PSAA Curriculum
Unit
Physical Science Systems
Problem Area
Energy and Power Systems
Resistance in Series and
Parallel Circuits
Lesson
Christmas Lights…
Series or Parallel Circuits?
 Before plugging in the lights, which string
is connected in series and which one is
connected in parallel?
 Now that the lights are lit up, if we pull a
bulb out, what is going to happen?
Different wiring sizes
 What are some common uses of the
various wires displayed?
 Why would someone want larger wires?
Smaller wires?
 Why is a large wire used by power
companies to bring electricity to your
home when only small wire is needed to
carry that same electricity to an appliance?
Learning Objectives
 Explain series and parallel circuits, the
advantages and disadvantages of each,
and how to connect series and parallel
circuits.
 Determine voltages for resistors in series
and parallel circuits, and determine total
current flowing through series and parallel
circuits.
Learning Objectives
 Explain the mathematical relationship that
exists in circuits containing more than one
resistor and use Ohm’s law in calculating
current, resistance, or voltage for different
circuits.
Terms
 Ampere
 Kirchoff’s First Law
 Kirchoff’s Second
Law
 Load
 Ohm
 Ohmmeter
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Ohm’s Law
Parallel Circuit
Resistance
Series Circuit
Series-Parallel Circuit
Volt
What are parallel and series
circuits?
 The series circuit is an electrical
connection method in which current flows
through all devices in a circuit before
returning to the source.
Series Circuit
Schematic Diagram
What are parallel and series
circuits?
 A parallel circuit is set up when the
resistors or capacitors are connected so
as to allow current to divide and flow
through each individual device, then to
combine and flow back to the source.
Parallel Circuit
Schematic Diagram
What are parallel and series
circuits?
 The two types of circuits can be combined
to form a series-parallel circuit.
 This circuit has at least one resistor
connected in series with at least two resistors
connected in parallel.
 The circuit is considered complete in its
loop or path when the current can flow
through the load before returning to the
source or point of origin.
Series-Parallel Circuit
Schematic Diagram
How do voltage and amperage
differ in their movement through
series and parallel circuits?
 In a series circuit, voltage is divided between
the devices connected in the circuit
proportionately to the resistors.
 Each device in a series circuit receives the same
amperage.
 There is only one path that the current flow may
follow.
How do voltage and amperage
differ in their movement through
series and parallel circuits?
 By contrast, current may follow more than
one path in a parallel circuit.
 Current (amperage) can flow through any
single device or all devices
simultaneously.
Movement through parallel
circuits.
 Devices connected in parallel receive the
same voltage.
 However, the total current flowing through
a parallel circuit splits proportionately at
each resistor and is equal to the sum of
the current flowing through each device.
Kirchoff’s Laws
 Kirchoff’s First Law states that current
entering a junction is equal to the current
leaving the junction.
 Considering the behavior of voltage in
these circuits, Kirchoff’s Second Law
follows up by stating that the sum of the
voltage drops around a complete circuit
loop or path is equal to the applied voltage
to the circuit.
What is Ohm’s Law and how is it
used to determine voltage, current,
and/or resistance for electrical
circuits?
 Ohm’s Law states that the current in an
electrical circuit is directly proportional to
the amount of voltage which causes the
current to flow, and inversely proportional
to the amount of resistance in the circuit.
Ohm’s Law
 One volt (electrical pressure) is needed to
push one ampere (electrical current)
through a conductor with one ohm
(electrical resistance) of resistance.
 Resistance is the opposition to the flow of
electrons.
Ohm’s Law (cont.)
 Ohm’s law can be mathematically
expressed as follows:
Current = Voltage
or
Resistance
Where: Current = I
Voltage = E
Resistance = R
I=E
R
Ohm’s Law (cont.)
 Examining the mathematical relationship,
it is easy to see that as the voltage is
increased, the amount of current flowing in
the circuit is increased.
 If the amount of voltage is decreased, the
amount of current flowing in the circuit is
decreased.
Ohm’s Law (cont.)
 The formula also shows that as the
resistance in the circuit is increased, the
amount of current in the circuit is
decreased.
 If the amount of resistance in the circuit is
decreased, the amount of current in the
circuit is increased.
Ohm’s Law (cont.)
 As the length of a conductor increases
there is more resistance to the flow of
electricity along the conductor which
results in a drop in voltage.
 The resistance of the circuit can easily be
measured across any device with an
ohmmeter.
Ohm’s Law Circuit Schematic
Diagram for Testing Resistors
Review/Summary
 What are parallel and series circuits?
 How do voltage and amperage differ in
their movement through series and parallel
circuits?
 What is Ohm’s Law and how is it used to
determine voltage, current, and/or
resistance for electrical circuits?