20.2 Resistors in Series
or Resistors in Parallel
pp 736- 745
Essential Questions
 Based on the orientation of resistors in a circuit,
how is the distribution of current and voltage
affected?
 What are the advantages or disadvantages of
arranging resistors in certain ways?
Objective(s): Students will be
able to…
 Calculate the equivalent resistance for a circuit
of resistors in series, and find the current in and
potential difference across each resistor in the
circuit.
 Calculate the equivalent resistance for a circuit
of resistors in parallel, and find the current in and
potential difference across each resistor in the
circuit.
Agenda:
 The plan through May 31.
 Check and review HW from 20.1 p 735 # 1-5
 Introduction to series and parallel circuits.
 Notes:
 Resistors in Series
 Resistors in Parallel
The Plan
 Final is Thursday, May 31. We have 10 class days
before then, including today.
 5/16-5/17: 20.2 Resistors in Series and Parallel.
 5/18: 20.3 Complex resistor combinations
 5/21: Lab
 5/22: Chapter 20 Review/ Lab Worksheet
 5/23: Chapter 20 Test
 5/25, 5/26, 5/29, 5/30: Review for Final Exam
Resistors in Series
 When there exists a circuit or portion of a circuit
that provides a single conducting path without
junctions, that circuit or portion of a circuit is
connected in SERIES.
Current in Series
 Think of current like cars (or
sheep if you’d prefer) on a track.
 If the cars are bumper-to-bumper
(like electrons), the current can’t
build up or lessen anywhere in
the track.
 This means the current is the
same everywhere in the circuit
(track).
 In other words, the current
flowing through each resistor in a
series is equal.
Equivalent Resistance in
Series
 It is easier to consider
multiple resistors as an
equivalent single resistor.
 Each resistor in a series
acts like a dam for the
current, so the equivalent
resistance is the sum of the
individual resistances.
 Note: the equivalent
resistance in series is
always larger than any
individual resistance.
Calculations in Series
 When the equivalent resistance is
known, current and voltage can be
calculated as if the entire circuit
contains just one resistor.
 I = ΔV/Req
 You try: A 9.0 V battery is connect to
four light bulbs in series with resistances
of 4.0, 7.0, 2.0, and 5.0 Ω respectively.
What is the current in the circuit?
 I = 0.50 A
Voltage Drop
 Each resistor in a circuit uses some of the voltage
(potential difference) supplied by the power
source.
 The amount of potential difference across each
resistor is called the VOLTAGE DROP.
 To calculate the voltage drop, use ΔV=IR.
 The voltage used by each resistor is equal to the
current running through it multiplied by its
individual resistance.
Series Circuits
 Because series circuits allow only one pathway
for current, no current flows if an element breaks.
Think of cheap seasonal lights.
 Advantages to series circuits:
 Each element helps to control current, to avoid
overheating.
 Some circuits succeed if they fail. Like burglar
alarms.
Resistors in Parallel
 When two or more elements in a circuit are
connected across common points or junctions,
providing multiple pathways for current, these
elements are in PARALLEL.
Voltage Drop in Parallel
 Parallel circuits behave such that each element
is directly connected to the incoming power
source.
 It is almost like each element is in its own circuit.
 Thus, the voltage drop across each branch of a
parallel circuit is the same.
Current in a Parallel Circuit
 Because each branch acts like its own circuit…
 The total current drawn in a parallel circuit is the
sum of the current in each of the branches.
Equivalent Resistance in
Parallel
 Each element in parallel adds another pathway
for current to flow. Like adding another lane for
traffic.
 As a result, when a resistor is added to a parallel
circuit, overall resistance DECREASES!
 The equivalent resistance is always smaller than
any individual resistance.
Calculations in Parallel
 Try the same elements as the
previous example, but now
arranged in parallel instead of in
series.
 A 9.0 V battery is connected to
four light bulbs in parallel with
resistances of 4.0, 7.0, 2.0, and 5.0
Ω respectively. What is the
current in the circuit?
 I = 9.8 A
Parallel Circuits
 Each branch of a parallel circuit provides a
separate path for current, so if one branch fails,
the rest can keep going.
 Think not-cheap seasonal lights.
 Household wiring is on parallel circuits because:
 We want to be able to turn things on an off
independently.
 We have a standard input voltage 120V to every
device, and can design those devices
accordingly.
Recap: Series
 In series circuits:
 The equivalent resistance is equal to the sum of the
individual resistances.
 Resistance increases and current decreases as
elements are added.
 Circuits fail if one element fails.
Recap: Parallel
 In parallel circuits:
 The reciprocal of the equivalent resistance is equal
to the sum of the reciprocals of the individual
resistances.
 Resistance decreases and current increases as
elements are added.
 The rest of the circuit still conducts electricity if one
device fails.
Recap Activity
 Clear your desks.
 Organize the slips of paper into the 2 columns in
which they belong: series or parallel.
 You have 5 minutes.
Answers
Series
Parallel
The current through each resistor is the
same
The voltage through each resistor is the
same.
As resistors are added the total resistance
increases.
As resistors are added, the total resistance
decreases.
As resistors are added, the current in the
circuit decreases
As resistors are added, the current in the
circuit increases.
If a device fails, current will cease to flow If a device fails, current will continue to
to other devices.
flow to other devices.
A flashlight is a good example of this
type of circuit.
Household wiring tends to be on this type
of circuit.
If you have a 6V battery connected to a
2Ω resistor and a 3Ω resistor, the total
current running through the circuit is 1.2
amps.
If you have a 6V battery connected to a
2Ω resistor and a 3Ω resistor, the total
current running through the circuit is 5
amps.
Homework
 Due Wed: p739 #1-4
 Due Thurs: p745 #1-6