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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.
SwitchesThe 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!
SwitchesThe 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
The circuit must be complete for the bulb
to light up.
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.
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 CircuitsIn 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 HomesHousehold appliances are wired in parallel with
each other.
How do we know this must be true?
Safety DevicesWhen 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.
ORA 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 QuizTwo arrangements of battery, bulb, and wire are shown below. Which of the two arrangements, if
either, will light the bulb? Explain.
