A Complete Circuit
Electrons flow from the source to the load and
then back to the source.
The battery is the
source that supplies
energy to the circuit.
The light bulb is the
load where energy
leaves the circuit.
The electrons are
recycled.
Switches
The function of a switch is to start or stop
the flow of electrons.
With the
switch closed,
electrons can
flow and the
light bulb is
on!
Switches
The function of a switch is to start or stop
the flow of electrons.
When the
switch is
opened, the
circuit is
incomplete
and electrons
cannot flow.
The light bulb
is off!
The Water Flow Analogy
Compare this with the electric circuit.
What part of
this plumbing
circuit is
comparable in
function to the
switch?
Anatomy of a Flashlight
Electrons flow from the
base of the battery to
the metal strip along the
side. If the switch is on,
then this strip presses
tightly against the side of
the light bulb. Electrons
return from the bulb to
the top of the battery.
The circuit must be
complete for the bulb
to light up.
Series Circuits
In a series circuit, there is only one path that
current may follow.
What happens to current in the other lamps if one
lamp in a series circuit burns out?
Parallel Circuits
In a parallel circuit, there are many routes that
current may follow.
What happens to the current in the other lamps if one
of the lamps in a parallel circuit burns out?
Schematic Diagrams
The series circuit.
The parallel
circuit.
Circuits in our Homes
Household appliances are wired in parallel with
each other.
How do we know this must be true?
Safety Devices
When too many devices are plugged into a parallel
circuit, it draws too much current from the source.
This is called an overload.
Fuses protect us
from overloads
by melting. They
break the circuit
before a fire can
start.
When Things Go Wrong
When electrical devices stop working, the
cause is usually one of two things.
An incomplete circuit.
OR
A short circuit.
When Things Go Wrong
Here are two examples of incomplete circuits.
In neither
case can the
electrons flow
from the
source to the
load and then
back to the
source.
When Things Go Wrong
Here are two examples of incomplete circuits.
Where
would you
add a wire to
complete
each circuit?
When Things Go Wrong
Why is this circuit incomplete?
What could
you do to fix
it?
Troubleshooting
In the real world, incomplete circuits
happens whenever something
breaks,
melts,
pulls apart, or
is not assembled correctly.
Troubleshooting
Contrast these two pictures.
good light bulb
burned-out bulb
When Things Go Wrong
In a short circuit, there is a path for
electrons to follow from the source to the
load and back again that has very little
electrical resistance.
This momentarily causes a
dangerously large amount of current to
flow. To prevent a disaster, a fuse
blows or a circuit breaker shuts the
circuit off.
When Things Go Wrong
Why is this a short circuit?
What will
happen in this
case?
How would
you fix it?
Quick Quiz
Two arrangements of battery, bulb, and wire are
shown below. Which of the two arrangements, if
either, will light the bulb? Explain.