Is it possible to imagine your life without electricity? Modern man densely surrounded himself with household appliances that help in life. We can no longer imagine ourselves and our lives without smart home helpers.
In technology, they are increasingly switching to the use of electricity. Even transport is gradually transferred to electric motors, which helps to reduce the significant damage 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 the voltage measured?
What is current?
At the dawn of the study of electricity, it was received by the friction of one body on 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 directional movement - electric current. Current Properties:
- conductor heating;
- chemical effect;
- mechanical action;
- magnetic effect.
They are used in everyday life and technology. A necessary condition for the existence of current is considered 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 on the nature of current pulses in animal organs. Prolonged observation of the behavior of frog legs attached to 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 fact that contributes to the flow of electricity, and the reaction of nerve endings is only the physiological effect of the current.
A unique discovery led to the creation of the world's first direct current source, called the Volta Pole. Heterogeneous metals (Volta claimed that they should be removed from each other in a series of chemical elements) are laid with paper impregnated with a liquid “second-kind conductor”.
This device became the first source of constant voltage. A unit for measuring voltage has immortalized the name of Alessandro Volta.
DC source
The main element of the electric circuit is the current source. Its purpose is to create an electric field, under the influence of which free charged particles (electrons, ions) come in directional motion. The charges accumulated on individual elements of the source (they are called poles) have different signs. The charge itself is redistributed inside the source under the influence of forces of 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 of the need for a closed circuit to create direct current.
Since the movement of the charge occurs in the sources under the action of forces of non-electric nature, it can be argued that these forces do the work. Let's call them third-party. The ratio of the work of external forces on charge transfer inside the current source to the magnitude of the charge is called electromotive force.
The mathematical record of this relation:
where E is the electromotive force (EMF), A st is the work of external forces, q is the charge transferred by external forces in the source.
EMF characterizes the ability of a source to create current, but sometimes the main characteristic of a source is electric voltage (potential difference).
Voltage
The ratio of the work of the field on the movement of the charge in the conductor to the magnitude of the charge is called electrical voltage.
To determine it, you need to divide the magnitude of the field by the magnitude of the charge. Let A be the work done by the electric field of the current source to move the charge q. U is the electrical voltage. Mathematical record of the corresponding formula:
Like any physical quantity, stress has a unit of measure. What is the 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 an international system, voltage is measured in volts (V).
A voltage of 1 V is considered to be the voltage of an electric field that performs work of 1 J per charge movement of 1 C.
- B = J / Cl = N • m / (A • s) = kg • m / (A • s 3 ).
In the basic units of the SI system, the unit of voltage measurement:
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 current in it is much more.
Different current sources have different voltages. Therefore, this value is extremely necessary.
Useful analogy
Understanding the physical meaning of electrical voltage comes when you delve into 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 equal pressure.
If the fluid current is compared with the flow of an electric charge, then the pressure difference of the liquid columns plays the same role as the potential difference in the current source.
While processes occurring inside the current source that accompany the charge redistribution at the poles, it is able to create 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
An electric current that periodically changes its direction is called alternating. It is created by an alternating voltage source. Most often it is a generator. Let's try to explain: what is the AC voltage measured in?
The principle of current generation itself is based on the phenomenon of electromagnetic induction. The rotation of the closed loop in a magnetic field leads to the appearance of a potential difference in the conductor. The voltage is measured in volts and in the case of a changing current.
Is it possible to say that the voltage does not change? It is obvious that 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 a certain maximum value, then falls again to zero. There is no need to talk about a certain amount. Enter the so-called effective voltage value:
What instrument measures voltage?
A device for measuring electrical voltage is a 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 a circuit with current rotates in a magnetic field. Depending on the magnitude of the current in the circuit, the rotation angle changes.
If an arrow is attached to the circuit, then it deviates from zero when current flows in the circuit (usually a coil). Depending on what voltage is measured, graduate the scale of the device. Perhaps the use of fractional and multiple values.
In the case of low values, the voltage is measured in millivolts or microvolts. On the contrary, in high voltage networks use multiple units.
Any voltmeter is connected in parallel with that part of the circuit on which the voltage is measured. The main property of the circuit of the device can be called high ohmic resistance. A voltmeter, no matter what the voltage is measured in, should not affect the amperage in the circuit. A small current is passed through it, which does not significantly affect the main value.
Stress table
Physical device | Voltage at its contacts, V |
Volt pole | 1,1 |
Flashlight battery | 1,5 |
Alkaline battery | 1.25 |
Acid battery | 2 |
City network | 220 |
High voltage power 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 contact school teachers.
Physics classrooms are equipped with laboratory and demonstration devices for measuring stress.
Use any voltmeter with caution, observing simple rules:
- The voltmeter has a maximum measurement limit. This is the highest value on its scale. Do not connect it to a circuit containing an element of higher voltage.
- If there is no other source or voltmeter, you can use the system of additional resistances. 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 connector of the voltmeter, the negative to the negative. If mixed up, the hands of the device may bend, which is extremely undesirable.
- All connections are made exclusively to a dead circuit.
Unhealthy
An electric current may be unsafe for humans. A voltage of less than 24 V is considered harmless.
The effect of current under the voltage of the city network (220 V) is quite noticeable. Touching the exposed contacts is accompanied by a significant “electric shock”.
The voltage during a thunderstorm passes such a high current through the human body that it threatens him with a fatal outcome. Do not risk your life and health.