Electrotechnics: Basics


Voltage. Electric potential difference

One of the most commonly used in electrical characteristics - is voltage. Very often, even for experienced professionals in the electrical causes difficulty to explain that there is the most voltage. It is understandable that, for practical needs of electric service is not required strong understanding of voltage, voltage enough knowledge within the meaning of Ohm's law.

The question then arises. In any case and under any circumstances must be a full understanding of the essence of the voltage? First of all it is necessary to understand the nature (physics) of electricity, as well as for the development of new electrical devices and new electrical materials. On the other hand, a full understanding of voltage contributes to self-knowledge.

Mental experiment with a flat capacitor

To go to the explanation of the essence of the voltage needed to understand what the electric field, electric field lines and the electric field intensity.

Also the lines of force in the description fields are present also equipotential lines, and thus there is another characteristic such as the potential of the electric field. Imagine the scene uniformly distributed electric field lines that intersect the equipotential lines, each such line will have its value field potential. For convenient use of such a representation of the electric field pattern flat capacitor having two plates and a fully charged up to a certain maximum value. In such a capacitor is induced an electrical charge, and the space between the plates so be filled with dielectric gas such as air. Each side of capacitor has a certain amount of charge Q. Since capacitor plates are made of metal in which the charge carriers are negative type charges - electrons on the one plate will be an excess of electrons and another drawback. Thus, you can designate one as the electrode is +Q, and the other as -Q, and the electric field lines will be directed according to the rules of +Q to -Q. As a result, we get a picture of the image below.

Mental experiment with a capacitor

Let's assume that the size of the capacitor greater human growth several times, let the plates will be a wall of two large high buildings, which are pasted metal sheets welded together into a single sheet. You can move freely within this capacitor from one electrode to another in any direction. Mentally, you can imagine where the field lines are shown, one fixed beam of dry wood, and on the ground potential lines set of stairs from the same material. As a result, you will be able to move freely in a space inside the capacitor. If you have the power of imagination, you can imagine such a structure without any problems. The size can be any, provided that the length and height of the plates is many times greater than the distance between the electrodes.

The electric field of the capacitor is fully charged in this case is static, that is constant in time, its characteristics do not change over time. What we have? We have two plates have a certain amount of charge of equal magnitude but opposite sign. These plates are attracted to each other according to Coulomb's law, but the electric force is compensated by the fact that the plates are firmly fixed on the walls of imaginary buildings. The picture of the electric field of the capacitor is represented by force and equipotential lines which are designated tangible objects such as wooden beams and stairs. You can travel freely within such a capacitor, and perform the necessary measurements. Neither of which an electric current, and even more so about the strength of the current speech here is not because there are no free charge carriers.

An experienced electrician can ask, and what will be the voltage on this capacitor? It is logical and fair question, but we should understand that this is the most voltage. Here we should recall the probe charge, which was used to explain the intensity of the electric field. Assume that such a charge and appeared to be freely movable in the space between the capacitor plates. What can it be? Imagine that you are the most probe charge and endure the long-range electrical forces. Of course, in real life this is unlikely, but in our imaginary experiment is quite acceptable.

Physical work probe charge in an electric field

So you become mentally probe electric charge q is much smaller than the charge Q on the capacitor plates and began their journey between the capacitor plates. Here you will experience the action of Coulomb forces. Let's say that you are the negatively charged particles like the electron, then you will attract to the side plates +Q, and you will repel from the plate with a charge -Q. The closer you are to one of the plates, the more you will feel the force of its action.

Suppose you went to the condenser side plate -Q and you immediately started to push away from her toward the plate +Q. You did not resist such an attack and decided not to oppose nature and move in complete agreement with the attraction. For these purposes are conveniently located just beams and staircases, which you can freely reach the plate +Q by any route. Since you operate electric Coulomb force, then you are free to start to pick up speed, as if the wind carries you. As a result, you have overcome the distance of the beam from one ladder to another in the direction from point A to point B (see figure above). Stairs - is equipotential lines, and thus, you have to overcome the distance from one value to another potential. In this case, you are moved by the potential that for you at a higher value to the fact that less. If you have to charge the other sign, ie +q, then the potential would have changed their signs and more would become less and less is more. Mathematically, this means multiplying the potential to -1.

You acted on force and the you have moved from point A to point B, in other words, you move from the potential φa (more) to the potential φb (smaller). It is just as if you were floating down the river on a raft, when you do not have oars to paddle and do not require a motor to move. You can say that you committed the mechanical work, which is calculated as the product of force over a distance. Having performed such a move, you lose part of the potential energy, which is transferred into kinetic (speed of your movement), and then select probably in the form of heat during braking. Having done the way back from point B to point A, you will move as if against the current, you will have to expend energy, rowing oars, used motor, etc. Moving back to you increase its potential energy, because the move to a point with great potential and able to increase your energy.

The difference between these two potentials φa and φb and will be an electric voltage. This is equivalent to the concept, but in practice often use electrical expression is a potential difference, and voltage. When considering the use of electrical circuits such as the expression of the voltage drop on the circuit, and a source of electricity for the same potential difference is defined as the electromotive force (EMF).

equipotential surfaces

The potential difference Δφ = φ12 always shows what work can make a carrier A charge q moving at this point charge from one point to the potential φ1 another potential φ2. In the calculation it is necessary to bear in mind that potentials can be either with a plus or minus sign.

Potential of electrostatic field

If charging for such a movement energy must be expended, and thus increase its capacity, then the work of A is the sign (-), and if the charge carrier moves from the high potential to the low potential, then the energy is released and the work of A will be with plus sign (+). Thus, the voltage - a energetic characteristic of the electric field and a potential difference Δφ. This means that it is wrong to assert that voltage - is the potential. electrical voltage - it is always the potential difference, and it is only possible between two points of an electric field. If there is one point in the space of an electric field, then only appropriate to talk about the potential of this point, but certainly not its electrical voltage.

It should be perfectly clear idea of what are the differences between concepts such as: the electric field intensity E, potential φ, and, of course, the potential difference - Voltage. Understanding these differences will be quite easy to understand that such an electric current.

Units of measurement voltage

Just like the electric field potential, the voltage is measured in volts and is often denoted by a symbol U, a symbol V. What is the voltage of 1 volt? It is equal to the mechanical work of 1 joule, which makes a charge of 1 Coulomb. Thus, if the potential difference is for example 12 volts, and the difference (the equipotential lines and surfaces) overcame charge, for example a 2-Coulomb, it should be said that work of was made 24 Joule (12 volts multiplied by 2 Coulomb).

Units of measurement voltage

When in electrical circuits there are an electric current, then there is a movement of charge carriers along the field lines of the electric field (direction depends on the sign), the source of which can be electric or chemical battery, then parts of the circuit voltage drop occurs (potential), and energy is released. The source current is the opposite, where energy is expended (or was spent) to create EMF.

Date: 01.30.2016

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© Valentin Grigoryev


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