Is it possible to imagine your life without electricity? Modern man has tightly surrounded himself with household appliances that help in life. We can no longer imagine ourselves and our lives without smart home assistants.
Technology is increasingly switching to the use of electricity. Even transport is gradually being transferred to electric motors, which can reduce significant harm to nature.
Today we will try to answer the following questions:
- What is electric current?
- What is electrical voltage?
- How to determine the voltage?
- What is voltage measured in?
What is current?
At the dawn of the study of electricity, it was obtained by rubbing one body against another. A larger supply of charge could be obtained during a thunderstorm using a natural discharge - lightning. It is known that this method cost the life of a student of M. V. Lomonosov - Richter.
The charge itself is difficult and irrational to use. It is necessary to obtain its directed movement - an electric current. Current properties:
- heating the conductor;
- chemical action;
- mechanical action;
- magnetic action.
They are used in everyday life and technology. A necessary condition for the existence of 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 conclusions of his compatriot Luigi Galvani about the nature of current impulses in animal organs. Long-term observation of the behavior of frog legs, fixed on metal hooks, allowed him to conclude that the source of electricity is not a living organism, but the contact of dissimilar metals. It is this circumstance that contributes to the flow of electricity, and the reaction of nerve endings is only a physiological effect of the current.
A unique discovery led to the creation of the world's first direct current source, called the Voltaic Pillar. Dissimilar metals (Volta argued that they should be removed from each other in a series of chemical elements) are laid with paper impregnated with a liquid "conductor of the second kind."
This device was the first source of constant voltage. The unit of electrical voltage immortalized the name of Alessandro Volta.
DC power supply
The main element of the electrical circuit is a current source. Its purpose is to create an electric field, under the influence of which free charged particles (electrons, ions) come into directed motion. Accumulated onindividual elements of the source charges (they are called poles) have different signs. The charge itself is redistributed inside the source under the action of forces of a non-electric nature (mechanical, chemical, magnetic, thermal, and so on). The electric field created by the poles outside the current source does the work of moving the charge in a closed conductor. Alessandro Volta spoke about the need for a closed circuit to create direct current.
Since the charge moves in sources under the action of non-electric forces, it can be argued that these forces do work. Let's call them outsiders. The ratio of the work of external forces to transfer charge inside the current source to the magnitude of the charge is called the electromotive force.
Mathematical notation for this ratio:
- Ε=Ast: q,
where E is the electromotive force (EMF), Astis the work of external forces, q is the charge carried by external forces in the source.
EMF characterizes the ability of a source to create a current, but the main characteristic of a source is sometimes considered an electrical voltage (potential difference).
Voltage
The ratio of the work of the field to move the charge in the conductor to the magnitude of the charge is called electric voltage.
To determine it, you need to divide the field work value by the charge value. Let A be the work done by the electric field of the current source to move the charge q. U - electrical voltage. Mathematical notation of the corresponding formula:
U=A: q
Like any physical quantity, voltage has a unit of measure. How is voltage measured? By the name of the inventor of the world's first direct current source, Alessandro Volta, this value was given its own unit of measurement. In the international system, voltage is measured in volts (V).
A voltage of 1 V is the voltage of an electric field that does work of 1 J to move a charge of 1 C.
V=J/C=N•m/(A•s)=kg•m/(A•s3).
In basic SI units, the unit of electrical voltage:
kg•m/(A•s3).
Required value
Why is it not enough, characterizing the current, to introduce the concept of current strength? Let's do a thought experiment. Let's take two different lamps: an ordinary household lamp and a lamp from a flashlight. When connecting them to different current sources (city network and battery), you can get exactly the same current value. At the same time, a household lamp gives more light, that is, the work of the current in it is much greater.
Different current sources have different voltages. Therefore, this value is essential.
Useful analogy
Understanding the physical meaning of electrical voltage comes from an interesting analogy. In communicating vessels, fluid flows from tube to tube if there is a pressure difference in them. The fluid flow stops in case of equalitypressure.
If the liquid 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 charge on the poles take place inside the current source, it is able to create a current in the conductor. Electric current voltage is measured in volts, the pressure difference has a unit of measurement - pascal.
Alternating current
Electric current, periodically changing its direction, is called variable. It is created by an alternating voltage source. Most often it is a generator. Let's try to explain: what is the measurement of AC voltage?
The very principle of current generation is based on the phenomenon of electromagnetic induction. The rotation of a closed circuit 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 varying current.
Can it be argued 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 generated voltage changes over time. Its value grows from zero to some maximum value, then falls again to zero. There is no need to talk about a certain value. Enter the so-called effective voltage value:
- Ud=U: √2.
What instrument measures voltage?
A device for measuring electrical voltage - a voltmeter. The principle of its operation is based on the interaction of a circuit with current and a magnetic fieldpermanent magnet. It is known that a circuit with current rotates in a magnetic field. Depending on the amount of current in the circuit, the angle of rotation changes.
If you attach an arrow to the circuit, then it deviates from zero when current flows in the circuit (usually the coil). Depending on what the voltage is measured in, the scale of the device is graduated. It is possible to use submultiples and multiples.
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 that part of the circuit where the voltage is measured. The main property of the device circuit can be called high ohmic resistance. A voltmeter, no matter what the voltage is measured in, should not affect the current strength in the circuit. A small current is passed through it, which does not significantly affect the main value.
Voltage table
| Physical device | Voltage on its contacts, V |
| Volt pole | 1, 1 |
| Flashlight battery | 1, 5 |
| Alkaline battery | 1, 25 |
| Acid battery | 2 |
| City network | 220 |
| High voltagepower lines | 500,000 |
| Between the clouds in a thunderstorm | Up to 100,000,000 |
Practical application of a voltmeter
To use the voltmeter effectively, you should learn how to use it. A curious experimenter can be advised to contact school teachers.
School physics classrooms are equipped with laboratory and demonstration instruments for measuring stresses.
Operate any voltmeter with caution, following simple rules:
- The voltmeter has a maximum measurement limit. This is the highest value on his scale. Do not connect it to a circuit containing a higher voltage element.
- If there is no other source or voltmeter, you can use a system of additional resistance. In this case, the scale of the voltmeter must also be changed.
- Electrical appliances are connected to the DC circuit depending on the indications 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 terminal. If mixed up, the arrows of the device may bend, which is highly undesirable.
- All connections are made exclusively to a de-energized circuit.
Unhe althy
The action of electric current may be unsafe for humans. Less than 24 V is considered harmless.
The action of the current under the voltage of the city network (220 V) is quite noticeable. Touching bare contacts is accompanied by a significant "shock".
Voltage during a thunderstorm passes such a high current through the human body that it threatens to kill him. Don't risk your life and he alth.
