Work, Energy and Power

Work:

A man pushing a car along a level road is said to be doing work. The amount of work done depends on the force exerted and the distance through which the force acts.

UNIT OF WORK: Since W = F x s, Unit of work = (unit of F) x (unit of s) = newton x metre (N m) = Joule

This unit is called the joule in honour of James Joule (1818—89) who did much early research in the area of work and the related of energy.

One Joule is the work done when a force of 1 Newton acts over a distance of 1 metre in the direction of the force: 1J = 1Nxm

Energy:

Energy is the ability to do work. The amount of energy something has is the amount of work it can do.

The unit of energy is the Joule.

Power:

A bulb in a desk lamp can convert electrical energy into heat and light at the rate of 40 joules per second; a bulb in a floodlight can convert electrical ener

gy into heat and light at the rate of 500 joules per second. The second bulb is more powerful than the first.

A builder brings 1000 blocks each of mass 20 kg from the ground, up a ladder, to the top of a house. It takes him 5 hours to do so. A crane lifts all the blocks

together from the ground to the top in a time of 10 seconds. Even though the man and the crane do the same work the crane does it

in a much shorter time. The cra

ne is much more powerful than the man.

Power is defined as the rate at which work is done. That is Power = Energy/time

UNIT OF POWER Since the U nit of Energy = joule and the Unit of time is the Second;

P = 1J / 1s

This unit is called the W

att (W) in honour of the English s

cientist James Watt.

DIFFERENT FORMS OF ENERGY

Since a body can do work for different reasons it can have energy for different reasons. It is useful to classify energy a ccording to the reasons that bodies have it.

KINETIC ENERGY: All moving objects have energy due to their motion. The energy a body has due to its motion is called kinetic energy.

POTENTIAL ENERGY: Potential energy is the energy a body

has due to its position in a force field.

In the picture shown on the right, a cyclist with Kinetic energy is moving up the hill and transforming the en

ergy into Potential energy

ELECTROMAGNETIC ENERGY: This i

s the energy transmitted from one point to another in the form of electromagnetic waves that travel at the speed of light. Example of a electromagnetic energy producer i

s a microwave.

INTERNAL ENERGY or heat energy: The molecules of every substance, whether solid, liquid or gas are in perpetual motion. If heat is supplied to a substance, the motion (the vibration) of its molecules increases as its temperature

rises. The kinetic energy of the molecules therefore increases. Thus heat is

a form of energy.

SOUND ENERGY: Any source of sound is always some object vibrating. As sound travels through a substance, the vibration from the source is passed on from molecule to molecule in that substance. Each molecule acquires energy in kinetic and potential form as the sound wave passes.

CHEMICAL ENERGY: The energy given out (or sometimes taken in) in the form of heat, light or kinetic energy during a chemical reaction is called chemical energy. For example, when something is burned, some of the chemical energy stored in it is changed into heat and light energy.

ELECTRICAL ENERGY When an electric current flows in a metal wire subatomic particles called electrons move in the wire. As they move, they lose energy called electric potential energy. This potential energy appears as

heat and other forms of energy in the wire.

NUCLEAR ENERGY The energy given out from the nuclei of certain atoms during nuclear reactions (fission or f

usion for example) is called nuclear energy

. This energy is used practically in the nuclear reactor

THE PRINCIPLE OF THE CONSERVATION OF ENERGY

Energy cannot be created or destroyed but can only be transferred from one body to another or changed from one for m to another. In every chemical or physical reaction, there is no net gain or loss of energy; any loss is always accompanied by an equal gain.

RENEWAB

LE AND NON-RENEWABLE SOURCES OF ENERGY

Every day we use energy for many purposes, e.g. transport and heating. The sources of this energy are many, e.g. oil, coal and hydroelectricity.

Sources of energy are classified as renewable or non-renewable. Examples of renewable energy sources are wind energy and solar energy, hydroelectric energy, wave energy and biomass. Non-renewable s

ources include oil, coal, peat and natural gas.

Energy is said to be renewable if the source of the energy is still there after energy is taken from it, and non-renewable if the source of the energy is gone after energy is taken from it.

Renewable energy sources will not run out. A problem with renewable energy sources is generating enough reliable energy to meet all our requirements. A lot of research is being done to find practical ways of using these ‘altemative

’ sources for the future.

Non-renewable ener

gy sources are fossil fuels, oil, gas, coal, peat or nuclear. They are usually easier to get energy from and store, but have a bigger environmental impact.

Energy pl

ant.

We need electrical energy to run all our appliances at home. We know that energy is everywhere around us, but how can we get it? There are several ways to transform energy into electrical energy. Energy plants have all these components to produce energy. Let's see a very simple one.

HYDROELECTRIC ENERGY

The store water behind an enormous dam flows through the intake and into a pipe called a penstock. The intake remains closed when no energy is asked for, for example, at night, as everybody is in bed. The water pushes against blades (the arms of the rotary mechanism), causing them to turn. The turbine spins and its shaft is connected to a generator that changes the mechanical spinning energy into electrical energy ( electricity ).

Advantage: Water power is reasonably easy to store ( in a reservoir ). Renewable energy.

Disadvantages: A lot of water is needed

P = V x I

If we want to transfer high power from the electric plant to the town where it can be used, we need high Voltage and Current ( Amps )values.

Remember that Amps mean electrons moving through conductors which, because of their collisions with atoms, produce heat.

The cables of transmission lines are made of aluminium ( cheaper ) or copper ( more expansive )

because they have a low resistance. So the higher the current, the higher the energy lost in the wires.

It is not good. All this work just to warm the bird's foot.

By the way, do you know how birds can sit safely on electric power lines?.

The answer is simple. Because the bird only touches one line, so electrons flow through the path of least resistance, the wire, and not its body.

So, the trick is to reduce the Current and increase the voltage. There is an electrical device that does just this. Its name is the transformer.

Electrical transformers are widely used to "transform" voltage from a lower voltage to a higher voltage or vice versa ( normally in appliances ). The principle of magnetic induction between coils makes it possible.

A transformer basically consists of a ferromagnetic core and two coils called "windings".

A changing current in the primary winding generates an alternating magnetic field in the ferromagnetic material.

The alternating magnetic field "goes and comes back" through the magnetic core ( the green line).

There is a relationship between voltage and electric field, so if a voltage is applied in the primary winding with 10 turns it generates a Magnetic field of value X , the same magnetic field generates a voltage in the secondary winding.

The voltage in the Secondary winding will be the same if N2 = 10

The voltage will be less if the secondary winding has less turns than the primary winding

In general, the simple formula that relates both voltages is:

V1 X N1 = V2 X N2

Example: If the output voltage in an alternator is 20000 volts and we want 200000 for the transmission lines, in a transformer with 100 turns in its primary winding:

20.000 X 100 = 20000 X N2 .....N2 = 2.000.000 / 20.000 = 1000 turns.

As Power is a constant, the current in the secondary winding will be less.

Now, everything is ready for the electricity to travels long distances to a substation. Substations are situated near factories and homes. Here transformers do their work in a inversely. I mean, they change the very high voltage electricity back into lower voltage electricity. In this case, the secondary winding will have less turns than the Primary Winding.

The next image illustrates the different elements of a transmission power system.

1º Step, The power plant, in this case a Hydroelectric plant.2º The transmission substation where the transformer increases the voltage 3º High voltage transmission, using long thick cables made of copper or aluminium with voltages from 200.000 v to 400.000 v4º The power substation, which will decrease the voltage from 400.000 to 20.000 5º Power poles take electricity to the Transformer Drum and from this element to homes with tension of 220 v

floodlight: light that is a source of artificial illumination having a broad beam; used in photography.

Crane: a device for lifting and moving heavy weights in suspension.

Blade: the arm of a propeller or other similar rotary mechanism, such as an electric fan or turbine.

shaft: a rotating, straight bar for transmitting motion, usually supported on bearings and carrying gears, wheels, such as a propeller shaft on a ship

Core: The piece of iron, or other ferrous material forming the central portion in an transformer .

Solar power. How to use the sun's energy

What are Electrical Transformers?

Electrical transformers are used to transform electrical energy. How electrical transformers do so is by altering voltage, generally from high to low. Voltage is simply the measurement of electrons, how many or how strong, in the flow. Electricity can then be transported more easily and efficiently over long distances.

While power line electrical transformers are commonly recognized, there are other various types and sizes as well. They range from huge, multi-ton units like those at power plants, to intermediate, such as the type used on electric poles, and others can be quite small. Those used in equipment or appliances in your home or place of business are smaller electrical transformers and there are also tiny ones used in items like microphones and other electronics.

Probably the most common and perhaps the most necessary use of various electrical transformers is the transportation of electricity from power plants to homes and businesses. Because power often has to travel long distances, it is transformed first into a more manageable state. It is then transformed again and again, or “stepped down,” repeatedly as it gets closer to its destination.

When the power leaves the plant, it is usually of high voltage. When it reaches the substation the voltage is lowered. When it reaches a smaller transformer, the type found on top of electric poles, it is stepped down again. It is a continuous process, which repeats until the power is at a usable level.

You have likely seen the type of electrical transformers that sit on top of electric poles. These, like most electrical transformers, contain coils or “windings” that are wrapped around a core. The power travels through the coils. The more coils, the higher the voltage. On the other hand, fewer coils mean lower voltage.

Electrical transformers have changed industry. Electric power distribution is now more efficient than ever. Transformers have made it possible to transfer power near and far, in a timely, efficient, and more economical manner. Since many people do not wish to live in close proximity to a power plant, there is the added benefit of making it possible for homes and businesses that are quite a distance from power plants to obtain dependable, affordable electricity. Much of the electricity used today will have passed through many electrical transformers before it reaches users.

Electrical transformers are used to "transform" voltage from one level to another, usually from a higher voltage to a lower voltage. They do this by applying the principle of magnetic induction between coils to convert voltage and/or current levels.

Electrical transformers are used to "transform" voltage from one level to another, usually from a higher voltage to a lower voltage. They do this by applying the principle of magnetic induction between coils to convert voltage and/or current levels.

Electrical transformers can be configured as either a single-phase or a three-phase configuration. There are several important specifications to specify when searching for electrical transformers. These include: maximum secondary voltage rating, maximum secondary current rating, maximum power rating, and output type. An electrical transformer may provide more than one secondary voltage value. The Rated Power is the sum of the VA (Volts x Amps) for all of the secondary windings. Output choices include AC or DC. For Alternating Current waveform output, voltage the values are typically given in RMS values. Consult manufacturer for waveform options. For direct current secondary voltage output, consult manufacturer for type of rectification.

Cores can be constructed as either a toroidal or laminated. Toroidal units typically have copper wire wrapped around a cylindrical core so the magnetic flux, which occurs within the coil, doesn't leak out, the coil efficiency is good, and the magnetic flux has little influence on other components. Laminated refers to the laminated-steel cores. These steel laminations are insulated with a nonconducting material, such as varnish, and then formed into a core that reduce electrical losses. There are many types. These include autotransformer, control, current, distribution, general-purpose, instrument, isolation, potential (voltage), power, step-up, and step-down. Mountings include chassis mount, dish or disk mount, enclosure or free standing, h frame, and PCB mount.

Wiring diagram

A wiring diagram is a simplified conventional pictorial representation of an electrical circuit. It shows the components of the circuit as simplified shapes, and the power and signal connections between the devices. A wiring diagram usually gives more information about the relative position and arrangement of devices and terminals on the devices, as an aid in construction the device. This is unlike a schematic diagram where the arrangement of the components interconnections on the diagram does not correspond to their physical locations in the finished device. A pictorial diagram would show more detail of the physical appearance, whereas a wiring diagram uses a more symbolic notation to emphasize interconnections over physical appearance.

A wiring diagram is used to troubleshoot problems and to make sure that all the connections have been made and that everything is present.

Architectural wiring diagrams

Architectural wiring diagrams show the approximate locations and interconnections of receptacles, lighting, and permanent electrical services in a building. Interconnecting wire routes may be shown approximately, where particuular receptacles or fixtures must be on a common circuit.

Wiring diagrams use standard symbols for wiring devices, usually different from those used on schematic diagrams. The electrical symbols not only show where something is to be installed, but also what type of device is being installed. For example, a surface ceiling light is shown by one symbol, a recessed ceiling light has a different symbol, and a surface fluorescent light has another symbol. Each type of switch has a different symbol and so do the various outlets. There are symbols that show the location of smoke detectors, the doorbell chime, and thermostat. On large projects symbols may be numbered to show, for example, the panel board and circuit to which the device connects, and also to identify which of several types of fixture are to be installed at that location.

A set of wiring diagrams may be required by the electrical inspection authority to approve connection of the residence to the public electrical supply system.

Wiring diagrams will also include panel schedules for circuit breaker panelboards, and riser diagrams for special services such as fire alarm or closed circuit television or other special services.

Electrical wiring

Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to describe common features of electrical wiring that should apply worldwide.

Wiring methods
Materials for wiring interior electrical systems in buildings vary depending on:

Intended use and amount of power demand on the circuit
Type of occupancy and size of the building
National and local regulations
Environment in which the wiring must operate.

Wiring systems in a single family home or duplex, for example, are simple, with relatively low power requirements, infrequent changes to the building structure and layout, usually with dry, moderate temperature, and noncorrosive environmental conditions. In a light commercial environment, more frequent wiring changes can be expected, large apparatus may be installed, and special conditions of heat or moisture may apply. Heavy industries have more demanding wiring requirements, such as very large currents and higher voltages, frequent changes of equipment layout, corrosive, or wet or explosive atmospheres. In facilities that handle flammable gases or liquids, special rules may govern the installation and wiring of electrical equipment in hazardous areas.

Early wiring methods

The very first interior power wiring systems used conductors that were bare or covered with cloth, which were secured by staples to the framing of the building or on running boards. Where conductors went through walls, they were protected with cloth tape. Splices were done similarly to telegraph connections, and soldered for security. Underground conductors were insulated with wrappings of cloth tape soaked in pitch, and laid in wooden troughs which were then buried. Such wiring systems were unsatisfactory because of the danger of electrocution and fire and the high labor cost for such installations.

Knob and tube

single conductors were run through cavities between the structural members in walls and ceilings, with ceramic tubes forming protective channels through joists and ceramic knobs attached to the structural members to provide air between the wire and the lumber and to support the wires. Since air was free to circulate over the wires, smaller conductors could be used than required in cables. By arranging wires on opposite sides of building structural members, some protection was afforded against short-circuits that can be caused by driving a nail into both conductors simultaneously.

Other historical wiring methods
Other methods of securing wiring that are now obsolete include:

Re-use of existing gas pipes for electric lighting. Insulated conductors were pulled into the pipes feeding gas lamps.
Wood moldings with grooves cut for single conductor wires, covered by a wooden cap strip. These were prohibited in North American electrical codes by 1928. Wooden molding was also used to some degree in England, but was never permitted by German and Austrian rules.
Metal molding systems, with a flattened oval section consisting of a base strip and a snap-on cap channel, were more costly than open wiring or wooden molding. Similar systems are still available today.

A system of flexible twin cords supported by glass or porcelain buttons was used near the turn of the 20th century in Europe, but was soon replaced by other methods.
During the first years of the 20th century various patented forms of wiring system such as Bergman and Peschel tubing were used to protect wiring; these used very thin fiber tubes or metal tubes which were also used as return conductors.

In Austria, wires were concealed by embedding a rubber tube in a groove in the wall, plastering over it and then removing the tube and pulling in wires in the cavity.

What is a Parallel Circuit?

A parallel circuit is one of the two basic types of electric circuit that can be found in electrical devices. "Circuit" refers to the total path of an electric current, or flow of electrical energy, and includes devices such as resistors, which control the flow of voltage, or difference in electrical charge, and capacitors, which store electrical charge. Circuits fall into one of two categories: series or parallel. In a series circuit, all the components of the circuit are lined up in a single path so that the current flows through each component in order.

In a parallel circuit, however, there are multiple pathways between the circuit’s beginning and end. As a result, since the current has more than one route to take, the circuit can still function if one path fails. This makes parallel circuits much more fail-resistant than series circuits which is why parallel circuits are common in everyday applications, such as household wiring. Regardless of how many different paths the circuit has, the total voltage stays the same, and all components of the circuit share the same common points. This set of common points is known as electrically common points. Every parallel circuit has two sets of them.

One thing to consider about parallel circuits is the current load that they carry. When a circuit has multiple paths for current, the circuit's total effective resistance drops. Since the voltage is equal to the current multiplied by the resistance — known as Ohm’s law, named for German physicist Georg Ohm — and the voltage does not change, this means the current has to increase. Thus, the more paths that a circuit has, the greater the current flow across each path will effectively become. This can lead to damage to the circuit or external equipment, which is why excessive use of outlet extenders or multi-plug inserts is considered hazardous. Parallel circuits are found in virtually all complex electrical devices. Many devices use both series and parallel circuits in conjoined and stand-alone configurations.

Another aspect of parallel circuits to keep in mind is that such circuits must be measured differently than series circuits. For example, when testing a parallel circuit using a voltmeter or multimeter, which tests multiple measurements, the multimeter must be connected in parallel to properly measure the voltage. Multiple branches means the load is distributed over more than one path, and measuring only one path will not present the full picture. If this isn’t done correctly, the measurement will be faulty, and the circuit may incorrectly be judged defective.

What Is an Electrical Circuit?

An electrical circuit is a closed loop formed by a power source, wires, a fuse, a load, and a switch. When the switch is turned on, the electricalelectricalelectrical circuit is complete and current flows from the negative terminal of the power source, through the wire to the load, to the positive terminal. Any device that consumes the energy flowing through a circuit and converts that energy into work is called a load. A light bulb is one example of a load; it consumes the electricity from a circuit and converts it into work — heat and light.

There are three types of circuits: series circuits, parallel circuits, and series-parallel circuits. A series circuit is the simplest because it has only one possible path that the electrical current may flow. If the electrical circuit is broken, none of the load devices will work. A parallel circuit has more than one path, so if one of the paths is broken, the other paths will continue to work.

A series-parallel circuit attaches some of the loads to a series circuit and others to parallel circuits. If the series circuit breaks, none of the loads will function. If one of the parallel circuits breaks, however, that parallel circuit and the series circuit will stop working, but the other parallel circuits will continue to work.

Many "laws" apply to electrical circuits, but Ohm's Law is probably the most well known. To understand Ohm's Law, it's important to understand the concepts of current, voltage, and resistance. Current is the flow of an electric charge. Voltage, or electrical potential difference, is the force that drives the current in one direction. Resistance is the opposition of an object to having current pass through it.

Ohm's Law states that an electrical circuit's current is directly proportional to its voltage and inversely proportional to its resistance. So, if voltage increases, for example, the current will also increase, and if resistance increases, current decreases. The formula for Ohm's Law is E = I x R, where E = voltage in volts, I = current in amperes, and R = resistance in ohms.

Source voltage is another important concept in electrical circuits. It refers to the amount of voltage that is applied to the circuit and is produced by the power source. Source voltage is affected by the amount of resistance within the electrical circuit and affects the amount of current. The current is affected by both voltage and resistance. Resistance is not affected by voltage or current, but it affects both voltage and current.

Electric motor:

An electric motor is a device using electrical energy to produce mechanical energy, nearly always by the interaction of magnetic fields and current-carrying conductors. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Traction motors used on vehicles often perform both tasks. In principle, all electric motors can run as generators and vice versa, although that is not practical with all types in all applications.
As a convention the term electric engine is not used for electric motors, but instead refers to a railroad electric locomotive.

Electric motors are found in a myriad of applications such as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and computer disk drives, among many other applications. Electric motors may be operated by direct current from a battery in a portable device or motor vehicle, or from alternating current from a central electrical distribution grid. The smallest motors may be found in electric wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide convenient mechanical power for industrial uses. The very largest electric motors are used for propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the thousands of kilowatts. Electric motors may be classified by the source of electric power, by their internal construction, and by application.

The physical principle of production of mechanical force by the interactions of an electric current and a magnetic field was known as early as 1821. Electric motors of increasing efficiency were constructed throughout the 19th century, but commercial exploitation of electric motors on a large scale required efficient electrical generators and electrical distribution networks.

The principleThe principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices, unsuited to practical applications due to limited power.

In 1827, Hungarian Ányos Jedlik started experimenting with electromagnetic rotating devices he called "electromagnetic self-rotors". He used them for instructive purposes in universities, and in 1828 demonstrated the first device which contained the three main components of practical direct current motors: the stator, rotor and commutator. Again, the device had no practical application.

The first electric motors:

The first British commutator-type direct current electric motor capable of turning machinery was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. His motors ran at up to 600 revolutions per minute, and powered machine tools and a printing press. Due to the high cost of the zinc electrodes required by primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.

In 1855 Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work.He built a model electric motor-propelled vehicle that same year. There is no evidence that this experimentation was communicated to the wider scientific world at that time, or that it influenced the development of electric motors in the following decades.[citation needed].

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.

In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.

The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of a relatively-small air gap between rotor and stator. Early motors, for some rotor positions, had comparatively huge air gaps which constituted a very-high-reluctance magnetic circuit. They produced far-lower torque than an equivalent amount of power would produce with efficient designs. The cause of the lack of understanding seems to be that early designs were based on familiarity of distant attraction between a magnet and a piece of ferromagnetic material, or between two electromagnets. Efficient designs, as this article describes, are based on a rotor with a comparatively small air gap, and flux patterns that create torque.

The armature bars are at some distance (unknown) from the field pole pieces when power is fed to one of the field magnets; the air gap is likely to be considerable. The text tells of the inefficiency of the design. (Electricity was created, as a practical matter, by consuming zinc in wet primary cells!).

In his workshops Froment had an electromotive engine of one-horse power. But, though an interesting application of the transformation of energy, these machines will never be practically applied on the large scale in manufactures, for the expense of the acids and the zinc which they use very far exceeds that of the coal in steam-engines of the same force. [...] motors worked by electricity, independently of any question as to the cost of construction, or of the cost of the acids, are at least sixty times as dear to work as steam-engines.

Although Gramme's design was comparatively much more efficient, apparently the Froment motor was still considered illustrative, years later. It is of some interest that the St. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor. Photo of a traditional form of the motor. The prominent bar magnets, and the huge air gap at the ends opposite the rotor. Even modern versions still have big air gaps if the rotor poles are not aligned.

Application:

The electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using shaft, belts, compressed air or hydraulic pressure. Instead every machine could be equipped with its own electric motor, providing easy control at the point of use, and improving power transmission efficiency. Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible. Today, electric motors consume more than half of all electric energy produced.

Pathophysiology:

A myocardial infarction occurs when an atherosclerotic plaque slowly builds up in the inner lining of a coronary artery and then suddenly ruptures, totally occluding the artery and preventing blood flow downstream.Main article: Acute coronary syndromeAcute myocardial infarction refers to two subtypes of acute coronary syndrome, namely non-ST-elevated myocardial infarction and ST-elevated myocardial infarction, which are most frequently (but not always) a manifestation of coronary artery disease.

The most common triggering event is the disruption of an atherosclerotic plaque in an epicardial coronary artery, which leads to a clotting cascade, sometimes resulting in total occlusion of the artery. Atherosclerosis is the gradual buildup of cholesterol and fibrous tissue in plaques in the wall of arteries (in this case, the coronary arteries), typically over decades. Blood stream column irregularities visible on angiography reflect artery lumen narrowing as a result of decades of advancing atherosclerosis.

Plaques can become unstable, rupture, and additionally promote a thrombus (blood clot) that occludes the artery; this can occur in minutes. When a severe enough plaque rupture occurs in the coronary vasculature, it leads to myocardial infarction (necrosis of downstream myocardium).

If impaired blood flow to the heart lasts long enough, it triggers a process called the ischemic cascade; the heart cells in the territory of the occluded coronary artery die (chiefly through necrosis) and do not grow back. A collagen scar forms in its place. Recent studies indicate that another form of cell death called apoptosis also plays a role in the process of tissue damage subsequent to myocardial infarction. As a result, the patient's heart will be permanently damaged. This Myocardial scarring also puts the patient at risk for potentially life threatening arrhythmias, and may result in the formation of a ventricular aneurysm that can rupture with catastrophic consequences.

Injured heart tissue conducts electrical impulses more slowly than normal heart tissue. The difference in conduction velocity between injured and uninjured tissue can trigger re-entry or a feedback loop that is believed to be the cause of many lethal arrhythmias. The most serious of these arrhythmias is ventricular fibrillation (V-Fib/VF), an extremely fast and chaotic heart rhythm that is the leading cause of sudden cardiac death. Another life threatening arrhythmia is ventricular tachycardia (V-Tach/VT), which may or may not cause sudden cardiac death. However, ventricular tachycardia usually results in rapid heart rates that prevent the heart from pumping blood effectively. Cardiac output and blood pressure may fall to dangerous levels, which can lead to further coronary ischemia and extension of the infarct.

The cardiac defibrillator is a device that was specifically designed to terminate these potentially fatal arrhythmias. The device works by delivering an electrical shock to the patient in order to depolarize a critical mass of the heart muscle, in effect "rebooting" the heart. This therapy is time dependent, and the odds of successful defibrillation decline rapidly after the onset of cardiopulmonary arrest.

Diagnosis:

The diagnosis of myocardial infarction is made by integrating the history of the presenting illness and physical examination with electrocardiogram findings and cardiac markers (blood tests for heart muscle cell damage). A coronary angiogram allows visualization of narrowings or obstructions on the heart vessels, and therapeutic measures can follow immediately. At autopsy, a pathologist can diagnose a myocardial infarction based on anatomopathological findings.

A chest radiograph and routine blood tests may indicate complications or precipitating causes and are often performed upon arrival to an emergency department. New regional wall motion abnormalities on an echocardiogram are also suggestive of a myocardial infarction. Echo may be performed in equivocal cases by the on-call cardiologist. In stable patients whose symptoms have resolved by the time of evaluation, technetium-99m 2-methoxyisobutylisonitrile (Tc99m MIBI) or thallium-201 chloride can be used in nuclear medicine to visualize areas of reduced blood flow in conjunction with physiologic or pharmocologic stress. Thallium may also be used to determine viability of tissue, distinguishing whether non-functional myocardium is actually dead or merely in a state of hibernation or of being stunned.

Diagnostic criteria:

WHO criteria[50] formulated in 1979 have classically been used to diagnose MI; a patient is diagnosed with myocardial infarction if two (probable) or three (definite) of the following criteria are satisfied:
Clinical history of ischaemic type chest pain lasting for more than 20 minutes Changes in serial ECG tracings Rise and fall of serum cardiac biomarkers such as creatine kinase-MB fraction and troponin The WHO criteria were refined in 2000 to give more prominence to cardiac biomarkers. According to the new guidelines, a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression or coronary intervention are diagnostic of MI.

Physical examination:

The general appearance of patients may vary according to the experienced symptoms; the patient may be comfortable, or restless and in severe distress with an increased respiratory rate. A cool and pale skin is common and points to vasoconstriction. Some patients have low-grade fever (38–39 °C). Blood pressure may be elevated or decreased, and the pulse can be become irregular.

If heart failure ensues, elevated jugular venous pressure and hepatojugular reflux, or swelling of the legs due to peripheral edema may be found on inspection. Rarely, a cardiac bulge with a pace different from the pulse rhythm can be felt on precordial examination. Various abnormalities can be found on auscultation, such as a third and fourth heart sound, systolic murmurs, paradoxical splitting of the second heart sound, a pericardial friction rub and rales over the lung.

Electrocardiogram:

12-lead electrocardiogram showing ST-segment elevation (orange) in I, aVL and V1-V5 with reciprocal changes (blue) in the inferior leads, indicative of an anterior wall myocardial infarction.The primary purpose of the electrocardiogram is to detect ischemia or acute coronary injury in broad, symptomatic emergency department populations. However, the standard 12 lead ECG has several limitations. An ECG represents a brief sample in time. Because unstable ischemic syndromes have rapidly changing supply versus demand characteristics, a single ECG may not accurately represent the entire picture.

It is therefore desirable to obtain serial 12 lead ECGs, particularly if the first ECG is obtained during a pain-free episode. Alternatively, many emergency departments and chest pain centers use computers capable of continuous ST segment monitoring. The standard 12 lead ECG also does not directly examine the right ventricle, and is relatively poor at examining the posterior basal and lateral walls of the left ventricle. In particular, acute myocardial infarction in the distribution of the circumflex artery is likely to produce a nondiagnostic ECG.

The use of additional ECG leads like right-sided leads V3R and V4R and posterior leads V7, V8, and V9 may improve sensitivity for right ventricular and posterior myocardial infarction. In spite of these limitations, the 12 lead ECG stands at the center of risk stratification for the patient with suspected acute myocardial infarction. Mistakes in interpretation are relatively common, and the failure to identify high risk features has a negative effect on the quality of patient care.

The 12 lead ECG is used to classify patients into one of three groups:
those with ST segment elevation or new bundle branch block (suspicious for acute injury and a possible candidate for acute reperfusion therapy with thrombolytics or primary PCI), those with ST segment depression or T wave inversion (suspicious for ischemia), and those with a so-called non-diagnostic or normal ECG. A normal ECG does not rule out acute myocardial infarction. Sometimes the earliest presentation of acute myocardial infarction is the hyperacute T wave, which is treated the same as ST segment elevation. In practice this is rarely seen, because it only exists for 2–30 minutes after the onset of infarction.

Hyperacute T waves need to be distinguished from the peaked T waves associated with hyperkalemia. The current guidelines for the ECG diagnosis of acute myocardial infarction require at least 1 mm (0.1 mV) of ST segment elevation in the limb leads, and at least 2 mm elevation in the precordial leads. These elevations must be present in anatomically contiguous leads. (I, aVL, V5, V6 correspond to the lateral wall; V1-V4 correspond to the anterior wall; II, III, aVF correspond to the inferior wall.) This criterion is problematic, however, as acute myocardial infarction is not the most common cause of ST segment elevation in chest pain patients.

Over 90% of healthy men have at least 1 mm (0.1 mV) of ST segment elevation in at least one precordial lead. The clinician must therefore be well versed in recognizing the so-called ECG mimics of acute myocardial infarction, which include left ventricular hypertrophy, left bundle branch block, paced rhythm, early repolarization, pericarditis, hyperkalemia, and ventricular aneurysm.

Cardiac markers:

Cardiac markers or cardiac enzymes are proteins that leak out of injured myocardial cells through their damaged cell membranes into the bloodstream. Until the 1980s, the enzymes SGOT and LDH were used to assess cardiac injury. Now, the markers most widely used in detection of MI are MB subtype of the enzyme creatine kinase and cardiac troponins T and I as they are more specific for myocardial injury. The cardiac troponins T and I which are released within 4–6 hours of an attack of MI and remain elevated for up to 2 weeks, have nearly complete tissue specificity and are now the preferred markers for asssessing myocardial damage. Elevated troponins in the setting of chest pain may accurately predict a high likelihood of a myocardial infarction in the near future. New markers such as glycogen phosphorylase isoenzyme BB are under investigation.

The diagnosis of myocardial infarction requires two out of three components (history, ECG, and enzymes). When damage to the heart occurs, levels of cardiac markers rise over time, which is why blood tests for them are taken over a 24-hour period. Because these enzyme levels are not elevated immediately following a heart attack, patients presenting with chest pain are generally treated with the assumption that a myocardial infarction has occurred and then evaluated for a more precise diagnosis.