Download 7. Electric Circuits

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What is an electric circuit?
An electric circuit is
a path for electric
charge to flow along.
Like all electrical
devices, this
portable MP3 player
contains electric
circuits to enable it
to play music.
What is needed for an electric circuit to work?
 source of energy
 a complete loop
 a component
 no short circuits
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How can we model electric circuits?
A model can help
us to understand
how current works
in an electric circuit.
In this model, the
moped riders
represent the flow
of charge and the
pizzas represent
the electrical
energy carried
around the circuit.
What do the pizza shop and the
house of party-goers represent?
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Using circuit diagrams
Which competitor is finding it easier to build a circuit?
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Why use circuit diagrams?
Electronic circuits are
usually very complex.
It is important to have a
clear way of showing how
different components are
connected together.
A standard set of symbols
is used to represent
different devices and to
draw schematic diagrams
of circuits, which show
how the components
are connected.
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How are wires shown in circuit diagrams?
Wires or leads are used to
carry the current around a
circuit between the
components.
This copper lead is made from
thin wires twisted together to
make the lead more flexible.
Copper is used as it is a very
good conductor of electricity.
The lead is covered with a
plastic sleeve, which prevents
a short circuit if the lead
touches other bare wires.
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Circuit symbol
for a wire
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Are the wires connected?
Circuit diagrams can be complex and it is important to be
able to distinguish between wires that are joined together
and those that cross without being joined.
Circuit symbol for
two wires that
are connected
Circuit symbol for
two wires that cross
(i.e. not connected)
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What is the source of energy in a circuit?
Mains electricity is often used
to power electric circuits.
When mains electricity is
unsuitable, cells or batteries
can be used as the source of
electrical energy.
This remote control uses batteries.
Circuit symbol
for a cell
Circuit symbol
for a battery
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V
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Circuit components
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Circuit symbols
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What is a short circuit?
Current will always pass along the path of least resistance.
The resistance of the wires in a circuit is low compared to
the resistance of components, such as bulbs.
If current can flow along a path without passing through the
components this part of the circuit is called a short circuit.
The current can only
pass through the bulb.
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These circuits both
contain a short circuit, so
the bulb will not be lit up.
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Which circuit diagram?
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What is current?
Current is a measure of the rate of flow of electric charge in
a circuit. Electric charge is measured in coulombs.
How is current measured?
Current is measured in amperes
(A) using an ammeter.
This unit is named after Andre
Ampere, one of the early
scientists to study electricity.
A current of 1 A is 1 coulomb of
charge flowing every second.
So, a current of 5 A is 5 coulombs
of charge flowing every second.
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A
An ammeter must be
connected in series
as it measures the
current flowing
through it.
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What is potential difference?
Potential difference is a measure of the difference in
electrical potential energy between two points in a circuit.
Potential difference is also called voltage.
Voltage is measured in volts
(V) using a voltmeter.
The voltage of a battery or
cell is a measure of the force
or “push” it gives the current.
A 1 V cell gives 1 joule (J) of
energy to each coulomb of
charge.
So, a 12 V battery gives 12 J of
energy to each coulomb of charge.
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V
A voltmeter must be
connected in parallel
as it measures the
potential difference
across the component.
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Water model of a circuit
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Are they cells or batteries?
A cell is a useful source of
electricity.
A chemical reaction takes
place inside the cell, which
produces a potential difference
(voltage) across the cell.
In everyday language, people often refer to cells as batteries.
However, there is a difference between a cell and a battery
and it is important to use the terms correctly!
Circuit symbol
for a cell
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What is a battery?
A battery consists of two or
more cells that are joined
together.
The potential difference
across a battery is the sum
of the potential differences
across the cells.
A 12 V car battery contains
six 2 V cells inside its case.
Circuit symbol
for a battery
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V
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How can battery voltage be increased?
Using more cells in a battery
increases the voltage.
If two or more cells are
connected in series, the
total voltage across the
battery can be found by
adding up the cell voltages.
So, if 2 cells with a voltage
of 1.5 V are connected
together, the voltage
across the battery is 3 V.
When three cells of 1.5 V are
connected, what is the
voltage across the battery?
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1.5 V
V
3V
V
4.5 V
V
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Current and voltage – true or false?
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What are conductors and insulators?
In some materials the electric charges are not free to move.
This means the current cannot flow.
Conductors are materials
that allow the flow of
electric charge. Metals are
good conductors.
Insulators are materials that
do not allow the flow of
electric charge. Plastic is an
example of a good insulator.
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How do metals conduct electricity?
It is the delocalized electrons involved in metallic bonding
that allow metals to conduct electricity.
The delocalized electrons are
free to flow through the metal
and so carry a current.
Insulating materials do not
contain free electrons and
so current is unable to flow.
Ionic solutions are also able
to conduct electricity
because they have mobile
charge-carrying particles.
delocalized electrons
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Does it conduct?
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Why do cables have plastic coatings?
Electricity and electric wires can be very dangerous.
Insulators can be used to make wires safer.
Metals, usually copper, are
used to make electrical
wires because they
conduct current.
To prevent electric shocks,
the metal wires are coated
with a thick layer of plastic.
How does this increase safety?
Plastic is an insulator and
does not conduct current.
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Conductors and insulators
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What is conventional current?
Before the discovery of the electron, scientists assumed that
current was due to positively-charged particles moving from
the positive terminal around a circuit to the negative terminal.
This way of representing the
direction of current is called
conventional current.
It is now know that charge is
carried by electrons, flowing
from the negative terminal
to the positive terminal.
This is called electron flow.
Today, both conventional current and electron flow can be
used to represent the direction of current.
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Representing current direction
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Representing current – activity
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What’s the difference between series and parallel?
Circuit components can be wired in series or parallel.
A series circuit has all its
components wired in the
same loop.
These tree lights are
wired in series.
A parallel circuit contains
junctions and so there is more
than one path for the current.
Car headlights are wired in
parallel. What would happen
if they were wired in series?
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Series circuits – experiment
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Current in a series circuit
In a series circuit, the current flows through one continuous
pathway.
So, the current is the same in all parts of a series circuit.
1A A
A
1A
A
1A
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Voltage in a series circuit
In a series circuit, the voltage supplied by the battery is
shared by the components. So, the sum of the potential
difference across the components equals the battery voltage.
V
V
3V
6V
V
3V
As more bulbs are added in series, each bulb has less
potential difference and so the bulbs become dimmer.
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Parallel circuits – experiment
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Current in a parallel circuit
In a parallel circuit, the current divides at the point where the
circuit branches and then recombines to complete the circuit.
So, the current is not the same in all parts of a parallel circuit.
A 2A
2A A
1A
A
A
1A
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Potential difference in parallel circuits
In a parallel circuit, the
potential difference across
each bulb is the same as
the potential difference
across the battery.
V
This means that all the
bulbs have the same
brightness, and they are
brighter than the same
number of bulbs in a
series circuit.
V
However, this also means that
the battery will run down
faster in a parallel circuit.
V
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6V
6V
6V
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Comparing circuits
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Glossary (1/2)
 ammeter – A device used to measure current.
 ampere – The unit used to measure current. Its symbol is A.
 battery – The scientific word for two or more cells that are
joined together.
 cell – A chemical source of voltage. The chemicals inside
this device react and electrons are pushed out into the circuit.
 circuit – A complete path that current can travel along.
 current – The rate of flow of electric charge in a circuit.
It is measured in amperes (A).
 electron flow – The movement of negatively-charged
electrons from a negative terminal around a circuit.
 parallel circuit – A complete circuit that branches into two
or more paths.
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Glossary (2/2)
 potential difference – The scientific name for voltage,
measured in volts (V). Measured across a component, it is
the difference in electrical potential energy between the two
sides of the component.
 series circuit – A complete circuit in which components
are wired one after the other in a continuous loop.
 volt – The unit used to measure potential difference
(voltage). Its symbol is V.
 voltmeter – A device used to measure potential
difference (voltage).
 voltage – Another name for potential difference. For a
cell, it is the amount of energy that the cell gives to each
electron pushed out into the circuit.
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Anagrams
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Multiple-choice quiz
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