HOW ELECTRICITY IS GENERATED
Electricity is a form of energy. It can’t be created or destroyed. It can only be converted from or to another form of energy. So let’s explore where the energy comes from.
Heat energy can be produced from the splitting of atoms in a nuclear reactor. Heat can also be produced by burning wood, coal, oil or gas. These sources of heat can be used to raise the temperature of water, and if enough heat is applied, we get steam. This steam and the pressure it creates may be used to turn machines called turbines. The heat energy has become mechanical energy. The turbines will then turn other machines called generators. It is in this last machine, the generator, in which the mechanical energy is converted to electricity.
Another form of energy that works without using heat is potential energy. An example of potential energy is the water in a lake behind a dam. This form of storage is used at a hydroelectric plant, where the water stored in the lake will be released through special channels in the dam. The falling water impacts turbine blades, releasing its potential energy, converting it to mechanical energy. The turbine then turns a generator, converting mechanical energy into electric energy (generating alternating current electricity).
To go further in our understanding of how to generate electricity, we must take a look at magnetism. Position a bar magnet with the North Pole on the left and the South Pole on the right. The poles were so named because if you suspended the bar in the center, the North Pole would swing roughly toward earth’s North Pole and the South Pole toward the earth’s South Pole. Take a small compass and move it about the bar magnet. We see that the needle points in different directions depending on its position. If we map the area around the bar magnet, indicating the directions our compass gives, we have a picture of a magnetic field. This field could also be mapped by placing a thin sheet of cardboard above the bar. Sprinkle a uniform covering of iron filings and then tap the sheet. The filings will arrange themselves into a field map quite similar to the first one.
The magnetic field is represented by magnetic flux lines. The direction of the flux lines were determined by your small compass. Note that the flux lines exits the north pole of the bar magnetic and reenters the South Pole. The strength of the field is greatest adjacent to the bar magnet. Its strength will vary inversely as the square of the distance from the pole. It finally reaches a position of no measurable strength some distance from the bar. In your map you can see where the field lines go out from the North Pole and fade as they have to travel farther and farther to get to the South Pole.
Take an insulated wire and run a current through it. Using our compass, and explore the area about the wire, we find that the field lines take the form of concentric circles around the wire and at right angles to it. The direction of the field can also be determined by the compass. It points one way when held above the wire and in a directly opposite direction when held below it. Also the famous right-hand rule can be used to point the way. Pretend to grasp the wire with the right hand thumb pointing in the direction of the flow of current (electron flow) and the fingers will encircle the wire in the direction of the field.
Form the wire into a coil with several turns. Explore the magnetic flux lines again. We find that the lines inside the coil are crowded together. The amount of flux has not changed from the single wire, but it is concentrated into a smaller area. The intensity of the field inside the loop has therefore increased (lines per unit area). The direction of the lines of flux in each loop is the same inside and outside the coil. The lines of flux between adjacent turns are opposed and cancel out.
Each flux line originates in each turn, in the same direction inside the coil. It exits from one end of the coil and reenters at the other end. An examination shows that they exit at the north pole of the magnetic coil. If you take time to map the flux field, we find we have an electric coil with current creating magnetic flux that is now the same as the bar magnet we started with.
An electric current has produced magnetism and we will now see where magnetism will produce an electric current. Michael Faraday, a British Physicist who was self taught, discovered this phenomenon in 1831. As an illustration of how this works, let us take a bar magnet of an appropriate size. Shape it into a square so that there is a gap between the north and South Pole on the bottom. Now instead of flux lines leaving the North Pole and spreading out as it did when the bar was straight, the flux lines go directly from the North Pole across the gap to the South Pole.
Cut the faces of the two poles so that they are curved and allow space for an armature shaped as a cylinder. In actual construction there would be a small gap between the pole faces and the armature. An armature is simply the structure that supports the conductors which cut the magnetic field, and carry the induced current. Support the armature cylinder on an axis through the center. In a real generator, the armature would be covered by a layer of conductors connected so that the two ends of the armature circuit come out to a pair of insulated rings mounted on the axis.
Picture an end view of this arrangement showing the North Pole on the left and the South Pole on the right. The armature is mounted between them fully immersed in the magnetic field. Allow it to rotate clockwise. Place one wire on the armature, forming a loop around the armature, end to end and parallel with the armature axis. Starting and stopping the wire on each of the insulated rings (slip rings). Using only one wire will greatly aid in showing what happens. Position it on the bottom of the armature at 6 o’clock. In this place it is traveling in a direction opposite the magnetic field and moving parallel to the flux lines. Therefore no flux lines are being cut. No voltage is induced, so the current is zero. Rotate the armature slightly clockwise and the wire begins to cut flux lines, diagonally. An emf (electromotive force or voltage) will be induced in the conductor in proportion to the rate the lines are cut and the angle at which the conductor intersects the flux lines. The current will begin to flow. Continue around the axis to 9 o’clock. The wire is now moving at right angles to the flux lines and a maximum emf is induced. Continue around clockwise to 12 o’clock. Now there are no lines being cut and the current is zero as at 6 o’clock. As the wire passes 12 o’clock, the direction of the emf and current is reversed. Previously the wire was cutting the flux lines in an upward direction. Now the wire is cutting lines in a downward direction. Going further to 3 o’clock and we have maximum lines cut. The wire is moving perpendicular to the flux lines.
If we plot the ac current verses a time axis or an angle axis (angle around the armature) we will see a sine wave. We won’t get into mathematics in this article, but a sine wave is a trigonometric function that allows us to express the antics of ac current in math equations. This was a tremendous breakthrough by Charles Steinmetz that allows analysis of electricity on paper. If you tie a rope to the barn and yank it up and down so that waves travel back and forth along the rope, you will see sine waves generated.
Note that we find zero current at 6 and 12. At 3 and 9 there are a maximum number of lines cut perpendicularly. Current at 12, 3, and 6 will flow opposite that at 6, 9, and 12. This change in direction is caused by the reversal in the relative direction of conductor movement.
The data in the previous paragraph can be plotted so that we recognize alternating current. The current flows from zero to a maximum and down to zero in one direction. Passing through the time (angle) axis it increases in a negative or reverse direction, increasing to a maximum then decreasing to zero. This is one complete cycle that is repeated over and over again. In the United States, 60 cycles (hertz) is the standard and in other countries 50 cycles are standard.
The energy being converted into electricity by the generator comes from the mechanical energy that is turning the armature. The bar magnet described above is referred to as the poles. In most applications, especially in large power plants, the poles will not be magnetic and will have windings on them. The magnet field is referred to as field excitation and can be provided separately from several possible sources.
When a magnetic flux and a conductor move relative to each other and the flux lines intersect the conductor, a voltage is induced in it. A flow of current will then follow. Faraday took this principle and placed the conductor on an iron core, thereby increasing the flux. The iron in the coil core greatly increases the magnetic paths ability to conduct the flux. He placed a similar coil on another iron core, with all the wiring insulated. When the two coils are placed in close proximity we can have a voltage transformed from the first coil to the second. By varying the number of turns on the second coil, the secondary voltage can be increased or decreased. At the same time the current will be reduced when the voltage increases. The current will increase when the voltage decreases. When the secondary voltage is reduced we have a step-down transformer. When it is stepped up we have a step-up transformer.
The alternating current and the transformer are the essential components that make the present power system across this country possible. With the tremendous versatility of this combination the possibilities are limitless.
http://helium.com/items/1091664/how-electrical-current-flows
http://helium.com/items/1036892/how-we-get-electric-power
