Download N:2-3 / Series and Parallel Circuits

It’s a cool, clear night as you
stroll by the harbor with your
family.
The night is dark, but the
waterfront is bright thanks to
the thousands of twinkling
white lights that outline the
tall ships.
They make a striking view.
•As you walk, you notice that
a few of the lights are
burned out.
•The rest of the lights,
however, burn brightly. If
one bulb is burned out, how
can the rest of the lights
continue to shine?
•The answer depends on
how the electric circuit is
designed.
•The parts of a circuit can be
arranged in series or in
parallel.
•If all the parts of an electric circuit are connected
one after another, the circuit is a series circuit.
•The photo and diagram below show a series
circuit. In a series circuit, there is only one path for
the current to take.
• For example, a switch and the device it controls
are connected in series with each other.
•A series circuit is very simple to design and build, but it
has some disadvantages.
•What happens if a bulb in a series circuit burns out?
•A burned-out bulb is a break in the circuit, and there is
no other path for the current to take.
•So if one light goes out, all the lights go out.
•Another disadvantage of a series circuit is that the
light bulbs in the circuit become dimmer as more
bulbs are added.
•Why does that happen?
•Think about what happens to the overall resistance
of a series circuit as you add more bulbs.
•The resistance increases.
•Remember that if resistance increases, current
decreases.
• So as light bulbs are added to a series circuit, the
current decreases.
•The result is that the bulbs burn less brightly.
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N:2-3 / Series and Parallel Circuits
•Could the lights on the ships
have been connected in
series?
•No—if the lights were part
of a series circuit, all of the
lights would have gone off
when one burned out.
•What you saw, however,
was that a few lights were
burned out and the rest
were brightly lit.
Parallel Circuits
•The ships’ lights were connected in parallel circuits.
•In a parallel circuit, the different parts of the circuit are on
separate branches.
•The photo and diagram below show a parallel circuit.
•In a parallel circuit, there are several paths for current to
take.
•Notice that each bulb has its own path from one terminal of
the battery to the other.
Parallel Circuits
•What happens if a light burns out in a parallel circuit?
•If there is a break in one branch, current can still move
through the other branches.
•So if one bulb goes out, the others remain lit.
•Switches can be placed along each branch so that
individual bulbs can be turned on and off without affecting
the others.
Parallel Circuits
•What happens to the resistance of a parallel circuit when
you add a branch?
•Although you might think that the overall resistance
increases, it actually decreases.
• To understand this, consider the flow of water once again.
• Suppose water is being released from a reservoir held by a
dam.
Parallel Circuits
•If the water is allowed to flow through one pipe, a certain
amount of water comes out.
•But if two pipes are used instead of one, twice as much
water flows.
•The water will flow more easily because it has two paths to
take.
•The same is true for a parallel circuit.
•As new paths, or branches, are added, the electric current
has more paths to follow, and so total resistance decreases.
Parallel Circuits
•What does this tell you about current?
•If resistance decreases, the current must increase.
• The increased current travels along the new
branch without affecting the original branches.
•So as you add branches to a parallel circuit, the
brightness of the light bulbs does not change.
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•A couple more things…
•Different meters are wired into circuits in different ways.
•A voltmeter is used to measure voltage.
•When you measure the voltage across some device, the
voltmeter and the device should be wired as parallel
circuits.
•An ammeter is used to measure current.
•If you want to measure the current through some device in
a circuit, the ammeter should be connected in series with
the device.
•Would you want the circuits in your home to be
series circuits?
•Of course you would not.
•With a series circuit, all the electrical devices in
your home would go off every time a light bulb
burned out or a switch was turned off.
•Instead, the circuits in your home are parallel
circuits.
•Electricity is fed into a home by heavy wires called lines.
•These lines have very low resistance.
•You can see in the illustration below that parallel branches
extend out from the lines to wall sockets, appliances, and
lights in each room.
•The voltage in these household circuits is 120 volts. Switches
are located in places where they can be used to control
one branch of the circuit at a time.
Comparison
Series Circuit
Parallel Circuit
Only one path for
Definition current to flow
Multiple paths for
current to flow
Total resistance
Resistance
increases
Examples
-Holiday lights
Total resistance
stays same
-Household
Current