Before we get started, let’s review:
Describe a Series Circuit
Draw a circuit diagram of a
closed series circuit with a 1.5V
battery and three light bulbs
What happens if there is a
break anywhere in a series
circuit?
Why are Christmas lights
wired in a series circuit?
A series circuit has only one
path for the current so
the current is the same at
any point in the circuit.
You can use Ohm’s law to
calculate the current in a
circuit if you know the
voltage and resistance.
Adding resistors in a
circuit is like adding
pinches in a hose.
The greater the
number of pinches
or resistors, the greater the
resistance to current (flow).
You can think of
adding resistances
adding pinches to a
hose. Each pinch
adds some resistance.
The total resistance is the sum
of the resistances from each pinch.
To find the total resistance in a series
circuit, you add the individual resistances.
A series circuit contains a 12-V battery
and three bulbs with resistances of
1Ω, 2 Ω, and 3 Ω. What is the current
in the circuit? (First draw a circuit diagram)
A string of 5 lights runs on a 9-V
battery. If each bulb has a resistance
of 2 Ω, what is the current?
(First draw a circuit diagram)
A series circuit operates on a 6-V
battery and has two 1 Ω resistors.
What is the current?(First draw a circuit
diagram)
Voltage Drops and Ohm’s Law
-Each separate bulb or resistor in a
circuit creates a voltage drop.
-As a result, the voltage gets lower
after each device that uses power. This
is known as the voltage drop.
Voltage drop and Ohm’s law
 The voltage drop across a resistance is
determined by Ohm’s law in the form V
= IR.
 The voltage drop (V) equals the current
(I) multiplied by the resistance (R) of
the device.
What voltage drop
should the
Multi-meter read if
each bulb has a
Resistance of 1 Ohm?
Why?
The law of conservation of energy also
applies to a circuit.
Kirchhoff’s voltage law - the total of
all the voltage drops must add up to
the battery’s voltage.
To calculate the voltage drop across a
resistance, Ohm’s Law (I=V/R) is
manipulated to solve for volts, V=IR.
This formula must be used for each
resistance in a circuit to calculate the
voltage drop.
This circuit contains a 9-volt battery, 1-ohm
bulb, and a 2-ohm bulb. Calculate the
circuit’s total resistance and current. Then
find each bulb’s voltage drop.
Parallel Circuits
Describe a parallel circuit
Parallel Circuits
Draw a parallel circuit with three
lightbulbs and two batteries.
Parallel Circuits
Parallel Circuits
 Unlike series circuits, parallel
circuits contain more than one path
for current to flow.
Parallel Circuits
 Every parallel circuit contains at least one
point where the circuit divides, providing
multiple paths for the current.
Parallel Circuits
 The point where the circuit divides is
called a juncture or branch point.
 The paths are called branches.
 The current through
a branch is called
the branch
current.
Parallel Circuits
 Because there are multiple branches, the
current may not be the same at all points
in a parallel circuit.
 Kirchhoff’s Current Law - At every branch
point, the current flowing out must equal
the current flowing in.
Parallel Circuits
The total current in the circuit is the sum of the currents
in all the branches.
Parallel Circuits
 In a parallel circuit:
-the voltage is the same across each
branch.
-the amount of current in each branch is
not necessarily the same.
 the resistance in each branch determines the
current in that branch.
 Branches with less resistance have larger
amounts of current than branches with more
resistance.
Voltage and parallel circuits

Parallel circuits have two advantages over
series circuits.
1.
Each device in the circuit has a voltage drop
equal to the full battery voltage.
2.
Each device in the circuit may be turned off
independently without stopping the current in
the other devices in the circuit.
Current and parallel circuits
 Each branch works
independently so the
total current in a
parallel circuit is the
sum of the currents in
each branch.
Parallel Circuits
 In series circuits, adding an extra
resistance increases the total resistance of
the whole circuit.
 The opposite is true in parallel circuits.
Parallel Circuits
 Adding resistance in parallel provides
another path for current, and more
current flows.
 When more current flows for the same
voltage, the total resistance of the circuit
must decrease.
Parallel Circuits
Parallel Circuits
A circuit contains a 2-ohm resistor and a 4ohm resistor in parallel. Calculate the
total resistance of the circuit.
Most electrical appliances have a label that
lists the power in watts or kilowatts (kW).
The kilowatt is used for large amounts of
power.
One kilowatt (kW) equals 1,000 watts.
To calculate power in a circuit:
A 12-volt battery is connected in series to
two identical light bulbs. The current in the
circuit is 3 amps. Calculate the power
output of the battery.
VIRP Table
Series
VIRP Table
Parallel