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:
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:
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:
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:
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.
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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.
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.
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.
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:
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.
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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.
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:
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).
Figure 1: The process of electrolysis
The process of electrolysis is put to beneficial use in a number of ways discussed next.
Electroplating is the process of coating a metal with a thin layer of a different metal. It is used for:
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.
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:
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.
There are three common ways in which an electric current passing through a material can cause the emission of visible light.
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.
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.
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. |
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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.
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:
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.
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.
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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. |
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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 ( )
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.
– heat
– magnetic field
– chemical reaction
– light.
– the amount of current,
– the duration of current flow,
– the path of current flow through body.
1 Magnitude of current.
Duration of current.
The path within the body through which the current passes.
2 Respiratory arrest
asphyxia
ventricular fibrillation.
3 Ventricular fibrillation is when the heart goes into rapid uncontrolled pumping spasms (fibrillation) stopping blood circulation.
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.
1 A contactor is an electrically operated switch that uses an electromagnet to close or open its contacts.
2 Contactor
Transformer
Lifting magnet.
3 Force exerted on cables
Interference to television and radio.
1 Because there is less possibility of water being present to form an electrolytic cell.
2 Decorative (jewellery etc)
Prevent corrosion
Building up worn metal components.
3 In a solid, conduction is by electron flow, in an electrolyte conduction is by ions (both positive and negative).
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.
1 Earthing
Correct insulation and enclosure
Fuses and circuit breakers
RCDs.
2 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.
1 (b)
2 (a)
3 (c)
4 (a)
5 · breathing
6 · cannot be seen
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)
Source: http://lrr.cli.det.nsw.edu.au/LRRDownloads/3661/1/3661.doc
Web site to visit: http://lrr.cli.det.nsw.edu.au
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