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Topic 3
RESISTING THE MOVEMENT OF
CHARGE
Resisting Movement of a
Charge
• Resistance is a property of a substance
that hinders motion of electric charge and
converts electric energy into other forms
of energy.
• If current is described as the movement of
electrons through conductors, and
voltage is what makes the electrons
move, then resistance is what opposes the
motion of the electrons.
• A good conductor has low resistance.
• Poor conductors have high resistance.
Resistance Roadblock
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Nichrome wire is a resistor.
The standard unit for resistance is the ohm ( Ω)
Resistance can be measured with an ohmmeter
Technicians are more likely to use a multi-purpose
electronic measuring instrument (a “multimeter”)
set to measure resistance
• Ohm’s law R=V/I ie. Resistance = Voltage (potential
difference)/ Current
• Other forms of the equation are: V = IR and I = V/R
Example
• What is the resistance of an electric
heater, if a current of 12.5 A runs
through it when it is connected to a 120
V wall outlet?
Practice
• Please work on the next four practice
problems in your notes independently.
Resistors
In electronic circuits, resistance is useful for:
• Changing the energy of moving charges
into heat.
• To control current or voltage to suit the
specific needs of other electric devices in
the circuit. Circuit resistors are usually
made of a heat-conducting ceramic core
coated with materials such as carbon,
metal oxides, metal film, or resistance wire
Variable Resistors
• Variable resistors change in response to
temperature, light, voltage, or some other
variable.
• Varying the length of a Nichrome wire can act
as a variable resistor. The longer the length, the
more resistance there is.
• Variable resistors are used in light dimmers,
motor speed controls, surge protectors, and
refrigerator and stove temperature controls.
• Variable resistors are also called rheostats.
Types of Circuits
• Series circuits have only one current
path.
• Parallel circuits have several current
paths.
Series Circuit
• As more bulbs are added in a series
circuit, the bulbs become dimmer
because there are more resistors for
the current to travel through-more
bulbs mean less current.
• In a series circuit all loads must be
functional & all switches closed for any
of the loads to work
Series Circuit
• Advantages of series circuits:
– Easy to construct
– Less wire
– Batteries last longer
• Disadvantages of series circuits:
– Additional bulbs mean dimmer lights
– If one bulb burns out, they are all out
Parallel Circuits
• Adding more bulbs to a parallel circuit does
not decrease the brightness of the bulbs.
More bulbs increases the current.
• In a parallel circuit not all loads need to be
functional for the circuit to work because
there are a number of paths the current can
travel
Parallel Circuits
• Advantages of a parallel circuit
– Additional bulbs does not reduce the
brightness
– Other loads continue to work even though one
may not be working.
• Disadvantages of a parallel circuit
– More wires to construct
– Wires heat up with increased current flow
– Batteries won’t last as long
House Wiring
• Practical electric circuits for the home
are parallel circuits. The voltage across
each load in a parallel circuit is the same.
Turning on one appliance will not reduce
the energy available to other loads.
House Wiring
• One potential problem does exist with
parallel circuits. Current through wires
connected to the source increases
whenever another branch in the circuit is
closed. When you turn on any appliance in
your home, the current in the wires closest
to the source increases. More current
means the temperature of the conducting
wire rises.
House Wiring
• The wires could become hot enough to
start a fire. To guard against an electrical
fire, household circuits always include fuses
or circuit breakers.