How is the voltage measured? Unit of measurement of electrical voltage

Is it possible to imagine life without electricity? Modern man tightly surrounded himself with household appliances that help in life. We can no longer imagine ourselves and our lives without intelligent domestic helpers.

In technology, more often switch to the use of electricity. Even transport is gradually transferred to electric motors, which allows to reduce the significant harm to nature.

Today we will try to answer the following questions:

• What is an electric current?
• What is electrical voltage?
• How to determine the voltage?
• How is the voltage measured?

What is a current?

At the dawn of the study of electricity, it was obtained by rubbing one body about another. A larger reserve of charge could be obtained during a thunderstorm, using a natural discharge - a lightning bolt. It is known that this method cost a lifetime to the student of MV Lomonosov, Richter.

The charge itself is difficult to use and irrational. It is necessary to obtain its directional motion - electric current. Current properties:

• Heating conductor;
• Chemical action;
• Mechanical action;
• Magnetic action.

They are used in everyday life and technology. A necessary condition for the existence of a current is the presence of a current source, free electric charges and a closed conductor.

Background

In 1792, the famous Italian physicist, physiologist and inventor Alessandro Volta became interested in the findings of compatriot Luigi Galvani about the nature of the current pulses in the organs of animals. A long observation of the behavior of the feet of frogs fixed on metal hooks allowed him to conclude that the source of electricity is not a living organism, but a contact of dissimilar metals. It is this circumstance that contributes to the flow of electricity, and the reaction of the nerve endings is only the physiological action of the current.

A unique discovery led to the creation of the world's first direct current source, known as the "Volta Pillar". Heterogeneous metals (Volta argued that they should be removed from each other in a series of chemical elements) are paved with paper impregnated with a liquid "conductor of the second kind."

This device became the first source of constant voltage. The unit for measuring the electrical voltage perpetuated the name Alessandro Volta.

DC Power Supply

The main element of the electrical circuit is the current source. Its purpose is to create an electric field, under the action of which free charged particles (electrons, ions) come in directional motion. The charges accumulated on separate elements of the source (they are called poles) have different signs. The charge itself is redistributed inside the source under the action of forces of non-electrical nature (mechanical, chemical, magnetic, thermal, etc.). An electric field created by poles outside the current source produces work on the movement of charge in a closed conductor. Alessandro Volta spoke of the need for a closed circuit to create a constant current.

Since in the sources under the action of forces of non-electric nature the charge moves, it means that it can be asserted that these forces are doing the work. Let's call them third-party. The ratio of the work of external forces on the transfer of charge inside the current source to the magnitude of the charge is called the electromotive force.

The mathematical notation of this relation:

• Ε = А _{ст ст:} q,

Where E is the electromotive force (EMF), A _st is the work of external forces, q is the charge carried by external forces in the source.

EMF characterizes the ability of the source to create a current, but the main characteristic of the source is sometimes considered to be the electrical voltage (potential difference).

Voltage

The ratio of the field work on the movement of charge in the conductor to the charge value was called the electric voltage.

To determine it, you need to divide the field work by the charge value. Let A be the work accomplished by the electric field of the current source in charge displacement q. U is the electrical voltage. The mathematical notation of the corresponding formula:

• U = A: q.

Like any physical quantity, the voltage has a unit of measurement. How is the voltage measured? According to the name of the inventor of the world's first constant current source Alessandro Volta, this magnitude was given its own unit of measurement. In an international system, the voltage is measured in volts (V).

A voltage of 1 V is the voltage of the electric field, which performs work in 1 J for the charge transfer in 1 Cl.

• B = J / K = N • m / (A • c) = kg • m / (A • c ³ ).

In the basic units of the SI system, the unit for measuring the electrical voltage:

• Kg • m / (A • s ³ ).

Required value

Why is it not enough, when describing the current, to introduce the concept of current strength? Let's conduct a mental experiment. Let's take two different lamps: an ordinary household lamp and a lamp from a flashlight. When connecting them to different sources of current (city network and battery), you can get an absolutely identical value of the current. In this case, the household lamp gives more light, that is, the work of the current in it is much greater.

Different sources of current have different voltages. Therefore, this quantity is extremely necessary.

A useful analogy

Understanding the physical meaning of an electrical voltage comes when you look at an interesting analogy. In communicating vessels, liquid flows from the tube into the tube if there is a pressure difference in them. The liquid flow stops when the pressures are equal.

If the fluid current is compared with the flow of electric charge, then the pressure difference of the liquid columns plays the same role as the potential difference in the current source.

As long as the processes accompanying the redistribution of the charge at the poles occur inside the current source, it is capable of creating a current in the conductor. The voltage of the electric current is measured in volts, the pressure difference has a unit of measurement - pascal.

Alternating current

The electric current, periodically changing its direction, is called a variable. It is created by a source of an alternating voltage. Most often it's a generator. Let's try to explain: in what is measured the AC voltage?

The very principle of current generation is based on the phenomenon of electromagnetic induction. Rotation of a closed loop in a magnetic field leads to the appearance of a potential difference in the conductor. Voltage is measured in volts and in the case of a varying current.

Can we say that the voltage does not change? Obviously, due to a change in the angle between the plane of the contour and the normal to it, the created voltage changes with time. Its value rises from zero to some maximum value, then drops again to zero. Talk about a certain amount is not necessary. Enter the so-called effective voltage value:

• U _d = U: √2.

What device measures voltage?

Device for measuring electrical voltage - voltmeter. The principle of its operation is based on the interaction of the circuit with the current and the magnetic field of a permanent magnet. It is known that the current loop rotates in a magnetic field. Depending on the magnitude of the current in the circuit, the angle of rotation changes.

If the arrow is attached to the contour, it deviates from the zero value when the current flows in the circuit (more often the coil). Depending on how the voltage is measured, the scale of the instrument is calibrated. It is possible to use lobular and multiple values.

In the case of low values, the electrical voltage is measured in millivolts or microvolts. On the contrary, multiple units are used in high-voltage networks.

Any voltmeter is connected in parallel to the part of the circuit on which the voltage is measured. The main property of the device circuit can be called high ohmic resistance. The voltmeter, no matter what the voltage is measured, should not affect the current in the circuit. A small current is passed through it, which does not significantly affect the fundamental value.

Stress table

Physical device	Voltage at its contacts, V
Volt post	1.1
Lantern battery	1.5
Alkaline battery	1.25
Acid battery	2
City network	220
High-voltage transmission lines	500 000
Between the clouds in a thunderstorm	Up to 100 000 000

Practical application of the voltmeter

To use the voltmeter effectively, you should learn how to use it. A curious experimenter can be advised to apply to school teachers.

School physics classrooms are equipped with laboratory and demonstration instruments for measuring voltages.

Operate any voltmeter with caution, following simple rules:

1. The voltmeter has a maximum measurement limit. This is the highest value on its scale. Do not connect it to a circuit containing a higher voltage element.
2. If there is no other source or voltmeter, you can use a system of additional resistances. In this case, the scale of the voltmeter should also be changed.
3. In the DC circuit, electrical appliances are connected depending on the indication of the charge sign on its terminals. The positive terminal of the current source must be connected to the positive terminal of the voltmeter, the negative terminal to the negative one. If to confuse, the hands of the device can bend, which is highly undesirable.
4. All connections are made exclusively to the de-energized circuit.

Unhealthy

The effect of an electric current can be unsafe for a person. A voltage less than 24 V is considered harmless.

The effect of the current under the voltage of the city network (220 V) is quite noticeable. Touching exposed contacts is accompanied by a significant "current shock".

Voltage during a thunderstorm passes such a high current through the human body, which threatens him with a fatal outcome. Do not risk your life and health.