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2
Voltage sources
and effects of an
electric current
This chapter outlines how electrical energy is
produced from other sources of energy. It also
explains the effects of an electric current, and
how these effects are used in everyday life.
Some effects are not wanted, including the
effect of an electric current on the human body,
so methods of protecting against the unwanted
effects are also given.
Chapter outline
2.1 Producing a voltage
2.2 Effects of an electric current
2.3 Protection against effects of an
electric current
CHAPTER 2 Voltage sources and effects of an electric current
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2.1 Producing a voltage
As explained further in Chapter 4, electricity is a form of energy. A basic law, called the Conservation
of Energy, says that energy cannot be created or destroyed. However, energy can be converted
from one form to another. This means that electrical energy has to come from other sources of
energy, by a conversion process. The most commonly used energy sources for this are:
• mechanical
• chemical
• heat
• light.
Producing electricity often involves several processes, such as first converting heat to
mechanical energy, then using the mechanical energy to produce electricity. The basic cycle is
shown in Figure 2.1, where an energy source is converted to electrical energy, which is then
converted by an electrical appliance into another form of energy, such as heat.
Heat and
light are
both forms
of ‘radiant’
energy
FIGURE 2.1
Electrical energy is
produced by converting
other forms of energy
As you saw in Chapter 1, a voltage is ‘electrical pressure’ and current is a flow of electrons.
The current does the work (lights a lamp, makes a motor turn), as a voltage by itself is simply a
pressure source. So, electrical energy is a combination of voltage and current. That is, electrical
energy is used (or more correctly, transformed) only when current flows.
This means that the energy source to produce the electrical energy is only being used when
current flows. For instance, a battery that’s not connected to anything is not supplying electrical
energy, even though there’s a voltage developed across its terminals. Here’s a brief look at the
various ways of producing electricity. All these methods convert one form of energy into electrical
energy.
Mechanical to electrical
Mechanical energy is movement. The three main ways to convert mechanical movement into
electrical energy are:
• friction between two materials
• mechanical stress applied to a piezo-electric element
• moving a conductor in a magnetic field.
Friction
Most of us have felt the effect of ‘static’ electricity. When you walk on a nylon carpet or slide over
a fabric-covered car seat you can be ‘charged up’ to a high voltage. This is because electrons have
either been transferred to you or taken from you.
Static electricity is produced when clouds move in the atmosphere, because of friction
between the cloud and the surrounding air. Once the voltage is high enough, an electric current
will flow through the air between two charged clouds. Rubbing certain types of materials
together can also produce static electricity. The available energy from static electricity is
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E l e c t r i c a l P RINCI P LES
rarely useful, although very high voltages can be produced for use in certain types of tests
and experiments.
A popular way of producing a high voltage using friction is with a Wimshurst machine,
invented in the 1880s. This machine has two rotating glass discs fitted with metal sectors,
with one disc made to rotate in the opposite direction to the other. Motive power is with
a crank handle. Two cross bars support metallic brushes that rub lightly against the discs.
When the voltage produced by the action of the rotating discs is high enough, a spark is
produced between two metal spheres. Some machines, such as that shown in Figure 2.2, use
Leyden jars (an early type of capacitor) to increase the accumulated spark energy. A typical
Wimshurst machine can produce sparks up to 80 mm in length, depending on the diameter
of the rotating discs.
FIGURE 2.2
http://en.wikipedia.org/wiki/File:Wimshurst.jpg
The Wimshurst machine
is hand powered, and
can produce high
voltages but not much
energy
The Wimshurst machine led to the invention in 1929 by American physicist Robert J. Van
de Graaff of a more advanced electrostatic generator which uses a moving belt to accumulate
very high electrostatic voltages on a hollow metal globe on the top of a stand. Modern Van
de Graaff generators can produce five million volts or more and are used in test laboratories.
A Wimshurst machine produces a lower voltage, but can produce more current than a Van de
Graaff generator.
Getty Images/Science Photo Library/Adam
Hart-Davis
(a) Typical Van de Graaff
generator, (b) lightning
discharge from the
large Van de Graaff
generator at the Boston
Museum of Science
Getty Images/Science Photo Library/Peter Menzel
FIGURE 2.3
(a)
(b)
CHAPTER 2 Voltage sources and effects of an electric current
27
Piezo-electric crystal
The piezo-electric effect was discovered in 1880 by the Curie brothers. (Piezo is Greek for pressure.)
They found that when mechanical stress was applied to crystals such as topaz, quartz, Rochelle
salt and cane sugar, electrical charges appeared, and that this voltage was proportional to the
applied stress. Materials that have this characteristic are called piezo-electric materials.
The piezo-electric effect occurs when the charge balance within the crystal lattice of a piezo
material is disturbed. When there is no applied stress on the material, the positive and negative
charges are evenly distributed so there is no potential difference. When the lattice is changed
slightly, the charge imbalance creates a potential difference, often as high as several thousand
volts. However, the current is extremely small and only causes a small electric shock.
Piezo elements are used in electronic cigarette lighters, gas lighters, guitar pick-ups, some
kinds of microphones and various types of sensors, such as accelerometers. When used in
electronic gas lighters, a trigger-operated mechanical mechanism is arranged to apply a short,
sharp force to the piezeo element, which produces a voltage that is high enough to cause a
spark, as in Figure 2.4. In a microphone, the variations in air pressure due to sound waves
cause the piezo-electric crystal element in the microphone to produce a very small voltage that
changes with the sound waves.
FIGURE 2.4
Piezo-electric effect,
where pressure is
applied to opposite
faces of a quartz crystal
to produce a voltage.
Pressure can be by
compressing, flexing
or distorting the piezo
element.
Magnetism
Magnetism plays a big role in the electrotechnology
industry, and the topics of magnetism and
electromagnetism are covered in Chapters 11
and 12. In principle, if a conductor is moved
through a magnetic field, a voltage is induced
in the conductor. This effect was discovered by
Michael Faraday in 1831, a discovery that led to
the electrical power generation industry of today.
Figure 2.5 shows a single conductor being moved
through a magnetic field, although the voltage it
produces will be very small (a few millionths of
a volt).
Faraday found that the faster the conductor
moves through a magnetic field, the higher the
voltage. It doesn’t matter whether the magnetic
field or the conductor moves – a voltage is produced
if either moves relative to the other. He also found
FIGURE 2.5
Producing a voltage
by using mechanical
energy to move a
conductor in a magnetic
field
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E l e c t r i c a l P RINCI P LES
FIGURE 2.6
A simple alternator has
a magnet that rotates
inside a coil. When
the magnet is rotated,
its magnetic field
intersects with the coil,
producing a voltage.
that a higher voltage would be produced by increasing
the strength of the magnetic field and also by forming
the single conductor into a coil.
Figure 2.6 shows a simplified arrangement in which
a rotating magnet provides the moving magnetic field
which intersects with the conductor, now formed as a
coil. This device is called an alternator, although the
drawing shows the basic principles only.
An alternator produces a voltage that regularly
changes polarity, called an alternating voltage. The
diagrams in Figure 2.7 show the principle, in which one
turn of the magnet causes the voltage to go from zero to
a positive value, back to zero, then to a negative value and
finally back to zero. This is the type of voltage produced
by power stations. If the magnet in Figure 2.7 is rotated
at 3000 revolutions per minute (RPM), the alternating
voltage will change polarity 50 times a second, the same
as the voltage supplied by a power station.
To get a DC voltage (one that doesn’t change polarity) an extra part called a commutator is
added to the alternator, making it into a DC generator (described in Chapter 13). The alternator
is covered in Chapter 24.
In a practical alternator, rotation is provided by a prime mover, such as a diesel or petrol
engine, flowing water, wind power, high pressure steam that turns a turbine and so on.
Coal-fired power stations have large alternators driven by high pressure steam turbines.
The steam is produced by burning coal which heats water under high pressure in a boiler.
Because coal produces greenhouse gases, nuclear, geothermal and solar energy sources are
being increasingly used to produce steam.
FIGURE 2.7
The voltage changes
polarity (goes from
positive to negative)
every revolution of the
magnet
Renewable energy sources to power alternators
Coal, gas and nuclear power stations use non-renewable energy sources. Coal- and gas-fired power
stations in particular produce carbon dioxide, a greenhouse gas that is now recognised as a factor
leading to climate change. For this and many other reasons, renewable energy sources are being
increasingly used around the world to produce the mechanical force that rotates an alternator to
generate electricity. Here’s a brief look at some of these energy sources:
• Geothermal energy is the heat contained within the Earth, and is used to heat water to produce
the steam required to power a turbine driving an alternator. A hydrothermal system uses
CHAPTER 2 Voltage sources and effects of an electric current
naturally occurring steam, such as that produced by a geyser. Geothermal energy is relatively
abundant in New Zealand, and produces around 10 per cent of the country’s electrical power.
In Australia, an experimental power station powered by hot rocks (rocks heated by radioactive
decay) is being constructed. Water is forced through the hot rocks, which are up to three
kilometres below the Earth’s surface. This type of power generation is still being developed to
reach a commercial scale.
• Solar energy, in which the heat of the sun is focused onto specially shaped pipes, thereby
heating the water flowing through the pipes and producing high pressure steam to power a
turbine. A large baseload solar power station is now operating in Whyalla (South Australia).
Solar energy is generally only available during sunlight, but in this power station, solar heat
energy is captured by a chemical process, allowing it to be released at any time to ensure power
is generated on a continuous basis.
• Hydro-electric power stations have long been in use, in which flowing water from a dam or
river turns the alternators. The best known example in Australia is the Snowy River Scheme
which currently provides around 67 per cent of all renewable energy in eastern Australia.
• Wind power is another increasingly popular source of energy. In 2010, around 2 per cent of
Australia’s power needs were supplied by over 60 wind farms, nearly half of these in South
Australia. The main components of a typical wind generator are the rotor, usually three blades
up to 80 or more metres in diameter, a tower and the nacelle (an aerodynamically shaped
housing), which houses the gearbox, alternator and yaw motors. These motors rotate the
nacelle assembly to keep the blades facing the wind.
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Renewable energy is
sustainable energy
F igure 2 . 8
Paul Anderson, UK
(a) Components that
make up a wind
generator (b) Gearbox,
rotor, shaft and brake
assembly being
raised into position
(photo courtesy Paul
Anderson, UK)
While most of the electricity we use today is generated from the electromagnetic effect, there
are other methods, all of which produce a DC voltage (one that always has the same polarity).
Chemical to electrical
The best known application of producing electricity by way of chemical energy is the torch
battery. (Batteries are explained in more detail in Chapter 25.) The principle was first discovered
by Allesandro Volta in 1800, who noticed that the legs of dead frogs twitched when they were
touching two different metals. He went on to find that when two different metals are put into a
liquid called an electrolyte (an acid or an alkali) a voltage is developed between the two metals. He
had discovered the voltaic effect, or the electric cell, the first useful way of producing electricity.
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E l e c t r i c a l P RINCI P LES
The basic electric cell (shown in Figure 2.9) has three components: the positive electrode,
the negative electrode and an electrolyte. The choice of the electrode metals and the electrolyte
determine the voltage the cell produces. You can make an electric cell able to produce about
0.8 V by putting a galvanised nail and a piece of copper wire into a lemon, because lemon juice is
an acid (citric acid).
FIGURE 2.9
An electric cell converts
chemical energy to
electrical energy
A number of different types of cells have been
developed, which fall into two categories: those
that can be recharged, and those that are disposed
of when the cell’s chemical energy has been fully
used. Rechargeable cells include lead-acid, nickelcadmium (nicad) and nickel-metal hydride (NiMH).
Disposable cells include carbon-zinc (alkaline and
conventional) and most types of ‘button’ cells.
A carbon-zinc cell produces 1.5 V and a leadacid cell produces 2 V. To get a higher voltage,
cells are connected (in series) so their voltages
add together to give the required voltage. A 6 V
lead-acid battery has three cells and a 12 V
battery has six, connected either internally or
externally with metal straps. A 9 V carbon-zinc
battery has six cells, stacked on top of each other.
FIGURE 2.10
The most common
types of electric cells
are the zinc-carbon and
the lead acid
Heat to electrical
Although heat is used in coal-fired (and nuclear) power stations, it doesn’t directly produce electricity.
Instead the heat is used to make the high pressure steam to drive a steam turbine. However, heat
can be used to directly produce a voltage using a device called a thermocouple which is simply two
different metal wires joined at a point called the junction. The principle was discovered in 1821
by German–Estonian physicist Thomas Seebeck, and is now known as the thermoelectric effect or
Seebeck effect.
If heat is applied to the junction as in Figure 2.11, a voltage is developed by the thermocouple.
How much voltage depends on the type of materials used in the thermocouple and the difference
in the temperature between the hot and cold ends of the thermocouple.
CHAPTER 2 Voltage sources and effects of an electric current
31
FIGURE 2.11
A thermocouple is
formed by two different
metals joined at one
point
The metals used in the thermocouple wires depend on the temperature the junction will be
exposed to. For example, an iron and constantan thermocouple is used for temperatures up to about
600 8Celsius (C), a chromel-alumel type for temperatures up to about 1300 8C. The terms ‘chromel’,
‘alumel’ and ‘constantan’ are manufacturers’ names for the metals the thermocouple wires are made
of. These metals are special alloys and are made by combining a number of different metals.
The voltage output of a thermocouple is only a few thousandths of a volt (millivolts). An ironconstantan thermocouple with a temperature difference of 500 8C between both ends of the
thermocouple produces a voltage of around 30 millivolts (0.03 volts). While this voltage is too
low to provide useful power, because the voltage produced by a thermocouple is proportional to
temperature, it can measure temperature. In Figure 2.11, the voltmeter is calibrated in degrees
Celsius. Thermocouples are also used in industrial temperature control systems, in which the
thermocouple voltage is used to control heating elements.
Light to electrical
Courtesy Aussie Solar Installations
A device that produces a voltage when exposed to light is called photo-voltaic. The best known
photo-voltaic device is the solar cell. These are typically constructed with crystalline silicon or the
more expensive gallium arsenide, which is produced exclusively for use in photo-voltaic (solar)
cells. A single solar cell produces about 0.4 V and cells are combined to form a solar panel that
typically produces a voltage of 12 V to 24 V DC and up to 200 watts of power. The power available
from a solar panel installation depends on the number of panels used. Typical power ratings for a
domestic installation range from 1000 watts (1 kilowatt) to 5000 watts (5 kilowatts). Commercial
installations produce even more power.
Solar panels are commonly used to charge batteries that power remotely located roadside
signs, telephones and weather stations. Solar powered calculators are now being joined by solar
powered mobile phones, computers and lamps. Satellites and the international space station are
powered by banks of solar panels. Tracking mirrors are often used in commercial installations to
direct the sunlight onto the solar panels at all times during the day. A solar cell doesn’t need any
maintenance and produces electricity whenever light is present.
FIGURE 2.12
Solar cells convert the
sun’s light energy to
electrical energy
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Summing up…
• Energy can neither be created nor destroyed, it can only be converted from one form to another.
• Electrical energy is produced by converting other sources of energy, including
mechanical, chemical, heat and light.
• Most electrical power is produced by alternators driven by some form of mechanical
energy.
• High voltages can be produced with friction between two materials, or by applying force
to a piezo-electric material.
• Batteries produce electricity by converting chemical energy to electrical energy.
• Photo-voltaic devices such as the solar cell produce electricity when exposed to light.
• Renewable energy sources include geothermal, solar, hydro, biogas and wind power.
2.2 Effects of an electric current
An electric current causes a number of effects which can be useful, or merely unwanted byproducts. Effects caused by an electric current include:
• heating
• magnetism
• chemical
• luminous
• physiological.
All of these effects are used in some way, although the physiological effect (the effect of an
electric current on a living organism) is generally a safety hazard.
Heating
Whenever current flows in a conductor, heat is produced. This effect is used in electric heaters, frypans,
cooking ranges and so on. Heat is produced by a conductor because of its resistance. All conductors
have a certain amount of resistance, so heat is always produced when current flows. The higher the
resistance, the greater the amount of heat produced (for a given value of current). Because of this, the
resistance of conductors supplying power to electrical equipment should be as low as possible.
If, however, heat is wanted, as in an electric radiator, the conductor used in the heating element
needs to have a certain amount of resistance. The material used is therefore designed to have
resistance, and to withstand the heat it produces without burning out. Materials for this purpose
include nichrome (a combination of nickel and chrome), which is used in electric radiator elements.
The material used to make the filament in an incandescent lamp is another example.
FIGURE 2.13
Heat is developed when
current flows through a
resistance
CHAPTER 2 Voltage sources and effects of an electric current
33
As further explained in Chapter 4, because heat is a form of energy, electrical energy is used
to produce the heat. Unless it is wanted, heat in an electric circuit is actually a loss, as energy is
needed to produce it (wasted energy). Efficiency is a measure of power in to power out, so the
more unwanted heat that is generated, the lower the efficiency. For example, an electric motor
usually gets hot after it has been running for a while. The more efficient the motor is, the less heat
it will produce (at its rated load).
Magnetism
The magnetic effect caused by an electric current is perhaps the most useful of all. The effect
was discovered in 1820 when Danish physicist Hans Orsted observed that a compass needle was
deflected when current from a battery was flowing in a nearby wire. On hearing of this discovery,
French physicist André-Marie Ampère presented a paper a week later that explained the effect,
and thereby opened up the field of electromagnetism. This topic is covered in Chapter 11.
A basic application of the magnetic effect is the electromagnet, which has a coil of wire wound
around a soft iron core. When current flows in the coil, the iron core becomes magnetised. A
common use for the electromagnet is in a device called a relay.
A simple relay is shown in Figure 2.14. The moving part, called the armature, is spring-loaded
and is pulled down by the magnetised core when the coil is connected to a voltage (energised). A
metal contact is attached to the armature so that it makes an electrical connection with a fixed
metal contact when the armature moves towards the coil. If these contacts are connected then
they switch power to a load (heater, motor), and the load will turn on.
FIGURE 2.14
A relay has electrical
contacts that close
when the relay coil
is energised, thereby
switching on the load
controlled by the relay
The main application of a relay is to switch loads such as motors, heating elements, lights
and so on. A small amount of power is needed to operate the relay coil, but the relay contacts can
switch large amounts of power. Relays are available in a range of sizes and ratings. Those shown
in Figure 2.15 all have a low voltage coil rated at 12 V. The contacts are rated to handle 230 V AC
and currents of 10 A or more.
FIGURE 2.15
Relays used to control
a 230 V AC load such
as motors, lights and
heating elements
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E l e c t r i c a l P RINCI P LES
Two other very common applications of the magnetic effect are the electric motor and the
transformer. As described in Chapter 21, a transformer converts an alternating voltage from
one value to another. Transformers are used extensively in electrical power distribution and
transmission, and are widely used in electronics and telecommunications. Electric motors are
also widely used in many forms of transport, machines and appliances, and are explained in
Chapters 22 and 23.
FIGURE 2.16
Motors and
transformers are
common applications of
the magnetic effect of
an electric current
Other uses of the magnetic effect are magnetic strips on plastic credit cards, storing computer
data on a hard disk and recording sound on a tape recorder. Sensors that detect magnetism, called
Hall effect devices, are used extensively in car engines to give timing signals. Large electromagnets
are used in metal scrap yards to lift heavy metal objects, or to sort magnetic materials from nonmagnetic materials.
Chemical effect
As explained in Chapter 1, current can flow in certain types of liquids called electrolytes. Salt water
is an electrolyte as it contains salt (sodium chloride) dissolved in water. Acids and alkalies are also
electrolytes. Current can flow, because an electrolyte contains ions, which are atoms with a positive
or a negative charge. A positively charged ion has less electrons (negative charge) than protons
(positive charge), and a negatively charged ion has more electrons than protons, as shown in
Figure 2.17. In this diagram, each ion has four protons and three neutrons, but different numbers
of electrons.
FIGURE 2.17
Ions are atoms in a
liquid that have lost or
gained electrons
CHAPTER 2 Voltage sources and effects of an electric current
35
In salt water, the sodium atoms become positive ions as they each lose one electron which
is taken by the chlorine atoms, making the chlorine atoms (which are now ions) negative.
However, the liquid isn’t charged because the number of positive ions equals the number of
negative ions.
Because the ions are charged, they can be attracted by a voltage. The rule is: like charges repel
and unlike charges attract. If two metal electrodes connected to a voltage source are placed in an
electrolyte, the positive ions will flow towards (be attracted by) the negative electrode (cathode)
and the negative ions will flow towards the positive electrode (anode), as shown in Figure 2.18. As
a result, an electric current flows in the liquid.
In other words, the electrolyte behaves as a conductor. The more ions there are in the liquid,
the better the liquid can conduct an electric current. If more salt is added to the liquid, more ions
are created and the resistance of the liquid is less. This is why salt water is a better conductor than
fresh water.
FIGURE 2.18
Current flow in an
electrolyte – the
positive ions move
to the cathode and
negative ions move
towards the anode, in
this case to produce
purified copper
It’s important to understand that an electric current does not itself have a chemical effect, but
that it causes a chemical effect when passed through an electrolyte. The two main useful chemical
effects are electrolysis and, as already explained, the voltaic effect (the electric cell).
Electrolysis
In the electrical industry, an important use of electrolysis is to produce pure copper (greater
than 99.95 per cent), called electrolytic copper. High purity is needed because small amounts of
impurity metals can seriously reduce copper’s ability to conduct electricity. For example, 0.05 per
cent of arsenic impurity reduces its conductivity by 15 per cent. Electric cables must be made of
very pure copper to prevent electrical losses in them, which is particularly important when the
cables run for long distances.
To purify copper electrolytically, the impure copper metal is made the anode (the positive
electrode) in an electrolytic cell, as in Figure 2.18. A thin sheet of previously purified copper is
used as the cathode (the negative electrode). The electrolyte is a solution of copper sulphate and
sulphuric acid. When current is passed through the cell, positively charged copper ions are pulled
36
E l e c t r i c a l P RINCI P LES
out of the anode into the liquid, and are attracted to the negative cathode, where they lose their
positive charges and stick tightly as neutral atoms of pure copper metal.
As the electrolysis continues, the impure copper anode dissolves and pure copper builds up
as a thicker and thicker coating on the cathode. Impurity metals such as iron, nickel, arsenic and
zinc leave the anode, but go into the solution. Impurities such as platinum, silver and gold are also
released from the anode, but as they are not soluble, they fall to the bottom forming a valuable
sludge, which can often cover the cost of the large amount of electricity used.
Another use of electrolysis is the production of hydrogen, in which water (H2O) is decomposed
into hydrogen (H2) and oxygen (O) by passing an electric current through the water. About
4 per cent of hydrogen gas produced worldwide is created by electrolysis, mainly as a by-product
of passing current through salt water to produce chlorine gas.
Aluminium is produced by electrolysis, in which powdered alumina, derived from bauxite,
is contained within a large vessel (pot) lined with carbon or graphite, and fitted with a carbon
electrode (anode). The carbon lining of the vessel is the cathode. When an electric current
passes through the mixture, the carbon of the anode combines with the oxygen in the alumina,
producing metallic aluminium and carbon dioxide. The aluminium settles to the bottom of the
vessel where it is syphoned off into crucibles while the carbon dioxide escapes. A DC current of
up to 200 000 amperes per pot is required, which is why aluminium smelters are located near
affordable power sources.
Electroplating
The main use of electroplating is to protect metals against corrosion. It is also used to build
up a worn metal surface. This is a process in which a metal object (e.g. a bumper bar) is coated
by electrolysis with a thin layer of a different type of metal (e.g. chrome). The part to be plated
becomes the cathode of the circuit (has a negative potential). Typically, the anode is made of the
metal to be plated onto the part. Both components are immersed in an electrolyte containing
dissolved metal salts and other ions that allow the flow of electricity.
The current flowing through the solution causes the anode to release atoms through
oxidisation into the electrolyte. The dissolved metal ions in the electrolyte are attracted by the
negative potential of the part being plated (cathode) and are deposited onto the surface of
the part. The anode is dissolved during the process at the same rate as the cathode is being
plated. Sometimes electroplating uses a non-consumable anode, such as lead. In this case the
electrolyte supplies the metal ions, which need to be periodically replenished as they are drawn
out of the solution.
Galvanic corrosion
Most metals corrode in some way due to being exposed to oxygen, which causes the surface to
oxidise. Rust is the formation of iron oxide, and, over time, iron in the presence of moisture will
corrode away completely. Other metals such as copper and aluminium form an oxide layer, which
helps protect the metal from further oxidisation.
Another form of corrosion is galvanic corrosion, caused by electrolysis. This occurs when two
different types of metals are in electrical contact and are exposed to an electrolyte, such as rain or
sea spray. Corrosion occurs because of the voltaic effect, in which the dissimilar metals and the
electrolyte form an electric cell. The current produced by the cell causes the metals to corrode, with
one of the two metals corroding before the other.
An understanding of this process led to the development of cathodic protection, a technique to
control the corrosion of a metal surface by making it work as the cathode of an electrochemical
cell. This is achieved by placing the metal in contact with another more easily corroded metal that
CHAPTER 2 Voltage sources and effects of an electric current
37
acts as the anode of the cell. The first application of this occurred in 1824 when Sir Humphry Davy
succeeded in protecting copper sheathing on a ship against corrosion from seawater by the use of
iron sacrificial anodes. Today, most storage hot water heaters (gas and electrical) have an internal
sacrificial anode to help prevent the boiler from corroding.
FIGURE 2.19
Sacrificial anodes in use
and the electrochemical
series listing metals
in the order of their
behaviour as a
sacrificial anode
Galvanising is a common example of cathodic protection, in which steel sheets are covered with
a thin layer of zinc. The zinc not only coats the steel, preventing it from oxidising (rusting), it forms
a galvanic cell when the zinc is scratched, protecting the exposed steel by acting as a sacrificial anode.
That is, the zinc corrodes before the steel, until eventually the zinc is completely corroded away,
allowing the steel to rust. Figure 2.19 lists the commonly used metals in their electrochemical series,
which shows that zinc is below iron, hence zinc will corrode in preference to iron.
Rechargeable batteries
Charging a battery is another example of the chemical effect, in which the charge current reverses
the chemical action caused when the battery is discharged. In effect, electrical energy is stored
in the battery during charging, and released by chemical action during discharge. This applies
to rechargable batteries like the lead-acid type used in cars, and to nickel cadmium cells, but
not to carbon-zinc batteries, which are not designed to be recharged. See Chapter 25 for more
information about batteries.
Luminous effect
One of the first applications of electricity was the development of the arc lamp. This device
has two carbon electrodes in open air, arranged so an electric arc is established between the
electrodes when they are brought in contact with each other, then separated. While this type of
lamp can produce high amounts of light, it also generates high levels of ultraviolet light and a
large amount of heat.
The principle of the arc lamp led to the development of the discharge lamp, which has a
glass envelope filled with a gas, and contains two metal electrodes. Gases used include neon,
argon, xenon, sodium, metal halide and mercury. Light is produced by passing an electric current
through the gas, which ionises the gas, creating a plasma. A starting circuit provides a high
voltage to cause the lamp to light, and a device (sometimes called a ballast) limits the current
flowing through the lamp.
The fluorescent lamp is the best known example of a gas-discharge lamp in which the ultraviolet
radiation created by the current flowing through the gas (mercury vapour) causes a fluorescent
38
E l e c t r i c a l P RINCI P LES
LED lights are energy
efficient and are
now used in homes
for general lighting
purposes
coating inside the tube to glow. These lamps are far more efficient than incandescent lamps, where
light is produced by heating a filament. Compact fluorescent lamps are now being increasingly
used because of their high efficiency.
The light emitting diode (LED) is a solid state device that produces light by way of the effect
called electroluminescence, which is the result of electrons releasing energy in the form of photons
(light). LED lighting is efficient, as not much heat is generated. The first practical visible-spectrum
(red) LED was developed in 1962. Today, LEDs can produce red, green, blue, yellow, white, infrared
and ultraviolet light. High power LEDs (over one watt) are used in traffic lights and vehicle lights,
and are gaining acceptance in general lighting applications. LEDs operate from a low DC voltage,
generally around two volts, they have a long life and maintain a consistent light output even in
cold temperatures, unlike traditional lighting methods.
FIGURE 2.20
LEDs operate from a
low DC voltage and
are being increasingly
used in domestic and
industrial lighting
applications
Physiological effect
F igure 2 . 2 1
Blood is a good
conductor of electricity
and is carried through
the body by the
capillary system
This effect is the result of passing an electric current through a living organism, such as a human
or an animal. It has a few medical uses, mainly to stimulate a heart by applying a short burst
of current that will shock the heart out of ventricular
fibrillation (explained shortly).
Otherwise, this effect is limited to stun-guns, cattle
prods, electric fences and to those countries that still use the
electric chair. The purpose of these items is either to produce
a short, sharp electric shock, or in the case of an electric
chair, to cause death. So it’s important you understand this
effect as your life may depend on it!
Life is kept going by two important body functions:
breathing (lungs) and blood circulation (heart-beat). If either
of these stop for more than a few minutes, the brain is starved
of oxygen, and there will be so much damage that death soon
follows. If an electric current passes through the body, the
current can interfere with the tiny electrical impulses that
travel through the body’s nervous system to control the heart
and lung muscles.
The amount of interference depends on the value of
the current, how long it flows and where it flows. Current
which flows through the brain, or the chest region where
CHAPTER 2 Voltage sources and effects of an electric current
the heart and lungs are, is more dangerous than a current passing through two fingers on the
same hand.
If breathing stops after an electric shock, the heart can go into what is called ventricular
fibrillation. This is when the electrical impulses from the brain controlling the heart are confused
enough to be out of step, making the heart quiver and stop pumping blood.
An electric current can also cause terrible burns and make muscles tighten severely. Death and
injury can not only result directly from electric shock, but indirectly. Electric shocks have caused
people to fall off ladders and power poles, flung them into dangerous chemicals, and made them
drop something heavy on themselves, such as the live appliance they were carrying at the time.
The value of the current causing the shock can determine whether it’s lethal. Table 2.1 shows
the effects of various values of current. Remember that a milliamp (mA) is one-thousandth of an
ampere, so the current values are quite small. A current as low as three or four-thousandths of an
ampere (3 to 4 mA) can cause pain and a current over 20 mA can stop the lungs from functioning.
These values of current will hardly even light a lamp, but could kill you. Skin has a relatively high
resistance, but blood is a good conductor. If your skin is wet, this reduces its resistance, and
current can flow through your skin to the low resistance path of the arteries.
Effects of an electric current passing through the body
Current
Effect
up to 2 mA
barely perceptible
2 mA to 8 mA
sensation becomes obvious and more painful
8 mA to 12 mA
muscle spasms and greater pain
12 mA to 20 mA
unable to let go the conductor, can’t control muscles
20 mA to 50 mA
if passing through the chest, breathing might stop
50 mA to 100 mA
if near the heart, causes ventricular fibrillation
100 mA to 200 mA
heart stops beating
above 200 mA
severe burns as well!
The value of current that flows during an electric shock depends on two things:
• the value of the voltage causing the shock
• the resistance your body offers to the current.
There is no such thing as a ‘safe’ voltage. However, voltages less than 50 V are unlikely to
deliver a lethal shock. The type of voltage also has different effects. Because an alternating (AC)
voltage regularly changes its polarity, it doesn’t have the same effect on the muscles as a direct (DC)
voltage. A shock with a DC voltage causes the muscles to contract, making it almost impossible to
let go the live conductor. An AC voltage could allow you to let go, although this would depend on
the value of the current.
Resistance is mainly that of the skin. Wet, sweaty hands have less resistance than dry skin. Standing
on a wet floor allows current to flow through you to the ground by way of the feet. Standing barefoot is
obviously very dangerous – always wear rubber-soled shoes when working with electricity.
Wet clothes are another hazard when working with electricity, as wet cloth can conduct an electric
current. The resistance of the cloth depends on the type of liquid soaked up by the material. Remember
that tap water with all its usual impurities will conduct electricity. As we’ve said before, electricity and
water is a dangerous combination! Here are some basic rules for avoiding an electric shock:
• Think of any appliance, conductor, terminal or equipment as live, until you have proved otherwise.
• Don’t work on an electrical appliance or circuit with wet hands.
TABLE 2.1
39
40
E l e c t r i c a l P RI N CI P L E S
• A wet floor is a good conductor of electricity – be extra careful.
• Don’t wear metal jewellery (rings, watch) when working with electricity.
• Remember that ‘familiarity breeds contempt’. Always have respect for electricity – it has no
respect for you!
2.3 Protection against effects
of an electric current
FIGURE 2.22
Direct contact with
a live conductor can
cause current to flow
through your body to
ground
The effects of an electric current can sometimes cause dangerous situations, the most obvious
being the danger of an electric shock. The Australian and New Zealand wiring rules (AS/NZS 3000)
specify ways of minimising these dangers. In regard to the physiological effect, the standards
identify two situations that must be protected against:
1 Direct contact under normal service conditions with a live terminal, conductor or part of
an electrical installation or circuit. Protection against this is achieved by a combination of
insulation, barriers or enclosures and placing exposed electrical contacts out of reach. To protect
the public, appliances are required to pass safety tests, and electrical wiring installations must
be to the standards given in AS/NZS 3000.
Those working on live electrical circuits are the most likely to receive an electric shock.
Normally a circuit is isolated from the electrical supply before doing work, but in some
cases this is not possible (e.g. when working on power lines or a live fuse box). Under these
conditions, you need to wear protective gloves and other items of personal protective
equipment, as well as understand the risk and take steps (control measures) to prevent
being electrocuted.
2 Indirect contact that can
occur under a fault condition.
This might happen if you
touch a metal-cased appliance
that, because of an internal
fault, is now live. Other faults
are exposed live conductors
because the insulation is
missing, water reaching live
terminals or electrical parts;
any fault that could cause an
electric shock to the user.
Protection against these
dangers is usually achieved by
automatic disconnection of
the electrical supply by way
of fuses, circuit breakers and
a unit called a residual current
device (RCD), generally known
as a safety switch. This is in
combination with a system of
earthing, in which metal parts
of an appliance are connected
to ground via the earth wire.
CHAPTER 2 Voltage sources and effects of an electric current
41
FIGURE 2.23
Faults that can cause
an electric shock
include touching a live
appliance or exposed
live conductors
faulty appliance
making it live
frayed lead exposing
bare conductors
The AS/NZS 3000 standards also require electrical installations to provide protection against
the thermal effects of an electric current. For example, electrical equipment that produces a lot of
heat must be properly ventilated and fire-proof enclosures should be used where appropriate. The
magnetic effect must also be protected against.
The problems caused by the chemical effect include corrosion and the possibility of creating an
electric cell by joining two dissimilar metals. In the case of connecting aluminium conductors, the
standards require specially designed components to make the connection, after first removing the
aluminium oxide film on the connecting surfaces.
Corrosion of outdoor metal fittings must be prevented by using specified coatings, such
as galvanising. As already explained, other methods to prevent corrosion include the use of
sacrificial anodes.
Chapter summary
• Electrical energy is produced by converting
other forms of energy (mechanical,
chemical, heat and light).
• Electrical energy is converted to the
required form of energy (e.g. heat, light or
mechanical energy) by the electric load.
• Large alternators driven by steam turbines
(mechanical energy) generate most of
Australia’s electrical energy.
• An alternator has coils of wire arranged so
they intersect with a moving magnetic field,
thereby producing electricity.
• Renewable energy sources that can drive an
alternator include geothermal, solar, hydro,
biogas and wind power.
• Solar panels are photo-voltaic devices that
convert light energy to electrical energy.
• Friction can be used to produce high
voltages. Examples are the Wimshurst
machine and the Van de Graaff
generator.
• The electric cell produces electricity by
converting chemical energy to electrical
energy.
• Charging a battery (a group of cells) reverses
the chemical action that took place when
the battery was being discharged.
• Electric motors, transformers and
electromagnets work by way of the
magnetic effect of an electric current.
• Electrolysis is the process of passing current
from one electrode to another through an
electrolyte. Electroplating is an example of
electrolysis.
• Discharge lamps are filled with a gas that
produces light directly, or in the case of
the fluorescent lamp, ultraviolet light that
excites phosphors inside the tube.
• Physiological effects of an electric current
are an unwanted by-product, and pose
dangers to all forms of biological life,
including muscle contraction, ventricular
fibrillation, burns and death.
• Effects caused by an electric current include
heating, magnetism, chemical, luminous
and physiological.
• Heat is produced in a conductor because of
its resistance.
42
E l e c t r i c a l P RINCI P LES
• Current flowing in a conductor creates
a magnetic field around the conductor.
Coiling the conductor makes an
electromagnet.
• Chemical effects include electrolysis and
voltaic effect (electric cell).
• Galvanic corrosion is caused by electrolysis
when two different metals are in electrical
contact, creating an electric cell.
• A metal surface can be protected from
corrosion with sacrificial anodes that
corrode in preference to the metal being
protected.
• Fluorescent lamps are an example of the
luminous effect on an electric current.
• LED lighting is an example of
electroluminescence.
Self-check
Answer these questions then check your
answers at the back of the book.
1 What are three forms of energy that can be
converted to electrical energy?
2 How can static electricity be produced?
Name two machines that operate on this
principle.
3 Give a common application of the piezoelectric effect.
4 How does a simple alternator work?
5 Name two non-renewable and two
renewable energy sources that are used to
produce steam to power alternators in a
power station.
6 Apart from steam turbines, what are two
renewable energy sources commonly used
to power the prime mover that rotates an
alternator?
7 What are the three components of a basic
electric cell?
8 What is the main application of a
thermocouple?
9 What is the name given to devices that
produce a voltage when exposed to light?
10Name four situations where solar panels are
used to provide electrical power.
11List four useful effects of an electric current.
12What causes heat to be produced in a
conductor when current is flowing?
13List three commonly used electrical devices
that work by the magnetic effect of an
electric current.
14Name the two main chemical effects of an
electric current.
15Which electrode are negative ions in an
electrolyte attracted to, the anode or the
cathode. Why is this so?
16Give three applications of electrolysis.
17What is meant by ‘cathodic protection’?
18What are the main components of a
discharge lamp?
19Why is it important to be extra careful
when working with electricity in damp
surroundings?
20What two things determine the value of
current flowing through your body when
you get an electric shock?
Online resources
For more information on the contents of this
chapter, visit these websites:
• http://www.australiancoal.com.au/
(click on ‘Coal and its uses’ on the home
page, then on ‘Electricity Generation’ for an
overview of producing electricity in a coalfired power station)
• http://www.suzlon.com/index.aspx
(wind power generators)
• http://science.howstuffworks.com/
wind-power1.htm
• http://science.howstuffworks.com/
electromagnet.htm
Also try Googling the following search terms:
• electricity (Wikipedia gives an overview of
some of the content of this chapter)
• power stations in Australia (Wikipedia lists
all power stations in Australia that are
currently operating or planned, including
wind farms, coal-fired and so on)
• electrolytic cell (Wikipedia gives an overview
of the terms electrolysis and the electric cell)
• renewable energy (numerous websites cover
this topic)
• piezo-electric (numerous websites give
information, in particular Wikipedia).