Transcript Electricity – Electric Circuits

KS4 Physics
Electric Circuits
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Contents
Electric Circuits
Circuit diagrams
Conductors and insulators
Current and potential difference
Series and parallel circuits
Summary activities
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Why use circuit diagrams?
Electronic circuits are
often very complex.
It is important to have a
clear way of showing how
different components are
connected together.
A standard set of symbols
are used to represent
different devices and to
draw schematic diagrams
of circuits, which show
how the components are
connected.
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Common components: wires
Wires or leads are used to
carry the current around a
circuit between the different
components.
This lead is made from thin
copper wires twisted together
to make the lead more flexible.
Copper is used as it is a very
good conductor of electricity.
The wire 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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Common components: wires
Circuit diagrams are often complex and it is important to be
able to distinguish between wires that are joined together
and those that cross without being joined.
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Circuit symbol for
two wires that are
connected
or
Circuit symbol for
two wires that cross
(i.e. not connected)
or
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Common components: switches
A switch is a break in a circuit that can be closed and opened,
controlling the flow of electricity around the circuit.
Circuit symbol
for a switch
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open
closed
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Common components: cells
A cell is a useful source of
electricity.
A chemical reaction happens
inside the cell, which produces
a potential difference (voltage)
across the cell.
In everyday life, people normally 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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Common components: batteries
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 volt car battery contains
six 2 volt cells inside its case.
Circuit symbol
for a battery
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Common components: filament lamps
Filament lamps are often
used in circuits in school.
They are useful as the
brightness of the bulb is
proportional to the current
flowing through the bulb.
The filament is a fine wire
that heats up and glows
when a current is passed
through it.
Circuit symbol
for a lamp
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or
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Common components: resistors
A resistors has a resistance to the current flowing through it.
A fixed resistor has a set
resistance that does not change.
Circuit symbol for
a fixed resistor
A variable resistor, or rheostat,
has a resistance that can be
changed by adjusting a slider.
Circuit symbol for
a variable resistor
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Common components: meters
Meters are measuring instruments.
In electronics, different types
of meters are used to measure
different quantities about a circuit
and its components.
An ammeter measures the
current flowing through the
part of a circuit that it is
connected to.
Circuit symbol for
an ammeter
A
A voltmeter measures the
Circuit symbol for
potential difference (or
a voltmeter
voltage) across a component.
V
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Contents
Electricity: Simple Circuits
Circuit diagrams
Conductors and insulators
Current and potential difference
Series and parallel circuits
Summary activities
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A broken circuit
Would you expect the lamp
to light in this circuit?
No! There is a gap in the
circuit.
Air has a very high resistance and so it does not conduct
under normal conditions. This means that the circuit is
incomplete and a current will not flow.
If the potential difference is very high, then air can be forced
to conduct electricity. The electricity can be seen as a spark
jumping between the wires.
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Does it conduct electricity?
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Why can metals conduct electricity?
An electric current is a flow of charge. For materials to be
able to conduct, they must be able to transfer a charge when
a potential difference is applied across them.
In metals, some electrons (which
are negatively charged) are free
to move between the atoms.
These are called free electrons
and are why metals are good
conductors of electricity.
Covalent solids, such as
plastic, do not contain free
electrons and so are poor
conductors of electricity.
These are called insulators.
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metal ions
Electrons are free
to move and carry
an electrical charge.
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Why can salts conduct electricity?
Salts and some other solids are made of ions. These ions
are electrically charged and can carry charge through the
substance.
In the solid form, the ions are all strongly bonded (joined)
together and so are unable to move.
When an ionic solid melts, the strong bonds between the
ions weaken and the ions are able to move about and carry
the charge.
When an ionic solid dissolves in water, the ions are also
separated and are free to move about. This is why solutions
of salts are good conductors of electricity. Pure water which
does not contain any dissolved salt is a poor conductor as it
does not contain many ions to carry charge.
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Contents
Electricity: Simple Circuits
Circuit diagrams
Conductors and insulators
Current and potential difference
Series and parallel circuits
Summary activities
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What is electric current?
An electric current is a flow of charge. In a wire, the electric
current is the flow of the free electrons through the metal.
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Conventional current
The first scientists to work with electricity did so before the
discovery of the electron. They carried out their work without
fully understanding current and thought that it flowed from
positive to negative. This is called conventional current.
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Electron flow in circuits
However, when the electron was discovered and the true
nature of current understood, it was discovered that the early
scientists had got it wrong! The electrons actually flow from
the negative end of the battery around to the positive end.
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Measuring current
The unit used to measure current is the ampere. It is named
after Andre Ampere, who was one of the early scientists to
work on electricity. Current is measured using an ammeter.
A
An ammeter must be
connected in series as
it measures the current
flowing through it.
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The unit ampere is often
shortened to amp and is
given the symbol A.
The current in a circuit
is the rate of flow of
charge in the circuit.
A current of 1 amp is
1 coulomb of charge
flowing every second.
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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 without passing through the components
in a circuit, this is called a short circuit.
The current can only
pass through the bulb.
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These circuits both
contain a short circuit.
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Potential difference
The potential difference (or voltage) across a component in a
circuit is the difference in electrical potential energy between
the two sides of the component.
Potential difference is
measured with a voltmeter.
The units of potential
difference are volts, given
the symbol V.
A 1V cell gives 1 J of energy
to each coulomb of charge.
A 12V 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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Model of an electric circuit
Current is the flow
of electric charge.
A model can help
to understand how
this works in an
electric circuit.
In this model, the
moped riders
represent the flow
of charge and the
pizzas represent
the electrical
energy carried by
this current.
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Contents
Electricity: Simple Circuits
Circuit diagrams
Conductors and insulators
Current and potential difference
Series and parallel circuits
Summary activities
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Current in series circuits
The current in a series circuit must flow through all parts of
the circuit. Therefore, the size of the current is the same in
all parts of a series circuit.
1A A
A
1A
A
1A
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Potential difference in series circuits
If the bulbs in a series circuit are identical, then the potential
difference is shared equally between them.
As more bulbs are added, each bulb has less potential
difference and becomes dimmer.
V
V
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3V
6V
V
3V
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Current in parallel circuits
The current in a parallel circuit divides at any point where the
wires divide. If the bulbs are identical, an equal current will
flow through each bulb.
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 of the bulbs is the
same as the potential
difference across the battery.
This means that all the bulbs
have the same brightness
and are brighter than the
same number of bulbs in a
series circuit.
However, this also means
that the battery will run down
faster in the parallel circuit.
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V
V
V
6V
6V
6V
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Comparing series and parallel circuits
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Contents
Electricity: Simple Circuits
Circuit diagrams
Conductors and insulators
Current and potential difference
Series and parallel circuits
Summary activities
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Glossary
 ammeter – A device used to measure current.
 battery – The scientific word for two or more cells that are
joined together.
 cell – A chemical source of current. The chemicals inside this
device react and electrons are pushed out into the circuit.
 current – A flow of charge, which is measured in amperes (A).
 potential difference – The scientific name for voltage,
which is measured in volts (V). It can be measured across a
component and is the difference in electrical potential energy
between the two sides of the component.
 voltmeter – A device used to measure potential difference.
 voltage – Another name for potential difference. When
measured across a cell, it is the amount of energy 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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