
 R=V÷I
 where R = resistance,
 V = voltage, and
 I = current.
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Ohm’s Law
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What Ohm’s law tells us is that the higher the
resistance the greater the energy it takes in from
the electrons (so the greater the voltage needed
to make it work).
Also Ohm’s law tells us that the greater the
resistance of the resistor of a circuit the slower
the electrons will be moving overall in the circuit.
The resistor acts as a bottleneck in the circuit.
So the higher the resistance, the lower the
current will be.
Keep in mind that: V=I*R and I=V÷R
Types of Resistors
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Resistors can either have a fixed or variable
resistance. Fixed Resistors are usually made of a
heat conducting material to control the current and
voltage levels. Resistors that do not change as other
variables change are also called Ohmic Resistors
Variable Resistors change their resistance in
response to a changing variable such as temperature,
light, or voltage. Examples include light dimmers and
stove element controls.
Rheostats are variable resistors that you can actually
change the resistance by turning a dial. They are used
in dimmer light switchers; Thermistors are variable
resistors that change resistance based on
temperature; and varistors are variable resistors that
change their resistance depending on the voltage.
Circuit Types
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There are three types of circuits: series,
parallel and combined. Most circuits in the
house are combined or parallel. An
example of series circuits are old
fashioned Christmas lights.
Series Circuits
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Series Circuit – The
current source and
resistors are all
connected together on
the same path. The
voltage from the battery
must be shared by the
resistors leading to lower
energy levels. Also, if
one resistor breaks down
the circuit becomes
broken. See fig. 4.19 on
p. 286.
Parallel Circuits
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Parallel Circuit – Each
resistor each have their own
current path and therefore
each is connected directly to
the battery. Because of this
direct connection the
resistors each have the
maximum voltage coming to
them. Another advantage is
that if one resistor breaks
down the other resistors can
still operate. Most circuits in
a building are parallel for this
reason. See fig. 4.20 on p.
286.
Combined Circuit
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Combined Circuit – A
combination of a
series circuit and
parallel circuit. At least
two resistors are
attached directly
together on the same
path, but there is at
least one circuit on its
own circuit branch.
Problems With Circuits
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A problem with parallel or combined
circuits is that having all the resistors/loads
working at the same time will put more of a
strain on the current source and may
cause it to overheat and catch fire. In a
house, circuit breakers and fuses break
the circuit if the temperature of the source
becomes too high.