Only a few are capable of realizing that alternating current and direct current are somewhat different. Not to mention specific differences. The purpose of this article is to explain the basic characteristics of these physical quantities in terms that are understandable to people without baggage of technical knowledge, and also provide some basic concepts related to this issue.
Visualization difficulties
Most people find it easy to deal with concepts such as “pressure,” “quantity,” and “flow,” because they are constantly confronted with them in their daily lives. For example, it is easy to understand that an increase in flow when watering flowers will increase the amount of water leaving the watering hose, while an increase in water pressure will make it move faster and with more force.
Electrical terms, such as “voltage” and “current”, are usually difficult to understand because you cannot see or feel electricity moving through cables and electrical circuits. Even a novice electrician is extremely difficult to visualize what is happening at the molecular level or even to clearly understand what an electron is, for example. This particle is outside the limits of human sensory capabilities, it is impossible to see and cannot be touched, except when a certain amount of them will not pass through the human body. Only then will the victim definitely feel them and experience what is usually called an electric shock.
However, the open cables and wires to most people seem completely harmless only because they cannot see the electrons, just waiting to take the path of least resistance, which is usually the earth.
Analogy
It is clear why most people cannot visualize what is happening inside conventional conductors and cables. An attempt to explain that something is moving through the metal runs counter to common sense. At the most basic level, electricity is not so different from water, so its basic concepts are quite easy to master if you compare the electric circuit with the water supply system. The main difference between water and electricity is that the first fills something if it manages to break out of the pipe, while the second needs a conductor to move the electrons. By visualizing a pipe system, it is easier for most to understand the special terminology.
Voltage as pressure
The voltage is very similar to the pressure of electrons and indicates how fast and with what force they move through the conductor. These physical quantities are equivalent in many ways, including their relation to the strength of the pipeline-cable. Just as too much pressure breaks the pipe, too high a voltage destroys or shields the conductor.
Current as flow
A current is a flow of electrons indicating how many are moving along the cable. The higher it is, the more electrons pass through the conductor. Just as large amounts of water require thicker pipes, higher currents require thicker cables.
The use of the water circuit model allows us to explain many other terms. For example, power generators can be represented as water pumps, and the electric load as a water mill, for the rotation of which requires the flow and pressure of water. Even electronic diodes can be thought of as water valves that allow water to flow in only one direction.
D.C
What is the difference between direct and alternating current, it becomes clear from the name. The first is the movement of electrons in one direction. It is very simple to visualize it using a water circuit model. It is enough to imagine that water flows through the pipe in one direction. Common DC devices are solar cells, batteries, and dynamos. Almost any device can be designed to be powered by such a source. This is almost the exclusive prerogative of low-voltage and portable electronics.
The direct current is quite simple, and obeys Ohm's law: U = I × R. The load power is measured in watts and is equal to: P = U × I.
Due to simple equations and behavior, direct current is relatively easy to comprehend. The first power transmission systems developed by Thomas Edison back in the 19th century used it only. However, soon the difference in alternating current and constant became apparent. The transmission of the latter over considerable distances was accompanied by heavy losses, so after several decades it was replaced by a more profitable (then) system developed by Nikola Tesla.
Despite the fact that commercial power networks all over the planet currently use alternating current, the irony is that the development of technology has made the transmission of high voltage direct current at very large distances and at extreme loads more efficient. Which, for example, is used when connecting separate systems, such as entire countries or even continents. This is another difference in alternating current and direct current. However, the former is still used in low-voltage commercial networks.
Direct and alternating current: difference in production and use
If alternating current is much easier to produce using a generator using kinetic energy, then batteries can only create direct current. Therefore, the latter dominates the power supply circuits of low-voltage devices and electronics. Batteries can only be charged with direct current, so AC mains is rectified when the battery is the main part of the system.
A widespread example is any vehicle — a motorcycle, car, or truck. The generator installed on them creates an alternating current, which is instantly converted into direct current using a rectifier, since there is a battery in the power supply system, and most electronics require a constant voltage to operate. Solar cells and fuel cells also produce only direct current, which can then be converted to AC using a device called an inverter, if necessary.
Direction of travel
This is another example of the difference between DC and AC. As the name implies, the latter is a stream of electrons that constantly changes direction. Since the end of the 19th century, sinusoidal alternating current has been used in almost all household and industrial electric systems around the world, since it is easier to receive and much cheaper to distribute, with the exception of very few cases of long-distance transmission, when power losses force the use of the latest high-voltage DC systems.
AC has another big advantage: it allows you to return energy from the point of consumption back to the network. This is very beneficial in buildings and structures that produce more energy than they consume, which is quite possible when using alternative sources, such as solar panels and wind turbines. The fact that alternating current provides a bi-directional flow of energy is the main reason for the popularity and availability of alternative power supplies.
Frequency
When it comes to the technical level, unfortunately, it is difficult to explain how alternating current works, since the water circuit model does not quite fit it. However, it is possible to visualize a system in which water quickly changes the direction of flow, although it is not clear how it will do something useful. Alternating current and voltage constantly change their direction. The rate of change depends on the frequency (measured in hertz) and for household electrical networks it is usually 50 Hz. This means that voltage and current change direction 50 times per second. Calculating the active component in sinusoidal systems is quite simple. It is enough to divide their peak value by √2.
When alternating current changes direction 50 times per second, this means that incandescent lamps turn on and off 50 times per second. The human eye cannot notice this, and the brain simply believes that lighting works constantly. This is another difference in alternating current and direct current.
Vector math
Current and voltage not only constantly change - their phases do not coincide (they are unsynchronized). The vast majority of AC power loads cause phase differences. This means that even for the simplest calculations, vector mathematics must be applied. When working with vectors, it is impossible to simply add, subtract or perform any other scalar math operations. With direct current, if 5A is supplied to one point through one cable and 2A to another, then the result is 7A. In the case of a variable, this is not so, because the result will depend on the direction of the vectors.
Power factor
The active load power powered by the AC mains can be calculated using the simple formula P = U × I × cos (φ), where φ is the angle between voltage and current, cos (φ) is also called the power factor. This is what distinguishes direct and alternating current: the first cos (φ) is always 1. Active power is needed (and paid for) by household and industrial consumers, but it is not equal to the complex power passing through the conductors (cables) to the load, which can be calculated by the formula S = U × I and measured in volt-amperes (VA).
The difference between direct and alternating current in the calculations is obvious - they become more complex. Even the simplest calculations require at least a mediocre knowledge of vector math.
Welders
The difference between direct and alternating current is also manifested in welding. The polarity of the arc has a great influence on its quality. Positive electrode penetrates deeper than negative electrode, but the latter accelerates metal deposition. With direct current, the polarity is always constant. With a variable, it changes 100 times per second (at 50 Hz). Welding at constant is preferable, as it is performed more evenly. The difference in welding with alternating and direct current is that in the first case, the movement of electrons is interrupted for a split second, which leads to ripple, instability and arc failure. This type of welding is rarely used, for example, to eliminate the wandering of the arc in the case of electrodes of large diameter.