|
||||
|
||||||||
|
PREPARED NOTES
INTRODUCTION:
Since our distant ancestors first walked
the planet and witnessed trees exploding in flames after a lightning strike,
many different sources of power have been used.
Fire has probably been known and used for several hundred thousand years.
Controlled burning of wood, oil, crop and animal wastes and the various
types of coal has been used for thousands of years.
The use of water power dates from ancient Greece and Rome, where
waterwheels were used for the milling of corn.
The power of steam became available with the development of the Steam
Engine, which was patented by James Watt in 1769.
Early in the 19th Century, gas lamps became available to light
town and city streets. At the turn
of the 20th Century, electric lights (both incandescent and
fluorescent) gradually took over from gas lamps.
In the 20th Century, petroleum, natural gas and electricity
have been used to supply power in a multitude of different applications. We can get some idea of how sources of
domestic power have changed over recent time by paying a visit to some of the
excellent re-creations of old Australian towns, such as Timbertown at Wauchope,
and Sovereign Hill at Ballarat or by exploring a museum
like the Power House Museum at Darling Harbour, Sydney.
At Timbertown, for instance, you can see what things were like in the
1880’s - how important such things as open fire places were for heating and
cooking, whale oil and kerosene lamps for lighting, steam power for driving
heavy machinery such as trains, pumps, steam driven saws and so on. Although electrical effects have been known since the time of the ancient Greeks at least, the development of electricity as a source of usable power has really happened only in the last 200 years or so. Luigi Galvani (1737-1798) was an Italian anatomist who discovered “animal electricity” (about 1786). Galvani was investigating the effects of electrostatic stimuli on muscle fibre in frogs, when he discovered that he could make the muscle twitch by touching the nerve with various metals without a source of electrostatic charge. He found that the best reaction was obtained when two dissimilar metals were used. He attributed the effect to 'animal electricity'. In other words, he believed that the electricity was produced by the animal. Alessandro Volta (1745-1827) was a French physicist
who invented the voltaic pile (the first battery) and thus provided
science with its earliest continuous electric current source.
Volta’s invention (about 1800) demonstrated that “animal
electricity” could be produced using inanimate materials alone, thus ending a
long dispute with Galvani, who insisted that it was a special property of animal
matter. Volta had always suspected
that the source of the electricity produced in Galvani’s experiment was the
interaction of the two dissimilar metals, rather than the animal. Over the last 200 years, society has become
increasingly dependent on electricity as a source of power.
Consider for a moment the difference that electricity has made to the
quality of life of people today compared to 200 years ago.
Think of how electricity is used today in lighting, heating or cooling,
refrigeration, food preparation, transport, communication, manufacture of goods
and materials, entertainment, data storage and manipulation, household cleaning
tasks, medical applications and building & construction industries to
mention just a few areas. Electricity is employed wherever possible
as a medium of energy transfer and use for several reasons: (1)
It can be efficiently transported from generators
to the point of use through a network of wires. (2)
It can be very efficiently converted into other usable energy forms, such as
heat, light, mechanical, and chemical energy.
(3) It is easily controllable at the point of use, requiring a simple
flick of a switch to turn an electrical device on or off. Indeed, the availability of large, inexpensive supplies of electricity is important to the maintenance and development of all modern countries. Without electricity, industry would grind to a halt, communications would cease, and our food supply would be seriously affected. The easy availability of electricity allows us to enjoy our present standard of living. Today, even people in remote areas of
Australia are able to enjoy the benefits of electricity.
Innovations and improvements in solar power and wind-driven
generators have led to effective and affordable power generation ideal for
these areas. All household
electricity needs, as well as the electricity requirements of equipment and
machinery at remote areas of farms and stations can be supplied by this
technology.
ELECTROSTATICS
As early as the 7th Century BC,
the ancient Greeks were aware that amber, when rubbed vigorously, could attract
dust and cloth from a distance. Today,
we say that such objects are “charged”.
For example, a plastic ruler when rubbed has the ability to pick up tiny
pieces of paper. Electrostatics is
the study of stationary charge. CHARGE
Experiments by William Gilbert (1544-1603), Benjamin Franklin (1706-1790) and others suggested the following rules regarding charge: ¨ There are only two kinds of charge – called positive and negative. These were originally called vitreous and resinous respectively, because of the materials which produced each type of charge. ¨
Like charges repel. ¨
Unlike charges attract. Charles Coulomb (1736-1806) discovered the following law governing the behaviour of charges: For two charges q1 and q2, distant r apart, the force between them varies directly as the product of the two charges and inversely as the square of the distance between them. Mathematically,
where
is 9 x 109 SI units and e0 is an experimentally determined constant called the permittivity of free space, with a value of 8.85 x 10-12 SI units. The force acts along the line joining the centres of the two charges. Each charge experiences the same sized force. The SI unit of charge is the coulomb (C). One coulomb of charge is equal to the charge on 6.25 x 1018 electrons.
ELECTRIC
FIELDS
A “field” in physics is a region of influence of some kind. If a stationary charge experiences a force in a particular region of space we say that there is an electric field present in that region. The magnitude of the electric field strength at a particular point in space is defined as the force per unit charge at that point. E = F/q where
E
= electric field strength, q = size of the charge and
F = force experienced by
q
at the point in question. Both
E and F
are vector quantities and thus, must
be specified in terms of size and direction. The SI units of electric field strength are NC-1. The direction
of the electric field at any point is defined as the direction in which a positive
test charge would move if placed in the field at that point. The
relative strengths and directions of different electric fields may be
represented diagrammatically by using lines of force.
The spacing of the lines of force indicates the strength of the field.
The closer the lines are together, the stronger the field. The direction of the field at a given point is indicated by
the direction of the tangent to the lines of force at the point in question.
Lines of force, also called field lines, are always drawn as
emanating from positive charges and as terminating at negative charges. The
following are examples of the electric field around various objects.
A dipole consists of a positive and negative charge separated by a short distance. Note that in (c) the field is uniform between the plates but non-uniform towards the edges.
As shown in (d) above, the electric field inside a conductor under electrostatic conditions is zero. Also note that charge tends to accumulate at narrow or sharp ends of objects. You may like to think about why this would be expected and any consequences that may follow from such a phenomenon. Once you think you have an explanation have a look at Explanation of E-Field Around A Pear-Shaped Conductor.
POTENTIAL DIFFERENCEConsider a charge of + q coulombs in a uniform electric field as shown below:
To move charge +q from A to B back against the field direction, we must do work. The amount of work, W, that we must do is found from:
where F = the force applied to move the charge & d = displacement moved by the charge in the direction of the applied force. The SI unit of work is the joule (J). In the E field, the force, F, on the charge is given by
Therefore we have:
as the work done. Since we have done work on q to move it from A to B, we can say that we have increased its potential energy (ie its ability to do work for us). Further, we can say that there is a difference in potential between points A and B, in the E field. In general, we can say that there is a potential difference between any two points in an electric field, whenever we have to do work to move a charge from one point to the other. By definition:
That is, the potential difference between A and
B, VAB,
equals the work done in moving the charge from A to
B, WAB, divided
by the size of that charge, q. Since
the work done is the change in potential energy of the charge, we can say that the
potential difference between two points is the change in potential energy per
unit charge moving from one point to the other. Note that another term often used for potential difference is “voltage”. The SI unit of potential difference is the volt (V). 1V = 1JC-1.
ELECTRODYNAMICS
Electrodynamics is the study of moving
charges. A current is
defined to be a flow of charge. By
definition, the direction of a current is taken to be the direction in which the
positive charge flows. This
is called a conventional current. This direction was chosen because the early
researchers in this field did not know whether the moving charges in a current
were positive or negative. Today we
know that it is the electrons that actually carry the charge in a
current, but for convenience we still use conventional current direction as the
direction of flow of a given current. Mathematically, we define current as the rate at which charge flows (ie the amount of charge flowing per unit time): The SI unit of current is the ampere (A). 1A = 1Cs-1. When current flows continually in one
direction it is called a direct current (DC). When a current consists of charges that periodically change
direction, backwards and forwards, it is called an alternating current (AC).
CONDUCTORS AND INSULATORSSubstances containing large numbers of
electrons that can move from one atom to another (free electrons) are called conductors,
since they can be used to conduct a stream of electrons from one point to
another. At ordinary temperatures,
silver is the best conductor but it is too expensive for most uses.
Copper is nearly as good a conductor as silver and far less expensive. No material used at ordinary temperatures
is a perfect conductor. There is
always some opposition to the flow of electrons.
This opposition results in the loss of energy from the moving
steam of electrons. This lost
energy appears in the form of heat, which warms the conductor.
If too much energy is lost the rise in temperature may melt or vaporize
the conductor. In many substances, including glass, most
plastics, rubber and wood, the outer or valence electrons are linked by chemical
bonds to the corresponding electrons of adjacent atoms.
In these substances the electrons are not free to move.
Since electrons cannot move from atom to atom within these materials,
they cannot conduct a flow of electrons. These
substances are called non-conductors or insulators. [Top]
RESISTANCE OF CONDUCTORSThe opposition that conductors offer to the
movement of electrons across them is called the resistance of the
conductor. Resistance is a property
of a body due to the arrangements of the atoms of the body.
Every material has a certain ability to resist the passage of an electric
current through it. Thus, every material has a certain resistance value. The resistance of a conductor is found
experimentally to depend on four physical factors: ¨ Type of material – different materials can have different atomic arrangements (different geometrical arrangements, different spacing between the atoms, different sized atoms etc). Silver, copper and aluminium are all metallic conductors used to conduct electricity in various applications. If all other factors are equal the three metals still have different resistance values because of their slightly different structures on an atomic scale. ¨
Length
of conductor – the longer a conductor, the higher the
resistance (resistance µ length). ¨
Cross-sectional
area of conductor – the larger the cross-sectional area,
A, of a conductor, the smaller the resistance (R µ
1/A) ¨
Temperature
– Temperature effects on conductors are quite complex. In general, the metals used as conductors suffer an increase
in resistance as their temperature increases.
A formula exists which allows the resistance values of conductors to be
determined for temperatures other than the reference temperature of 20oC.
This formula is beyond the scope of the current syllabus.
OHM’S LAWConsider a current, I, flowing through a metal conductor, the potential difference across its ends being V.
In 1826 George Ohm found that for a given conductor at constant temperature, the ratio of the potential difference across its ends to the steady current flowing through it was a constant. This constant is called the resistance of the conductor. This relationship is now called Ohm’s
Law. The SI unit of resistance is the ohm (W). A conductor is said to have a resistance of one ohm if when the potential difference across its ends is one volt, the current flowing through it is one ampere. It is worth noting that Ohm’s Law does not apply to all conductors. Those conductors obeying Ohm’s Law are called ohmic conductors. A conductor may obey Ohm’s Law over a particular temperature range and be non-ohmic outside that range. [Top]
ELECTRIC CIRCUIT DIAGRAMSThe following symbols are used in circuit
diagrams to represent various circuit components shown:
COMBINATIONS OF RESISTORS(i) RESISTORS IN
SERIES:
Resistors joined end to end, so that the current only has one path along which it may travel, are said to be connected in series. For the circuit segment shown below the potential difference between points A and B is V.
Clearly, the current through each resistor is the same. Also, the total potential difference across the segment is equal to the sum of the potential differences across each resistor (Kirchhoff’s Voltage Law). Therefore, the total resistance, R, of the segment is found from: IR = IR1 + IR2 + IR3 IR = I.(R1 + R2 + R3) R = R1 + R2 + R3
Thus, the effective
resistance of a number of resistors in series is equal to the sum of the
resistances of the individual resistors.
(ii) RESISTORS
IN PARALLEL:
Resistors in parallel provide two or more different paths by which the current can travel through the circuit. In the following diagram the total current, I, splits into three components I1, I2 and I3, such that I = I1 + I2 + I3 (Kirchhoff’s Current Law).
The ends of each
resistor are connected to the same points, A and B, in the circuit.
It follows that the potential difference across each resistor is the same
and in each case is equal to V. Since I = I1 + I2 + I3, we can write (from Ohm’s Law):
Thus, the reciprocal of the effective resistance of a number of resistors in parallel is equal to the sum of the reciprocals of each individual resistance. [Top]
KIRCHHOFF’S LAWS OF DC CIRCUIT ANALYSISKirchhoff's Laws are not mentioned specifically by the Stage 6 Syllabus but an understanding of these laws is essential for successful circuit analysis.
(i) KIRCHHOFF’S CURRENT LAW:The sum of the currents flowing into a particular point in a circuit equals the sum of the currents flowing out of that point. See the section of the parallel resistor circuit above where I splits into three components. Kirchhoff’s Law says that the sum of those components must equal I.
Kirchhoff’s Current Law is really a consequence of the Law of Conservation of Charge, which states that the total net charge of any system is constant.
(ii) KIRCHHOFF’S VOLTAGE LAW:
In a closed loop, the sum of the voltage sources equals the sum of the voltage drops.
For a simple example of this, observe the
circuit segment above showing three resistors in series, with a total voltage
across the segment of V volts. We
can make this segment into a loop by imagining the ends of the segment are
attached to the positive & negative terminals of a battery supplying V volts
across the loop. A certain amount
of energy is needed to force current (moving charges) through each resistor.
This energy is supplied by the potential difference (voltage) applied
across the loop. A certain amount of this total voltage is used by the moving
charges to force their way through each resistor.
The amount of voltage used to force current through a resistor is
called the voltage drop across the resistor. Kirchhoff’s Voltage Law says that if we
add up all the voltage drops that occur around the loop, the total will be equal
to the total voltage applied across the loop.
Nothing is “left over”. All
of the available voltage is used up by the time the current has passed right
around the loop. Kirchhoff’s Voltage Law is really a
consequence of the Law of Conservation of Energy, which states that energy can
neither be created nor destroyed but merely transformed from one form into
another.
MEASURING CURRENT & VOLTAGE IN CIRCUITSAn ammeter is used to measure the current flowing in an electrical circuit or in part of a circuit. The ammeter is placed in series in a circuit to enable it to sample the current that it is to measure. The ammeter is designed so that it has a very low resistance, so that it does not alter the current flowing in the circuit. A voltmeter is used to measure the potential
difference across an electrical circuit or across elements in a circuit.
The voltmeter is placed in parallel with an element to enable it
to measure the difference in potential between one end of the element and the
other. The voltmeter is
designed with a very high resistance to ensure that it does not change
the current in the element across which it is connected.
If it changed the current in the element, it would have changed the
voltage across the element, which is what it was trying to measure.
THE
WORK DONE BY A CURRENT
In crossing a conductor, work must be done by the electrons to overcome the resistance of the conductor. The energy expended by the electrons is transformed into heat. Thus, a conductor gets hot when current passes through it. Consider a current of I ampere flowing through a conductor of resistance R ohm, with a potential difference of V volt across its ends. Remember that the work done in taking q coulombs of charge between two points differing in potential by V volts is given by: W = qV. So, the energy expended, W = qV
= VIt, since I = q/t
= RI2t, since V =
IR
= V2t/R, since I
= V/R Clearly, the total amount of energy used by an electrical component or circuit depends on the length of time the current is flowing.
POWER IN ELECTRICAL CIRCUITS
Power is the rate at which
energy is transformed from one form into another. By definition, power is equal to the rate at which energy is
expended.
Therefore
So clearly, the power dissipated by an electrical component is determined by multiplying the current through the component by the voltage across the component. The SI unit for power is the watt (W). Or using Ohm’s Law to re-arrange the equation, P = I2R or P = V2/R.
UNITS
FOR ELECTRICITY CONSUMPTION
If you
take a look at your parents’ electric power bill you will notice that
the amount of electrical energy consumed by your household is quoted in
units of kWh – kilowatt-hours. This
seems strange considering that the correct SI unit for energy is the joule
(J). Let’s see why the kWh is
used in place of the joule in this instance. Consider
an electric radiator. The amount of
electrical energy used by a radiator can be calculated as follows:
Say the radiator has a power rating of 2000 watts.
Using the definition of power, that means the radiator will use 2000
joules of energy every second. Thus,
the total energy used in a time of t seconds
will be 2000 t
joules, since W
= P t. Let’s
assume we use the radiator for 90 days at an average of 4 hours use per day. The
amount of energy used will be:
Energy
= 2000 x 4 x 60 x 60 x 90 = 2 592 000 000 joules. Rather
large isn’t it? And remember this
is just the energy used by one radiator for a few hours each day for a quarter
of the year. Imagine how large the
total energy usage for a whole household would be! Here we have the main reason as to why the kilowatt-hour is
used to measure electricity consumption rather than the joule.
The joule is a very small unit of energy, while the kilowatt-hour is a
much larger unit. Electrical energy authorities in Australia use the kWh as the
unit for energy simply because it produces more friendly, easily understood
energy consumption figures. The kilowatt-hour is defined as the
amount of energy used in one hour by an appliance rated at 1 kW (1000 W).
The equivalent energy in joules is:
1
kWh = (1 x 103 watts x 60 x 60 seconds) = 3.6 x 106 J Clearly, the kilowatt-hour is a much larger unit than the joule. So, for our example of the radiator, the amount of energy used in kWh
is: Energy
= 2 kilowatts x 360 hours = 720 kWh I think most people would agree that this is a much more manageable
figure than the roughly 2.6 billion joules calculated above.
MAGNETIC
FIELDS
Electric currents produce magnetic fields. In fact, any moving charge has a magnetic field associated with it. These magnetic fields are the same as those produced by ordinary bar magnets. We know that if we bring the north poles of two bar magnets close together, they repel one another. The same thing happens if we bring two south poles close together. If we bring a north pole and a south pole close together, they attract one another. In summary, like poles repel and unlike poles attract. Just as we did in the case of electric fields, we can use lines of force to represent a magnetic field. The direction of the field is indicated by arrows on the lines. The strength of the field is indicated by the separation of the lines. By definition, a magnetic field is said to exist at a point if a compass needle (small bar magnet) placed there experiences a force. The direction of the field is the direction of the force on the north pole of a compass needle placed at the point in question. The shape of the magnetic field around a bar magnet is as shown below. Note that the field lines emerge from the north pole and re-enter the magnet at the south pole. The magnetic field lines themselves are continuous. They travel through the magnet. Note also that no example of a single magnetic pole (monopole) existing on its own has ever been found. (Some experimental physicists are still looking for magnetic monopoles – certain theories on the nature of matter in the universe suggest that they could exist.)
GENERATION OF NATURAL MAGNETIC FIELDS Bar magnets and other so-called permanent magnets are made out of a material called ferromagnetic material. Iron, cobalt and nickel and the many alloys made from these are all ferromagnetic. This implies that these substances are all attracted strongly by a magnet. Ferromagnetic materials derive their magnetic properties from the spin motion of electrons in atoms. The spinning of an electron makes it behave like a little current loop, which has a magnetic field like that of a bar magnet, but on a much smaller scale. In most materials, the field from one electron cancels that from another, the net effect being no magnetic field. Ferromagnetic materials, however, consist of small regions (10-12 to 10-8 m3 volume) called magnetic domains in which the spins of electrons line up with each other to produce north and south poles. In the absence of an external magnetic field, these domains point in random directions. In the presence of a weak magnetic field, the domains line up in a particular direction and produce a net magnetic effect. If a magnetic field is required to keep the domains aligned, the magnet is called a temporary magnet (eg soft iron). If the domains remain aligned, the magnet is called a permanent magnet (eg hard steel). Note that even in permanent magnets, the domains will eventually relax into a random orientation, once out of the influence of the weak external magnetic field. This relaxation may take many, many years.
MAGNETIC FIELDS CAUSED BY CURRENTS Since every moving charge has a magnetic field associated with it, a current must also have a magnetic field associated with it. In fact, for a current moving through a straight conductor, the magnetic fields of the component charges add together to produce circular magnetic field lines concentric about the conductor. See below.
The direction of the field is given by the Right Hand Grip Rule, which states: Hold the thumb of the right hand in the direction of the conventional current flow through the conductor. The direction in which the fingers of the right hand naturally curl around the conductor, is the direction of the magnetic field. In the example below, the X in the middle of the conductor indicates that the current is flowing down into the page, perpendicular to the page. The field is then clockwise, looking from above the page, by the RH Grip Rule.
Try this java demonstration of the field around a current-carrying conductor: http://www.walter-fendt.de/ph14e/mfwire.htm
SOLENOIDSA solenoid is simply a coil of insulated wire. If we pass a current through a solenoid, we find that the solenoid has a magnetic field similar to that of a bar magnet. This field can be intensified greatly by adding a soft iron core inside the solenoid. Such an arrangement is called an electromagnet.
Another way of representing a
solenoid is to draw it in cross section, as shown below.
In the diagram above, the solenoid has been
cut through vertically. The current
is coming up out of the page through the bottom row of conductors
(indicated by the dot in the middle of each conductor) and down into the
page through the top row of conductors. Using the RH Grip rule, the magnetic field direction is as
shown. We can understand why a solenoid has such a magnetic field
by realizing that the fields due to each turn of wire in the coil, simply add
together to produce the typical bar magnet field. Note that at points inside the solenoid and reasonably far
from the wires, the magnetic field is fairly uniform and parallel to the
solenoid axis. In the limiting
case of adjacent, square, tightly packed wires, the solenoid becomes essentially
a cylindrical current sheet and the requirements of symmetry then make the
previous statement necessarily true.
APPLICATIONS OF MAGNETIC FIELDS IN HOUSEHOLD APPLIANCES In the home various
appliances make use
of magnetic fields. The electric
motors that drive many labour saving items of electrical equipment rely on
magnetic fields for their operation. Entertainment
devices, such as the TV and stereo require magnetic fields for the operation of
their speakers and various other components.
Cassette and video tapes use magnetic tape to store music only and pictures &
music respectively.
Computers are now common household appliances used for entertainment or
work. They use magnetic means of
storing and manipulating data (eg hard disk drives). Some
people use magnetic devices for controlling household pests, such as
cockroaches. The effectiveness of
such devices is still a matter of some controversy.
Some people use magnetic bracelets and amulets as a treatment for all
sorts of medical conditions eg arthritis. In
the home, applications of magnetic fields have resulted in improvements in the
standard of living. They save time
and human energy; they provide entertainment and relaxation; and they may have
other uses in keeping houses free from insect pests and in treating various
medical conditions. EXERCISE: Explain ONE application of magnetic fields in household appliances.
HOUSEHOLD
CIRCUITS
In Australian homes electrical energy is
available from the mains supply at a voltage of 240V AC* and a frequency
of 50Hz. This electricity is
supplied by the nearest substation. Two
wires carry the electricity into each house.
One wire is called the active and carries one of the available three
phases of electricity supplied by the substation. The other wire is called the neutral and is connected
to the ground at the substation. At
the house there is a third wire, called the earth wire, which is also
connected to the ground, via a copper rod literally driven into the ground. The active wire is connected to the Main Switch at the Meter Box. The neutral wire and the earth wire are connected to the neutral bar in the Meter Box. From the Meter Box, a number of circuits branch out through the house for different purposes. The Meter Box contains switches to electrically isolate the whole house or parts of it, an electricity meter that measures the amount of electrical energy taken from the Power Station, a mains fuse and a fuse (or circuit breaker) for each of the separate circuits in the house. For houses with off-peak electric hot water systems, there is also a separate electricity meter and timer to turn the water heater off and on. The number of separate circuits branching
out from the Meter Box depends on the size and design of the home, including the
number of electrical appliances to be used. There is a limit to the amount of electrical energy that
can be safely carried by household circuits.
If there are too many power points to wire into one circuit, one or more
other circuits will be used. There
will always be at least two different circuits – the lighting and power
circuits (to power points and fixed appliances).
These are kept separate since the lighting circuit usually requires a
smaller fuse than the power circuit. * Note
that the 240V value of the mains supply is really an average value.
It is really the RMS (root mean square) voltage, which is the DC
equivalent potential difference, which would be required for a direct current to
deliver the same energy to a circuit as the changing AC supply.
The actual voltage varies from 339 V to –339V during each AC cycle.
CONDUCTORS USED TO SUPPLY HOUSEHOLD ELECTRICITY Copper is the most common conductor used to provide household electricity. It is relatively cheap and a better conductor than all metals other than silver. Consequently, copper is used in most household wiring. Silver is occasionally used in some high quality electronic equipment due to its higher conductivity but it is not used widely due to its high price. Gold is also used sometimes for electrical contacts not because it is the best electrical conductor but because it is perhaps the least chemically reactive of metals. Aluminium is not as good a conductor as silver or copper but it is used in the wires for overhead power line distribution because of its light weight. The light weight allows the supporting structures to be placed further apart and this reduces the overall cost.
ELECTRICAL
SAFETY
As mentioned above, fuses and
circuit breakers are common devices found in household electrical circuits.
Both devices are designed to protect the house wiring from overload and
thereby prevent fires. For each separate household circuit, a fuse or circuit
breaker is placed in the meter box, in series between the external power supply
and the internal house wiring. In
the case of a fuse, if too much current is drawn for too long a time, the fuse
simply melts, thus breaking the circuit and protecting the wiring. In the case of a circuit breaker, if too much current is drawn for
too long, the circuit breaker opens, breaking the circuit and protecting the
wiring. Circuit breakers are rapidly becoming more common than fuses,
as they can simply be reset after use. All household electrical appliances are either earthed or double insulated (explained below). Many appliances are earthed by connecting a conducting wire from the metal body of the appliance to the earth wire of the household. As mentioned previously, this earth wire is connected to the ground. If a fault within the appliance results in current from the active wire leaking to the metal body of the appliance, two things will happen almost simultaneously. Firstly, the current will flow safely to the ground via the earth wire. Secondly, due to the large increase in current flowing in this household circuit via the short-circuit to ground, the fuse in this household circuit will blow or the circuit breaker in this household circuit will open, as the case maybe. Insulators play an important part in making household electrical appliances safe to use. Individual electrical conducting wires are covered with insulating material such as PVC (polyvinyl chloride) to prevent leakage of current. Power cables that enclose sets of insulated wires connected to appliances are also made from PVC or similar material. Light switches and power point plates are made from hard plastics. Fuse wires are held in place in household circuits using porcelain plugs. The internal insulation of electrical equipment may be made of mica or glass fibres with a plastic binder. Many small electrical appliances are double insulated which means that not only are the wires inside insulated but also the body itself, being made of plastic, is an insulator. Desk lamps, battery rechargers, electric drills, hair driers, electric mixers and electric razors are just a few examples. Such appliances have only two wires connected to them and a plug with two pins, one for the active and one for the neutral. Any metal screws or pins used to hold parts together are totally enclosed in plastic tubes. There are no electrically conductive parts that give a path for a current to the outside, even if a fault inside puts the body in direct contact with the active wire. Having mentioned some of the safety
features present in household electrical circuits and appliances, it is
appropriate to consider the dangers of electricity.
Electricity can kill a person in two ways: ¨
It can cause the muscles of the heart and
lungs (or other vital organs) to malfunction; or ¨
It can cause fatal burns. Even a small electric current can seriously disrupt body cell functions. When the electric current is 0.001A or higher, a person can feel the sensation of shock. At currents ten times larger, 0.01A, a person is unable to release the electric wire held in his/her hand because the current causes his/her muscles to contract violently. Currents larger than 0.02A paralyze the respiratory muscles and stop breathing. Unless EAR is started immediately the victim will suffocate. A current of 0.1A passing through the region of the heart, will shock the heart muscles into rapid, erratic contractions (ventricular fibrillation) so the heart can no longer function. Death would usually follow in a matter of a few minutes. Currents of 1A and higher through body tissue cause serious burns. Typically, the 240V AC mains supply causes a 25mA (milliampere) current in the body, which can easily cause death. This is the reason why some countries use 110V AC as their mains supply voltage – it is safer in the event of an electric shock. With AC, the frequency of the supply also affects the damage that the current causes. Since heart muscle is most sensitive to electricity of frequency 30-100Hz, the Australian mains frequency of 50Hz is ideal for inducing fibrillation. Higher frequencies, DC electrical current, and AC which does not pass through the heart do not cause fibrillation but rather heat up and burn the muscle they flow through, sparing skin and fat. Overall, the most important quantity to control in preventing injury is the electric current. Voltage is important only in that it can cause current to flow. Even though your body can be charged to a potential thousands of volts higher than the metal frame of your car, simply by sliding across the car seat, you feel only a harmless shock as you touch the door handle. Your body cannot hold much charge on itself, and so the current flowing through your hand to the door handle is short-lived and the effect on your body cells is negligible.
|
Last updated:
© Robert Emery Catholic Schools Diocese of Maitland-Newcastle 2002 - view the Terms of Use of this site.