Download Magnetic effect of current

Contents
Introduction
3
Physiological effect of current
4
Heating effect of current
6
Magnetic effect of current
8
Applications of the magnetic effect of a current
8
Unwanted magnetic effects of a current
9
Chemical effect of current
10
Applications of electrolysis
10
Corrosion
12
Lighting effect of current
14
Protection from damaging effects of current
16
Summary
20
Answers
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EEE042A: 6 Explain the effects of electrical current
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Introduction
When electric current passes through an object, whether it is a conductor or
insulator, solid liquid or gas, or even a living organism, different effects of
that current are observed. Some of the end results of electric current are
beneficial and intended. Other end results of current can be harmful to
people or equipment and are either accidental or unavoidable.
Four effects of an electric current are:

heating (always present)

magnetic (always present)

chemical

physiological.
The different effects of electric current and ways of minimising harmful
consequences will be explored in this section. After completing this topic,
you should be able to:

describe the physiological effects of current and the fundamental
principles (listed in AS/NZS 3000) to prevent such currents

outline the basic principles by which electric current can result in the
production of heat; the production of light; the production of magnetic
fields; a chemical reaction

list typical uses of the effects of current

describe the mechanisms by which metals corrode

describe the fundamental principles (listed in AS/NZS 3000) for
protection against the damaging effects of current.
EEE042A: 6 Explain the effects of electrical current
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Physiological effect of current
Electric shock caused by human contact with ‘live’ electrical equipment,
typically causes 70 to 80 deaths per year in Australia. Contact with voltages
as low as 32 V ac or 115 V dc can lead to:

respiratory arrest

asphyxia

ventricular fibrillation.
Any of these can be fatal unless medical assistance is immediate and
effective.
In the human body, hundreds of muscles control blood circulation,
breathing, digestion, reflex actions and many other vital areas within the
body. All muscles are driven by nerve impulses, small electrical signals sent
by the brain of about 70 mV (0.07V). An electric shock exposes the nervous
system to large voltages causing a massive overload and erratic response of
the body’s most important muscle, the heart.
Respiratory arrest (cessation of breathing) can be caused by a current
passing through the head in the region of the respiratory centre at the back of
the skull. Respiratory arrest can be brought on by even a fairly mild electric
shock in this area of the brain.
Asphyxia, is a choking effect caused by contraction of the chest and throat
muscles and brought on by contact with live parts across the chest region. In
most cases, this is caused by contact of the two hands, or the hands and the
feet, to different electrical potentials. Consciousness is lost after a minute or
so, followed by death a few minutes later.
Ventricular fibrillation is the major cause of deaths due to electric shock.
The heart goes into rapid uncontrolled pumping spasms (fibrillation)
stopping blood circulation. Without a freshly oxygenated blood supply, the
brain cells begin to die within about five minutes. Permanent brain damage
occurs after five minutes and death occurs after about ten minutes.
Electric shock can prove fatal with current as low as 50 mA. The overall
effect depends on several factors including the:
4

magnitude of the current

duration of the current flow

path within the body through which current passes that is, what muscles
EEE042A: 6 Explain the effects of electrical current
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and organs are affected.
Safe handling of electricity is of the utmost importance, and will be stressed
throughout your course. It is also very important that electrical students
learn resuscitation techniques.
Activity 1
1
What factors affect the severity of an electric shock?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2
What are the three possible effects of electric shock on the human body?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
3
Describe the condition of the heart termed ‘ventricular fibrillation’.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Heating effect of current
When electric current flows in a conductor, work must be done and energy
used in overcoming the resistance. All the energy used is released in the
form of heat.
In the case of good conductors the heat produced often escapes notice
because it is only small and can be dissipated into the surrounding air. The
result of this is that the temperature of the conductor hardly rises. But if
there is an excessive current for the size of conductor used, heat will be
produced more rapidly than it can be dissipated and the temperature of the
conductor will rise. The temperature may become very high, perhaps even
melting the wire. This happens of course with a fuse where ‘normal’ current
is carried with only minor heating but ‘overload’ current causes the fuse to
melt.
We must remember, however, that we can utilise this heating effect of a
current as, for example, in the heating elements of appliances. Radiators,
toasters, jugs and hotplates are perfect examples of this. The filaments
of incandescent lamps when heated to white heat give off white light.
The heating effect is given by the power equation P=I2R. Therefore the
amount of heat produced per second is proportional to the resistance and to
the square of the current. Therefore, to limit the losses in cables due to
heating we can decrease the resistance, or reduce the current. One way to
reduce the current (while providing the same power) is to increase the
voltage. This is done in power transmission, where very high voltages are
used to reduce the heating losses in the power lines.
In summarising then, the heating effects of current may be either a problem
or a benefit. We try to limit the heating of conductors, but at the same time
utilise the heating effect in elements and fuses.
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Activity 2
1
What effect of current does a fuse use?
_____________________________________________________________________
2
The flexible cord connecting an electric jug to a power outlet gets warm. Explain why
this occurs.
_____________________________________________________________________
_____________________________________________________________________
3
The elements in a radiator glows red hot, yet the cord supplying the radiator is
relatively cool. Explain why this is so.
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Magnetic effect of current
When an electron moves it creates a tiny magnetic field, and electrical
current creates a magnetic field around the conductor. The magnetic field of
an electric current is used in many types of electrical and electronic
equipment.
Applications of the magnetic effect of a
current
The magnetic field of an electric current is utilised in an electromagnet. This
effect is used in steel scrap yard cranes, which use an electromagnet to lift
iron and steel scraps. When the current is switched off, the scrap drops from
the magnet.
An electrical relay is a switch that uses a small electromagnet to close or
open its contacts. An example of this is in switching on a large electric
motor. The push-button switch used to start the motor cannot itself carry the
current drawn by the motor. Instead the push-button switch is used to
energise an electromagnet (called the ‘coil’) of a contactor, which in turn
closes a set of contacts to connect the supply to the motor.
Another use of the electromagnet is in solenoid valves, which use an
electromagnet to open and/or close a valve carrying fluid.
Some other common uses of the magnetic effect of current are:
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
An electric motor uses the interaction between several magnetic
fields to rotate the shaft of the motor

An electric generator rotates an electromagnet past a set of stationary
coils to generate electrical current. This is the magnetic effect in
reverse, where a current is created from a changing magnetic field.

Transformers use electromagnetic effect to convert a high voltage
into a lower voltage or vise versa.

Circuit breakers used to protect circuits from overloads, short
circuits, and other faults use electromagnetism in their tripping
mechanisms.
EEE042A: 6 Explain the effects of electrical current
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Unwanted magnetic effects of a current
The larger is the electrical current, the larger is the associated magnetic
field. If the magnetic field around conductors is strong enough it will
actually force the conductors to move. This is especially damaging when
overhead transmission lines or conductor in a duct or on a cable tray have to
carry abnormally large currents due to a fault in the system.
Devices which exploit the magnetic effects of current can cause interference
to sensitive electronic equipment such as television receivers, radio
receivers, and electronic heart pacemakers. This interference may occurs due
to the magnetic field itself, the high transient currents that occur when these
devices start or stop, and by the electromagnetic radiation caused by these
abrupt changes.
Activity 3
1
Explain how a contactor is used to control electrical current.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2
Name three items of equipment that use the electromagnetic effect.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
3
Name two unwanted effects of electromagnetism.
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Chemical effect of current
The chemical effect of current occurs when passing a current through an
electrolyte. An electrolyte is a fluid containing ions, which may be water
with other chemicals dissolved in it, or perhaps a molten salt. Current flow
in an electrolyte consists of a transfer of charge by ions, and results in
chemical change.
These chemical changes are utilised in:

The charging of batteries

The production of many materials by electrolysis, including
aluminium, magnesium, titanium, chlorine, and sodium.

Electroplating (for example chromium plating).
Applications of electrolysis
When two metal plates or rods are inserted into an electrolyte and a battery
or dc supply connected across the metals as shown in Figure 1 then the
process of electrolysis takes place. The positive metallic ions are attracted to
the negative plate and negative non-metallic ions are attracted to the positive
plate.
Various chemical processes will occur at the positive and negative plates. In
electroplating or aluminium refining, the positive metal ions in solution are
attracted to the negative plate (the cathode), and combine with electrons to
become solid metal.
In the electrolysis of water, hydrogen ions combine with electrons to become
hydrogen gas, and meanwhile oxygen is produced at the positive electrode
(the anode).
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Figure 1: The process of electrolysis
The process of electrolysis is put to beneficial use in a number of ways
discussed next.
Electroplating
Electroplating is the process of coating a metal with a thin layer of a
different metal. It is used for:

decorative reasons: for example gold or silver plating of jewellery items,
chrome plating of car parts

protection: plating with a metal such as zinc or cadmium will give anticorrosion protection to many common steel items such as nuts and bolts

repairs of worn shafts and the like can be built up using electroplating.
Electrical refining
Refining copper ore is mainly done using the electrolytic process. Impure
copper is connected to the positive electrode and the negative electrode
consists of an initially thin piece of pure copper. Copper ions move through
the solution and deposit on the negative plate as pure copper. Any impurities
fall to the bottom of the tank.
Aluminium is also refined in a similar manner, but in a molten electrolyte
called cryolite.
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Corrosion
The process of electrolysis can cause corrosion in a number of ways
discussed below.
The processes that take place in the corrosion of metals are similar to those
in an electrolytic cell. The electrolyte may be salt water or even rain water
with small amounts of dissolved acids or salts. The electrodes may be
different metals, or even different crystals in the same piece of metal.
Where two dissimilar metals are joined and a drop of moisture is in contact
with both metals, a miniature battery is formed which causes a small
localised circulating current which in turn causes corrosion.
Examples of this are:

where a copper cable is terminated at an aluminium bus-bar

where roofing iron comes in contact with lead flashing

corrosion of ships’ propellers that are made of brass, where the hull is
steel.
Corrosion can be made worse by electric current flowing in the structure.
Electric trams and trains use their rails as one part of the circuit that supplies
them with current to drive their motors. Any joints in the rail that also allow
ingress of air can form an electrolytic cell, and so a point of corrosion.
Another example is the terminals batteries. You may have noticed the
corrosion forming at the terminals of batteries in cars, flashlights, and in fact
any piece of equipment that uses batteries.
Corrosion can also be prevented by applying an electric current in the
opposite direction to that which would cause corrosion. A ‘sacrificial anode’
of a reactive metal like magnesium can be buried in the soil, or for marine
applications, suspended in the water. This electrode will corrode and
‘sacrifice’ itself in preference to the structure that is to be protected.
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Activity 4
1
If a copper busbar in a switchyard is bolted to an aluminium cable, the conductors will
probably become corroded. The corrosion is less likely to happen inside a building.
Explain why.
_____________________________________________________________________
_____________________________________________________________________
2
Name three applications of electro-plating.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
3
Explain the difference between current in an electrolyte and a solid conductor.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Lighting effect of current
There are three common ways in which an electric current passing through a
material can cause the emission of visible light.
Incandescent lighting
When a solid conductor is heated to a high temperature by the passage of an
electric current it produces light as well as heat. An example of this is the
common household light bulb. This type of lamp has a thin tungsten
filament through which a current is passed, causing it to glow and emit a
white-yellow light. This type of lighting is called incandescent lighting.
Discharge lighting
When gases are exposed to a large electric field, electrons are stripped from
some of the atoms, and ions are formed. The electrons and ions are
accelerated in the field, bumping into and exciting more atoms. The gas
becomes conductive. In this state the electrons orbiting the atoms of gas are
excited into higher energy levels. When the electron returns to its normal
state, it emits a photon of light energy with energy specific to the gas being
used. In some gases this wavelength corresponds to visible light. For
example, neon lights are red, and sodium lights are yellow. This principle is
used in the manufacture of discharge lighting. Some examples of discharge
lighting include fluorescent lamps, mercury vapour lamps, and sodium
vapour lamps. Because this type of lighting does not rely on heating a solid
element, it operates at a much cooler temperature and is more efficient than
incandescent lighting.
Light emitting diodes
Semiconductors can be used to manufacture a device called a light emitting
diode (LED). A current passing through an LED is converted efficiently to
light. The process again involves the transition of electrons between an
excited state and a lower-energy state, emitting photons of light in the
process. Red, green and blue light emitting diodes are used as indicating
lamps on all types of electronic devices, and more recently as very efficient
sources of white light.
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If you have Hampson, read the ‘Effects of an electric current’ section on
page 28 through to page 32, taking note of the review questions listed.
If you have Jenneson, refer to Section 2.16, ‘Effects of electricity’ on page
49 for a brief summary of key issues.
Activity 5
1
Explain how an incandescent lamp produces light.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2
What do the initials ‘LED’ stand for?
_____________________________________________________________________
3
How is light produced in a fluorescent lamp?
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
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Protection from damaging effects of
current
To protect people and equipment from the damaging effects of electricity
there are legal requirements placed on manufacturers and installers of
electrical equipment. These requirements are covered in AS/NZS 3000
known as the Wiring Rules.
Some of the means by which exposure to electric shock and equipment
damage is minimised and which are covered in AS/NZS 3000 include:

Earthing.
The connection of any exposed metal of electrical equipment to earth. In
the event of a fault where the exposed metal becomes live, the current is
diverted to ground rather than to the person handling the faulty
equipment. The low resistance path to earth ensures that the circuit
protection operates. This protects the wiring from excessive current, and
also protects people from electric shock.

Correct insulation and enclosure of wiring and equipment

Fuses and circuit breakers
These are designed to disconnect the supply to a piece of equipment
if excessive current is drawn due to an earth fault or short circuit.

Residual current devices (RCDs)
These are sometimes known as safety switches and are designed to
protect people from electric shock. RCDs are designed to disconnect the
supply from a circuit very quickly when a fault occurs. This will happen
if even a very small imbalance between the currents in the active and
neutral conductors occurs. A situation like this arises if a person
accidentally comes in contact with a live conductor and completes a
path to earth.
If you have Hampson, read the ‘Physiological effects of current’ section on
page43 through to page 45, noting the sensations listed and the topic review.
If you have Jenneson, refer to section 2.16.4 of ‘Effects of electricity’ on
page 49 through to section 2.16.7 on page 51 for the key issues.
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Activity 6
1
List three ways in which people and equipment can be protected from the damaging
effects of electric current.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
2
What is an RCD used for?
_____________________________________________________________________
3
Explain how earthing an item of equipment can protect the person handling that
equipment in the event of a fault.
_____________________________________________________________________
_____________________________________________________________________
4
Have a look around your place of work or at home and identify six pieces of electrical
equipment. Note these below and then try and identify what effect of electric current is
used in their operation.
Note: some items may use more than one effect of current.
Equipment
Effect of electric current used
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Check your progress
In questions 1–4, write your answer in the brackets provided.
1
A chemical reaction occurs when current flows through:
(a) a solid conductor
(b) an electrolyte
(c) an insulator
(d) a capacitor
2
(
)
(
)
Current passing through a copper conductor will cause a:
(a) magnetic field around the conductor
(b) chemical reaction within the conductor
(c) decrease in conductor temperature
(d) reduction in conductor resistance
3
Besides heat, the other effect that is always present when current flows in a conductor
is:
(a) physiological
(b) chemical
(c) magnetic
(d) electrolytic
4
(
)
(
)
An electric motor rotates due to the:
(a) magnetic effect of the current
(b) chemical effect of the current
(c) heating effect of the current
(d) electrolytic effect of the current
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5
List two vital body functions that stop if a person receives a severe electrical shock.
_____________________________________________________________________
_____________________________________________________________________
6
List three reasons why electricity is dangerous.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
7
Identify with arrows three corrosion sites on the following bus-bar assembly.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
8
Briefly explain how the passage of electric current through a gas can produce light.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
9
Name three possible applications of electro plating.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of the section.
EEE042A: 6 Explain the effects of electrical current
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Summary

There are four effects which may occur when current flows in a circuit.
Of these, two are always present, and two may occur depending on
circumstances.

Electric current can produce:
–
–
–
–
heat
magnetic field
chemical reaction
light.

Heat is produced by electrons bouncing off the atoms while moving
along the conductor. Even in a very good conductor, a small amount of
heat will be produced.

This ‘heating’ principle is applied to produce light and heat for domestic
and industrial purposes.

Whenever current flows in a conductor, a magnetic field is set up around
the conductor.

This ‘magnetic’ principle is used in the generation of most of the
electricity consumed, and in components such as electromagnets,
solenoids, circuit breakers, transformers and motors.

‘Electrolyte’ is the name given to conducting liquids including molten
salts or solutions of ionic compounds.

The chemical changes which take place when an electrolyte carries a
current are called ‘electrolysis’.

The principle of electrolysis is used in electroplating and in commercial
production of chlorine, aluminium, sodium hydroxide, and other
substances.

Electric shock can be fatal with currents as low as 50 mA.

The effect on a person of electric shock depends on:
– the amount of current,
– the duration of current flow,
– the path of current flow through body.
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Answers
Activity 1
1
Magnitude of current.
Duration of current.
2
3
The path within the body through which the current passes.
Respiratory arrest
asphyxia
ventricular fibrillation.
Ventricular fibrillation is when the heart goes into rapid uncontrolled
pumping spasms (fibrillation) stopping blood circulation.
Activity 2
1
Heating effect
2
Energy used in overcoming the resistance of the cord
3
The element has a lot more resistance than the cord therefore
a lot of heat produced in overcoming the resistance.
Activity 3
1
A contactor is an electrically operated switch that uses an electromagnet
to close or open its contacts.
2
Contactor
3
Transformer
Lifting magnet.
Force exerted on cables
Interference to television and radio.
Activity 4
1
Because there is less possibility of water being present to form an
electrolytic cell.
2
Decorative (jewellery etc)
EEE042A: 6 Explain the effects of electrical current
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Prevent corrosion
3
Building up worn metal components.
In a solid, conduction is by electron flow, in an electrolyte conduction is
by ions (both positive and negative).
Activity 5
1
A thin tungsten filament is heated to white heat by the passage of a
current through it producing light.
2
Light Emitting Diode.
3
Gas in the fluorescent tube become ionised when a current is passed
through it driving the electrons into higher energy levels and in so
doing radiate light.
Activity 6
22
1
Earthing
2
Correct insulation and enclosure
Fuses and circuit breakers
RCDs.
A small imbalance in currents in the supply conductors as when a
person accidentally comes in contact with a live conductor and
completes a path to earth, causes the RCD to disconnect the supply
from that circuit.
3
In the event of a fault, the current is diverted to ground rather than to the
person handling the faulty equipment. The low resistance path to earth
ensures that the circuit protection operates
4
If you require feedback on this activity, attach your results for this
activity to your assignment.
EEE042A: 6 Explain the effects of electrical current
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Check your progress
1
(b)
2
(a)
3
(c)
4
(a)
5
 breathing
 heart.
6
 cannot be seen

cannot be smelled

cannot be heard.
7
8
Electric current excites electrons of gas atoms into higher energy orbits.
On returning to their normal orbits energy is given off in the form of
light.
9

protective (against corrosion)

cosmetic (appearance of product)

electrolytic refining.
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