Alternating Current AC and Direct Current DC a Simple Explanation


The simplest explanation would be dc current flows in one direction. Alternating current flows first one way then the other. But this really doesn’t teach us anything. If we are going to spend time looking at how this comes about. I think that will still be a simple explanation.

Let’s start with direct current (dc) since it is the simplest. This is the current supplied by batteries. Take a storage battery like the one that starts your car. It has two terminals, one positive and one negative. There is an excess of electrons (carries a negative charge) at the negative terminal. There is a lack of electrons at the positive terminal. Electric current, of course is a flow of electrons. So if we connect the two terminals with a wire or cable, there will be a flow of electrons from the negative terminal through the wire to the positive terminal. Never hook it up that way however. The wire, being of very low resistance, will heat up. Run the current through a load, such as a heater, or coil anything that consumes power. These things have resistance and will limit the magnitude of the current.

Close the switch and the current will increase to a maximum that is dependent upon the voltage of the battery, the internal resistance of the battery and the resistance of the wire and whatever load it supplies. It will flow steadily in one direction, from the negative pole to the positive pole. As the battery charge is reduced, the current decreases, but always in the same direction.

Moving on to alternating current (ac), we find this is a little more complex. As the name implies, the current (same flow of electrons) flows first in one direction, building up to a maximum. It then decreases to zero and still flowing in the same direction. At zero the direction of the current reverses, flowing to a maximum in the opposite direction then decreases again to zero.

To go further in our understanding of the alternating current, we must take a look at magnetism. The Danish scientist Oersted found the connection between electricity and magnetism. Consider 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, the north pole would swing roughly toward earth’s north. Taking a small compass and moving it around the bar magnet, we see where the needle points in different directions depending on its position. With some attention we could map the area around the bar magnet. This is called the 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 map of the same field.

The direction of the magnetic field is represented by magnetic flux lines. The direction of the flux lines was 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. 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 the same compass, in an exploring mode, 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 another when held below it. Also the famous right-hand rule will 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 field lines again. We find that the flux lines inside the coil are crowded together. The amount of flux has not changed from the single wire, but it is concentrated into a small area. The intensity of the field inside the loop has therefore increased. The direction of the lines of flux in each loop is the same inside and outside the coil. They are opposed between the individual turns 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. We find we have an electric coil with current creating magnetic flux and are now the same as the bar magnet we started with. You could prove this by mapping the field with your compass.

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 starting and stopping 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 the 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 then zero. Rotate the armature slightly clockwise and the wire begins to cut flux lines, diagonally. An emf will be induced in the conductor in proportion to the rate the lines are cut, starting the current 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 break through by Charles Steinmetz that allows analysis of electricity on paper. Trust me, this was a big deal and opened up the door to all sorts of research. 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 between 12, 3, and 6 will flow opposite that between 6, 9, and 12. This change in direction is caused by the reversal in the relative direction of conductor movement. The resulting curve will be a sine wave.

Looking at the curve we recognize alternating current. The current flows from zero to a maximum and down to zero in one direction. Passing the time(angle) axis it increases in a negative or reverse direction, increasing to a maximum decreasing to zero. The cycle is repeated over again.

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. Usually the poles have windings on them and the magnet field referred to as field excitation is provided separately.

In the United States, 60 cycles (hertz) is the standard and in other countries 50 cycles are standard. A cycle is from one zero point, current increasing to the next zero point where the current is again increasing.

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.