Necessary and sufficient conditions for the passage of electric current in a conductor. Electricity. Conditions for the existence of a current. Basic concepts. What is electric current
Ohm's law for a circuit section states that current is directly proportional to voltage and inversely proportional to resistance.
If we increase several times the voltage acting in electrical circuit, then the current in this circuit will increase by the same factor. And if you increase the resistance of the circuit several times, then the current will decrease by the same amount. Likewise, the flow of water in a pipe is greater, the greater the pressure and the less resistance the pipe exerts to the movement of water.
Electrical resistance- a physical quantity that characterizes the properties of the conductor to prevent the passage electric current and equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it.
Any body through which an electric current flows, has a certain resistance to it.
Electronic theory explains the essence of the electrical resistance of metal conductors in this way. When moving along a conductor, free electrons encounter atoms and other electrons countless times on their way and, interacting with them, inevitably lose part of their energy. The electrons experience, as it were, resistance to their movement. Various metal conductors having different atomic structure, have different resistance to electric current.
The resistance of the conductor does not depend on the current strength in the circuit and voltage, but is determined only by the shape, size and material of the conductor.
The greater the resistance of the conductor, the worse it conducts electric current, and, conversely, the lower the resistance of the conductor, the easier it is for the electric current to pass through this conductor.
2 question. Visible movements of celestial bodies. The laws of planetary motion.
BUT) On a dark night, we can see about 2500 stars in the sky (taking into account the invisible hemisphere 5000), which differ in brightness and color. It seems that they are attached to the celestial sphere and, together with it, revolve around the Earth. To navigate among them, the sky was divided into 88 constellations. A special place among the constellations was occupied by 12 zodiac constellations through which the annual path of the Sun passes - the ecliptic. astronomers use to navigate among the stars various systems celestial coordinates. One of them is the equatorial coordinate system (Fig. 15.1). It is based on the celestial equator - the projection of the earth's equator onto the celestial sphere. The ecliptic and the equator intersect at two points: the spring and autumn equinoxes. Any star has two coordinates: α - right ascension (measured in hours), b - deviation (measured in degrees). The star Altair has the following coordinates: α = 19 h 48 m 18 s; b = +8° 44 ‘. The measured coordinates of the stars are stored in catalogs, they are used to build star maps that astronomers use when searching for the right stars. Mutual arrangement the stars in the sky don't change, they commit diurnal rotation together with celestial sphere. The planets, along with the daily rotation, move slowly among the stars, and are called a wandering star.
The apparent movement of the planets and the Sun was described by Nicolaus Copernicus, using geocentric system peace.
B) The movement of planets and other celestial bodies around the Sun occurs according to Kepler's three laws:
Kepler's first law- under the influence of the force of attraction, one celestial body moves in the gravitational field of another celestial body along one of the conic sections - a circle, an ellipse, a parabola or a hyperbola.
Kepler's second law- each planet moves in such a way that the radius vector of the planet covers equal areas in equal time intervals.
Kepler's third law- the cube of the semi-major axis of the body's orbit, divided by the square of the period of its revolution and the sum of the masses of the bodies, is a constant value.
and 3 / [T 2 * (M 1+ M 2)] = G / 4P 2 G is the gravitational constant.
Moon moving around Earth in an elliptical orbit. The change of lunar phases is determined by the change in the type of illumination of the side of the moon. The movement of the moon around the earth is explained by the lunar and solar eclipses. The phenomena of ebbs and flows are due to the attraction of the Moon and the large size of the Earth.
First, let's answer the question, what is an electric current. A simple battery standing on a table does not create current by itself. And a flashlight lying on the table will not create current through its LEDs just like that, for no reason at all. For a current to appear, something must flow somewhere, at least start moving, and for this, the circuit of the LEDs of the flashlight and the battery must be closed. Not for nothing, in the old days, electric current was compared with the movement of a certain charged liquid.
In fact, we now know that electric current is the directional movement of charged particles, and that a more realistic analog would be a charged gas, a gas of charged particles moving under the influence of an electric field. But first things first.
Electric current is the directed movement of charged particles
So, electric current is the movement of charged particles, but even the chaotic movement of charged particles is also movement, but it is not yet a current. Similarly, the molecules of a liquid, which are always in thermal motion, do not create currents, because the total displacement of the entire volume of a liquid at rest is exactly zero.
In order for a liquid to flow, a total displacement must occur, that is, the general movement of the liquid molecules must become directed. So the chaotic movement of molecules will be combined with the directed movement of the entire volume, and a flow of the entire volume of the liquid will arise.
The situation is similar with electric current - the directed movement of electrically charged particles - there is an electric current. The speed of thermal motion of charged particles, for example, in a metal, is measured in hundreds of meters per second, however, with directed motion, when a certain current is set in the conductor, the speed of the general motion of particles is measured in fractions and units of millimeters per second.
So, if a direct current equal to 10 A flows in a metal conductor with a cross section of 1 sq. mm, then the average speed of the ordered movement of electrons will be from 0.6 to 6 millimeters per second. This will be electric current. And this slow movement of electrons is enough for a conductor, for example, from nichrome, to warm up well, obeying.
The speed of particles is not the speed of propagation of the electric field!
Note that the current begins in the conductor almost instantly throughout the entire volume, that is, this “movement” spreads along the conductor at the speed of light, but the movement of directly charged particles themselves is 100 billion times slower. You can consider the analogy with a pipe through which a liquid flows.
1. Electric current requires charged particles
Electrons in metals and in vacuum, ions in electrolyte solutions - serve as charge carriers and ensure the presence of current in various substances. In metals, electrons are very mobile, some of them can freely move from atom to atom, like a gas filling the space between nodes. crystal lattice.
In vacuum tubes, electrons leave the cathode in the process of thermionic emission, rushing under the influence of an electric field to the anode. In electrolytes, molecules break up in water into positively and negatively charged parts, and become ions - free charge carriers in electrolytes. That is, wherever an electric current can exist, there are free charge carriers that can move. This is the first condition for the existence of an electric current - presence of free charge carriers.
2. The second condition for the existence of an electric current is that external forces must act on the charge
If we now look at the conductor, let's say it is a copper wire, then we can ask ourselves: what is needed in order for an electric current to arise in it? There are charged particles, electrons, they are able to move freely.
What will make them move? It is known that an electrically charged particle interacts with an electric field. Therefore, it is necessary to create an electric field in the conductor, then a potential will arise at each point of the conductor, a potential difference will occur between the ends of the conductor, and the electrons will move in the direction of the field - in the direction from "-" to "+", that is, in the direction against electric field strength vector. The electric field will accelerate the electrons, increasing their (kinetic and magnetic) energy.
As a result, if we consider simply an electric field applied outside to the conductor (we placed the conductor in an electric field along the lines of force), then the electrons will accumulate at one end of the wire, and a negative charge will appear at this end, and since the electrons have shifted from the other end of the wire, then it will have a positive charge.
As a result, the electric field of a conductor charged by an externally applied electric field will be in such a direction as to weaken the external electric field by its action.
The process of redistribution of charges will proceed almost instantly, and upon its completion, the current in the conductor will stop. The resulting electric field inside the conductor will become equal to zero, and the intensity at the edges will be equal in absolute value, but opposite in direction to the electric field applied outside.
If the electric field in the conductor is created by a direct current source, such as a battery, then such a source will become a source of external forces for the conductor, that is, the source that will create a constant EMF in the conductor and will maintain a potential difference. Obviously, in order for the current to be supported by a source of external forces, the circuit must be closed.
Charge in motion. It can take the form of a sudden discharge of static electricity, such as lightning. Or it could be a controlled process in generators, batteries, solar or fuel cells. Today we will consider the very concept of "electric current" and the conditions for the existence of an electric current.
Electric Energy
Most of the electricity we use comes in the form of alternating current from the electrical grid. It is created by generators that work according to Faraday's law of induction, due to which a changing magnetic field can induce an electric current in a conductor.
Generators have spinning coils of wire that pass through magnetic fields as they spin. As the coils rotate, they open and close relative to magnetic field and create an electric current that changes direction at every turn. Current passes through full cycle forward and backward 60 times per second.
Generators can be powered by steam turbines heated by coal, natural gas, oil or nuclear reactor. From the generator, the current passes through a series of transformers, where its voltage increases. The diameter of the wires determines the amount and strength of current they can carry without overheating and wasting power, and voltage is limited only by how well the lines are insulated from ground.
It is interesting to note that the current is carried by only one wire, not two. Its two sides are designated as positive and negative. However, since the polarity of alternating current changes 60 times per second, they have other names - hot (main power lines) and grounded (passing underground to complete the circuit).
Why is electricity needed?
There are many uses for electricity: it can light up your house, wash and dry your clothes, lift your garage door, boil water in a kettle, and power other household items that make our lives so much easier. However, the ability of the current to transmit information is becoming increasingly important.
When connected to the Internet, a computer uses only a small part of the electric current, but this is something without which a modern person cannot imagine his life.
The concept of electric current
Like a river current, a stream of water molecules, an electric current is a stream of charged particles. What is it that causes it, and why doesn't it always go in the same direction? When you hear the word flow, what do you think of? Perhaps it will be a river. It's a good association, because that's the reason the electric current got its name. It is very similar to the flow of water, only instead of water molecules moving along the channel, charged particles move along the conductor.
Among the conditions necessary for the existence of an electric current, there is an item that provides for the presence of electrons. Atoms in a conductive material have many of these free charged particles that float around and between the atoms. Their movement is random, so there is no flow in any given direction. What does it take for an electric current to exist?
The conditions for the existence of electric current include the presence of voltage. When it is applied to a conductor, all free electrons will move in the same direction, creating a current.
Curious about electric current
Interestingly, when electrical energy is transmitted through a conductor at the speed of light, the electrons themselves move much more slowly. In fact, if you walked leisurely next to a conductive wire, your speed would be 100 times faster than the electrons are moving. This is due to the fact that they do not need to travel huge distances to transfer energy to each other.
Direct and alternating current
Today, two different types of current are widely used - direct and alternating. In the first, the electrons move in one direction, from the "negative" side to the "positive" side. The alternating current pushes the electrons back and forth, changing the direction of the flow several times per second.
Generators used in power plants to produce electricity are designed to produce alternating current. You probably never noticed that the light in your house is actually flickering as the current direction changes, but it happens too fast for the eyes to recognize.
What are the conditions for the existence of direct electric current? Why do we need both types and which one is better? These are good questions. The fact that we still use both types of current suggests that they both serve specific purposes. As far back as the 19th century, it was clear that efficient transmission of power over long distances between a power plant and a house was possible only at very high voltages. But the problem was that sending really high voltage was extremely dangerous for people.
The solution to this problem was to reduce the stress outside the home before sending it inside. To this day, direct electric current is used for transmission over long distances, mainly because of its ability to easily convert to other voltages.
How electric current works
The conditions for the existence of an electric current include the presence of charged particles, a conductor, and voltage. Many scientists have studied electricity and found that there are two types of it: static and current.
It is the latter that plays an important role in Everyday life any person, as it represents an electric current that passes through the circuit. We use it daily to power our homes and more.
What is electric current?
When electric charges circulate in a circuit from one place to another, an electric current is produced. The conditions for the existence of an electric current include, in addition to charged particles, the presence of a conductor. Most often it is a wire. Its circuit is a closed circuit in which current flows from a power source. When the circuit is open, he cannot complete the journey. For example, when the light in your room is off, the circuit is open, but when the circuit is closed, the light is on.
Current power
The conditions for the existence of an electric current in a conductor are greatly influenced by such a voltage characteristic as power. This is a measure of how much energy is being used over a given period of time.
There are many different units that can be used to express this characteristic. However, electrical power is almost measured in watts. One watt is equal to one joule per second.
Electric charge in motion
What are the conditions for the existence of an electric current? It can take the form of a sudden discharge of static electricity, such as lightning or a spark from friction with a woolen cloth. More often, however, when we talk about electric current, we mean a more controlled form of electricity that makes lights and appliances work. Most of the electrical charge is carried by the negative electrons and positive protons within the atom. However, the latter are mostly immobilized inside atomic nuclei, so the work of moving charge from one place to another is done by the electrons.
Electrons in a conductive material such as a metal are largely free to move from one atom to another along their conduction bands, which are the higher electron orbits. A sufficient electromotive force or voltage creates a charge imbalance that can cause electrons to move through a conductor in the form of an electric current.
If we draw an analogy with water, then take, for example, a pipe. When we open a valve at one end to let water enter the pipe, we don't have to wait for that water to work its way all the way to the end of the pipe. We get water at the other end almost instantly because the incoming water pushes the water that is already in the pipe. This is what happens in the case of an electric current in a wire.
Electric current: conditions for the existence of an electric current
Electric current is usually viewed as a flow of electrons. When the two ends of the battery are connected to each other with a metal wire, this charged mass flows through the wire from one end (electrode or pole) of the battery to the opposite. So, let's call the conditions for the existence of an electric current:
- charged particles.
- Conductor.
- Voltage source.
However, not all so simple. What conditions are necessary for the existence of an electric current? This question can be answered in more detail by considering the following characteristics:
- Potential difference (voltage). This is one of the prerequisites. Between the 2 points there must be a potential difference, meaning that the repulsive force that is created by charged particles in one place must be greater than their force at another point. Voltage sources are generally not found in nature, and electrons are distributed in environment fairly evenly. Nevertheless, scientists managed to invent certain types of devices where these charged particles can accumulate, thereby creating the very necessary voltage (for example, in batteries).
- Electrical resistance (conductor). This is the second important condition that is necessary for the existence of an electric current. This is the path along which charged particles travel. Only those materials that allow electrons to move freely act as conductors. Those who do not have this ability are called insulators. For example, a metal wire will be an excellent conductor, while its rubber sheath will be an excellent insulator.
Having carefully studied the conditions for the emergence and existence of electric current, people were able to tame this powerful and dangerous element and direct it for the benefit of mankind.
Electric current - ordered in the direction of the movement of electric charges. The direction of the current is taken to be the direction of movement of positive charges.
The passage of current through the conductor is accompanied by the following actions:
* magnetic (observed in all conductors)
* thermal (observed in all conductors except superconductors)
* chemical (observed in electrolytes).
For the occurrence and maintenance of current in any medium, two conditions must be met:
* the presence of free electric charges in the environment
* creating an electric field in the environment.
The electric field in the medium is necessary to create a directed movement of free charges. As is known, a charge q in an electric field of strength E is affected by a force F = q* E, which forces the free charges to move in the direction of the electric field. A sign of the existence of an electric field in the conductor is the presence of a non-zero potential difference between any two points of the conductor,
However, electric forces cannot sustain an electric current for a long time. The directed movement of electric charges after some time leads to equalization of the potentials at the ends of the conductor and, consequently, to the disappearance of the electric field in it.
To maintain the current in the electric circuit, the charges, in addition to the Coulomb forces, must be affected by non-electrical forces (external forces).
A device that creates external forces, maintains a potential difference in the circuit and converts various types of energy into electrical energy is called a current source.
For the existence of electric current in a closed circuit, it is necessary to include a current source in it.
Main characteristics
1. Current strength - I, unit of measure - 1 A (Ampere).
The current strength is a value equal to the charge flowing through the cross section of the conductor per unit time.
I = Dq/Dt.
The formula is valid for direct current, at which the current strength and its direction do not change with time. If the strength of the current and its direction change with time, then such a current is called variable.
For AC:
I = limDq/Dt ,
Dt - 0
those. I = q", where q" is the derivative of the charge with respect to time.
2. Current density - j, unit of measurement - 1 A/m2.
The current density is a value equal to the strength of the current flowing through a single cross section of the conductor:
j = I/S .
3. The electromotive force of the current source - emf. (e), the unit is 1 V (Volt). E.m.f. is a physical quantity equal to the work done by external forces when moving along an electric circuit of a single positive charge:
e \u003d Ast. / q.
4. Conductor resistance - R, unit - 1 ohm.
Under the action of an electric field in a vacuum, free charges would move at an accelerated rate. In matter, they move uniformly on average, because part of the energy is given to particles of matter in collisions.
The theory states that the energy of the ordered movement of charges is dissipated by the distortions of the crystal lattice. Based on the nature of electrical resistance, it follows that
R \u003d r * l / S,
where
l - conductor length,
S - cross-sectional area,
r is a proportionality factor, called the resistivity of the material.
This formula is well confirmed by experience.
The interaction of conductor particles with charges moving in the current depends on the chaotic motion of particles, i.e. on the temperature of the conductor. It is known that
r = r0(1 + a t) ,
R = R0(1 + a t) .
The coefficient a is called the temperature coefficient of resistance:
a = (R - R0)/R0*t .
For chemically pure metals a > 0 and equal to 1/273 K-1. For alloys, temperature coefficients are less important. The dependence r(t) for metals is linear:
In 1911, the phenomenon of superconductivity was discovered, which consists in the fact that at a temperature close to absolute zero, the resistance of some metals drops abruptly to zero.
For some substances (for example, electrolytes and semiconductors), the resistivity decreases with increasing temperature, which is explained by an increase in the concentration of free charges.
The reciprocal of resistivity is called electrical conductivity s
s = 1/r
5. Voltage - U, unit of measurement - 1 V.
Voltage is a physical quantity equal to the work done by external and electric forces when moving a single positive charge.
U \u003d (Ast. + Ael.) / q.
Since Ast./q = e, and Ael./q = f1-f2, then
U = e + (f1 - f2) .
The directed (ordered) movement of free charged particles under the action of an electric field is called electric current.
Conditions for the existence of a current:
1. The presence of free charges.
2. The presence of an electric field, i.e. potential differences. Free charges are present in conductors. The electric field is created by current sources.
When current passes through a conductor, it does the following:
Thermal (heating of the conductor by current). For example: the operation of an electric kettle, iron, etc.).
· Magnetic (appearance of a magnetic field around a current-carrying conductor). For example: the operation of an electric motor, electrical measuring instruments).
Chemical ( chemical reactions during the passage of current through some substances). For example: electrolysis.
You can also talk about
Light (accompanies thermal effect). For example: the glow of the filament of an electric light bulb.
Mechanical (accompanies magnetic or thermal). For example: deformation of the conductor when heated, rotation of the frame with current in a magnetic field).
Biological (physiological). For example: electric shock to a person, use of the action of current in medicine.
The main quantities that describe the process of passing current through a conductor.
1. Current I- a scalar value equal to the ratio of the charge that has passed through the cross section of the conductor, the time interval during which the current flowed. The current strength shows how much charge passes through the cross section of the conductor per unit of time. The current is called permanent if the current does not change with time. In order for the current through the conductor to be constant, it is necessary that the potential difference at the ends of the conductor is constant.
2. Voltage U. The voltage is numerically equal to the work of the electric field in moving a single positive charge along the field lines of force inside the conductor.
3. Electrical resistance R- a physical quantity numerically equal to the ratio of the voltage (potential difference) at the ends of the conductor to the strength of the current passing through the conductor.
60. Ohm's law for a chain section.
The current strength in a circuit section is directly proportional to the voltage at the ends of this conductor and inversely proportional to its resistance:
I=U/R;
Ohm found that the resistance is directly proportional to the length of the conductor and inversely proportional to its cross-sectional area and depends on the substance of the conductor.
where ρ is the resistivity, l is the length of the conductor, S is the cross-sectional area of the conductor.
61. Resistance as an electrical characteristic of a resistor. The dependence of the resistance of metal conductors on the type of material and geometric dimensions.
Electrical resistance- a physical quantity that characterizes the properties of the conductor to prevent the passage of electric current and is equal to the ratio of the voltage at the ends of the conductor to the strength of the current flowing through it. Resistance for AC circuits and for alternating electromagnetic fields is described in terms of impedance and wave resistance.
Resistance (often denoted by the letter R or r) is considered, within certain limits, a constant value for a given conductor; it can be calculated as
Where R is the resistance; U is the difference in electrical potentials at the ends of the conductor; I is the strength of the current flowing between the ends of the conductor under the action of a potential difference.
The resistance of a conductor is the same characteristic of a conductor as its mass. The resistance of the conductor does not depend on the current strength in the conductor, nor on the voltage at its ends, but depends only on the type of substance from which the conductor is made and its geometric dimensions: , where: l is the length of the conductor, S is the cross-sectional area of the conductor, ρ is the specific resistance of the conductor, showing what resistance a conductor 1 m long and a cross-sectional area of 1 m 2 made of this material will have.
Conductors obeying Ohm's law are called linear. There are many materials and devices that do not obey Ohm's law, such as a semiconductor diode or a gas discharge lamp. Even for metal conductors at sufficiently high currents, a deviation from Ohm's linear law is observed, since the electrical resistance of metal conductors increases with increasing temperature.
The dependence of the conductor resistance on temperature is expressed by the formula: , where: R - conductor resistance at temperature T, R 0 - conductor resistance at a temperature of 0ºС, α - temperature coefficient of resistance.
