Series circuits
D
o you remember how the parts of the torch on pages 272–3 were connected
together? The circuit contained several components, connected one after
the other. Conductors, like the metal strip and the battery cases, linked the
components together. The circuit was a single, complete loop. This type of circuit
is called a series circuit.
The good thing about series
circuits is that they are simple to
put together. But if any part of a
series circuit (such as a light
globe) doesn’t work, the circuit
is broken. A series circuit will
not work if even just one part of
it breaks down.
Both of these circuits are series circuits.
The ammeter
30
0
500
20
DC 30
mA
0
0
10
40
50
A
200
100
0
40
An ammeter measures
electric current. Current refers to
the number of electrons flowing
through a circuit per second. The
more electrons that flow through
the circuit per second, the higher
the current and the higher the
ammeter reading.
Electrons pass through every
conductor that is placed in a series
circuit. In fact, the same number of
electrons pass through each
conductor (per second) — the
current is the same throughout a
series current.
An ammeter is always connected
in series. This way, the electrons
that flow through the circuit will
also flow through the ammeter.
The ammeter then measures how
many electrons have passed
through it.
The circuit symbol for an
ammeter is shown below.
The negative terminal of the ammeter is connected so that it is closer
to the negative terminal of the battery
than it is to the positive terminal.
—
50
0m
A
50
The positive terminal of the
ammeter is connected closer to the
positive terminal of the battery. Select the
positive terminal with the highest value
first. If the current is too small to register
on this scale, select the smaller scale.
mA
There are usually two
or more scales on an
ammeter. Your selection of
positive terminals tells
you which scale to read.
An ammeter is easily damaged. If the current reading
goes off the scale, open the circuit immediately. Always check the
terminal connections before closing the circuit to make sure the
ammeter is placed correctly in series.
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A series of batteries
Many portable CD players, calculators and toys run on more than one
battery. If this is the case, the batteries are likely to be connected one
after the other, in series. When batteries are
connected in series, electrons flowing
through the circuit must flow through
each battery. The electrons are given
electrical energy from each battery.
REMEMBER
1. Describe how the
components in a series
circuit are arranged.
2. To which terminal of a
battery should the positive
terminal of an ammeter be
connected?
3. Why is it important to check
the connections to an
ammeter before closing the
circuit?
4. The two circuits below
contain identical globes and
batteries. What should the
reading of the ammeter in
the second circuit be?
Explain your answer.
The two batteries in this case are 1.5 V each. In
total, these batteries provide 3 V of energy to the
electrons flowing through them. Note that the positive
terminal of one battery is joined to the negative terminal of
the other. Batteries must always be connected this way.
Using an ammeter
You will need:
6 V battery or other power supply
ammeter
2 light globes (6 V)
switch
connecting wires.
• Connect a series circuit that contains a battery, an ammeter, a switch,
two globes and connecting wires.
Remember to check that the
A
terminals are connected correctly.
Connect to the highest scale of
the ammeter first.
A
10 mA
1. (a) What is the reading on the
ammeter before the switch is
closed?
(b) How much current is flowing
through the circuit before the switch is closed?
A
• Complete the circuit by closing the switch. Record the reading shown
on the ammeter.
• Change the circuit so that the
ammeter is in the position shown
in the diagram at right.
5. What is a disadvantage of
connecting circuit
components in series?
• Close the switch. Record the
ammeter reading.
PREDICT
2. What do you notice about the
ammeter reading in the second
position?
6. Predict what would happen
to globe A if globe B ‘blew’
in this circuit.
A
B
• Predict the reading on the ammeter if it were to be placed in the
position shown in the diagram below.
• Test your prediction.
• Compare your results with those
of other members of the class.
What do you notice?
A
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12. Electrical energy
✓ learning
3. Complete this sentence:
The current in a series circuit
.
A
I CAN:
describe what a series circuit is
connect batteries and other
components in series
describe a disadvantage of series
circuits.
!
Parallel circuits
magine what would happen if the electrical appliances in your home were all
connected in series? Every time a light blows out, no other electrical appliance
would work. A series circuit will not work if a part of the circuit breaks.
To avoid this problem, most circuits are connected in parallel. A parallel circuit
works even when one part of it breaks down.
I
At this point, the electrons can
either move along the first
path or the second path. If the
paths are identical, half of the
electrons will take the first
path and the other half will
take the second path.
Although the number of
electrons is divided between
the two paths, each electron
will continue to carry the
same amount of energy
(voltage).
Second path
First path
A parallel circuit drawn
as a circuit diagram.
A parallel circuit has more than one path for the electricity to
follow. If one of the paths has a break in it, the other paths
will still work. Only appliances in the broken part of the
circuit will stop working.
The voltmeter
The voltmeter tells us how much of the electron’s energy is
turned into light energy. This light globe uses almost all of the
total energy given to the electrons.
This battery gives 6 V
of energy to the
electrons as they leave
the negative terminal
of the battery.
6V
–
+
5
0
10
0
1
Volt
s
2
15
3
The energy of electrons
moving around a circuit comes
from a battery or other power
supply. As the electrons move
around the circuit, some of their
energy is transformed into other
forms of energy by the appliances
in the circuit. The energy that is
transformed along the circuit is
measured in volts with a voltmeter.
Like an ammeter, a voltmeter is
placed in a circuit with its positive
terminal closer to the positive
terminal of the power supply than
the negative terminal. But, unlike
an ammeter, a voltmeter is
connected in parallel to the
appliances in a circuit.
The symbol for a voltmeter is
shown below.
V
Electrons
Electrons
—
15
v
3v
Voltmeters
measure how
much energy the
electrons lose as they pass through the appliance.
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REMEMBER
1. What unit is electrical energy measured in?
2. When a voltmeter is used to take readings of an
appliance, is it placed in series or parallel with
the appliance?
3. To which terminal of the battery is the negative
terminal of a voltmeter connected more closely?
THINK
4. In the experiment ‘Parallel circuits’, why did the
remaining light globes still work when one was
removed?
5. If a 9 V battery is used to power a circuit with
four globes in parallel to each other, how many
volts of electrical energy will each globe receive?
6. A battery is connected to two identical globes
that are connected in parallel. The current
flowing through one globe is 400 mA. What
current flows through:
(a) the other globe?
(b) the battery?
7. Why are all of the appliances in a house
connected in parallel?
These lights are connected in parallel. Why?
Parallel circuits
You will need:
6 V battery or power supply
3 light globes (6 V)
switch
connecting wires.
• Connect the circuit as shown
in the diagram at right.
• Close the switch. Record
the reading on the voltmeter.
SKILL BUILDER
8. Using the correct
symbols, draw this
circuit as a circuit
diagram.
V
• Disconnect the voltmeter.
Reconnect it across the next
globe as shown in the
diagram below.
• Close the switch. Record the
reading shown on the
voltmeter.
5
0
• Repeat the process for the
third globe.
2
15
3
15
v
3v
V
4. Complete this sentence: The voltage through each
branch of a parallel circuit is
.
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12. Electrical energy
✓ learning
9. (a) Use the correct symbols to draw a circuit
diagram containing two light globes in
parallel with each other. Make sure you
include a battery in your circuit
diagram.
(b) Redraw the circuit so that a single switch
turns the whole circuit on or off.
(c) Redraw the circuit with switches placed
so that the light globes can be turned
on separately.
• Remove one of the globes.
3. Compare the readings on
the voltmeter in this
experiment with the voltage
of the battery or power
source used. What do you
find?
1
Volts
—
1. How did the readings on the
voltmeter change for each
globe?
2. Do the remaining globes still
work if one globe is
removed?
10
0
I CAN:
use a voltmeter to measure the
energy transformed by an
appliance
describe and construct simple
parallel circuits.
!
Comparing circuits
ave you ever connected last year’s Christmas lights only to find that they don’t
work? You’re sure that they are plugged in and that the switch is closed, but
still nothing happens. It could be that the globes are connected in series and one
globe has burnt out. Now comes the tricky part, finding out which globe has burnt!
Not all Christmas lights are connected in series. Those connected in parallel will
still work even if some of the globes have burnt out. And it’s easy to tell which
globes need to be replaced.
H
Comparing circuits
You will need:
3 identical light globes
6 V battery
switch
PART B
• Connect a parallel
circuit like the one
shown.
connecting wires
voltmeter
burnt-out light globe.
• Close the switch. Note
the brightness of the
globes.
PART A
• Connect a
series circuit
with three
identical,
working globes,
a battery and a
switch.
• Use a voltmeter to
measure the energy used
by each of the globes.
Complete a table like the
one shown below.
A
B
C
Globe
• Close the switch. Note the brightness of the globes.
B
C
Reading on voltmeter (V)
A
• Use a voltmeter to measure the amount of energy
used by each globe. Record the voltage in a table
like the one below.
Globe
A
B
C
Reading on voltmeter (V)
4. How does the energy used by each globe in a
parallel circuit differ from the energy used by each
globe in a series circuit?
A
B
5. How does the brightness of the globes in the parallel
circuit differ from the globes in series?
C
• Replace one of the globes with the burnt-out globe.
• Close the switch.
• Replace one of the globes in the parallel circuit with
the burnt-out globe. Close the switch.
1. Describe what happens in the circuit with the burntout globe.
6. Describe what happens in the parallel circuit when
one light globe does not work.
2. How much current flows through the series circuit
containing the burnt-out globe?
7. How does the brightness of the remaining globes
change?
• Remove the burnt-out globe and one of the working
globes.
8. List two advantages of a parallel circuit over a series
circuit.
3. How does the brightness of one globe compare with
the brightness of three globes in series?
9. Give one advantage of a series circuit over
a parallel circuit.
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Science Alive for VELS Level 5
!
Go to worksheet 12.3:
Modelling circuits
What affects brightness?
The brightness of each globe depends
on both the voltage and the electric
current. The experiment on page
280 shows how voltage affects
the brightness of globes. In
series circuits, the electrons
share their energy (voltage)
among all of the globes in
the circuit. The more globes
there are, the more the
energy needs to be shared
and the less brightly they glow.
The branches of a parallel
circuit do not share the energy
carried by each electron. So,
identical
globes placed in parallel glow
A 60 watt globe
with equal brightness. No matter how
many branches are added to a parallel circuit,
the brightness of each identical globe is the same.
What about in your home?
All globes are connected in
parallel, but because they are
not all identical, some may
glow more brightly than
others. The electrons
flowing through each
globe receive the same
amount of energy.
Voltage is a measure
of how much energy
electrons receive —
for homes in Australia,
it’s 240 V. However,
some globes allow more
electrons through than
others.
REMEMBER
1. Why are the appliances in
houses and other buildings
connected in parallel? Give
at least two reasons.
2. Describe a disadvantage of
parallel circuits.
3. What do numbers printed
on a globe refer to — for
example, 80 W?
4. Why does a 70 W globe
glow more brightly than a
60 W globe, even though
each electron passing
through them receives the
same amount of energy?
THINK
5. Describe how the
brightness of a globe is
affected by:
(a) voltage
(b) current.
SKILL BUILDER
6. Are the circuits below the
same or different? Explain
your answer.
The numbers printed on light globes
tell us how quickly the globes use energy.
A 75 watt (W) light globe uses energy more
quickly than a 60 W light globe. The
electrons that pass through these globes
carry the same amount of energy (240 V).
But more electrons pass through the 75 W
globe than the 60 W globe in any given
time to supply energy more quickly. So,
current, as well as voltage, affects
brightness. (Remember that current refers
to the number of electrons flowing
through a circuit per second.)
Christmas lights — if these lights are connected in
parallel, when one light blows out, the others still
work.
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12. Electrical energy
✓ learning
A 75 watt globe
I CAN:
explain how voltage affects
brightness
explain how current affects
brightness.
!