The magnetic field is generated by electric current (moving charges)

Electricity

Electric current flows through the wires.
Moreover, it “flows”, almost like water. Let's imagine that you are a happy farmer who decided to water his garden with a hose. You opened the tap slightly, and water immediately ran through the hose. Slowly, but still she ran. The jet force is very weak. Then you decided that more pressure was needed and opened the tap to its fullest. As a result, the stream will flow with such force that not a single tomato will be left unattended, although in both cases the diameter of the hose is the same.

Now imagine that you are filling two buckets from two hoses. One of them has stronger pressure, the other weaker. The bucket into which water is poured from a hose with strong pressure will fill faster. The thing is that the volume of water for an equal period of time from two different hoses is also different. In other words, a much larger number of water molecules will run out of the green hose than from the yellow hose in the same period of time.

If we take a conductor with current, the same thing will happen: charged particles will move along the conductor, just like water molecules. If more charged particles move along the conductor, the “pressure” will also increase.

• Electric current is the directed movement of charged particles.

Current strength

There immediately arises a need for a quantity with which we will measure the “pressure” of the electric current. Such that it depends on the number of particles that flow through the conductor.

Current strength is a physical quantity that shows how much charge has passed through a conductor.

Current strength is measured in Amperes. The unit of measurement was chosen for a reason.

Firstly, it is named after the physicist André-Marie Ampère, who studied electrical phenomena. And secondly, the unit of this quantity was chosen based on the phenomenon of interaction of two conductors.

Here, unfortunately, it is impossible to draw an analogy with a water supply system. Hoses with water do not attract or repel each other close to each other (which is a pity, it would be funny).

When current flows through two parallel conductors in the same direction, the conductors attract each other. And when in the opposite direction (along the same conductors) they repel.

The unit of current 1 A is taken to be the current at which two parallel conductors 1 m long, located at a distance of 1 m from each other in a vacuum, interact with a force of 0.0000002 N.

Task

Find the current strength in the circuit if a charge equal to 300 mC passes through it in 2 seconds.

Solution:

Let's take the formula for current strength

I = q/t

Let's substitute the values

I = 300 mC / 2 s = 150 mA

Answer: the current in the circuit is 150 mA

The magnetic field is generated by electric current (moving charges)

F=1 C/V

1. The magnetic field is generated by electric current (moving charges).

2. A magnetic field is detected by its effect on electric current (moving charges).

Like the electric field, the magnetic field really exists, regardless of us, of our knowledge about it.

Magnetic induction is the ability of a magnetic field to exert a force on a current-carrying conductor (vector quantity). Measured in T.

The direction of the magnetic induction vector is taken to be the direction from the south pole S to the north pole N of the magnetic needle, which is freely established in the magnetic field. This direction coincides with the direction of the positive normal to the closed loop with current.

The direction of the magnetic induction vector is established using the gimlet rule:

if the direction of translational movement of the gimlet coincides with the direction of the current in the conductor, then the direction of rotation of the gimlet handle coincides with the direction of the magnetic induction vector.

Magnetic induction lines.

A line at any point of which the magnetic induction vector is directed along a tangent - the magnetic induction line. A uniform field is parallel lines, a non-uniform field is curved lines. The more lines, the greater the strength of this field. Fields with closed lines of force are called vortex fields. The magnetic field is a vortex field.

Magnetic flux. – a value equal to the product of the magnitude of the magnetic induction vector by the area and the cosine of the angle between the vector and the normal to the surface.

Ampere's force is equal to the product of the magnetic induction vector by the current strength, the length of the conductor section and the sine of the angle between the magnetic induction and the conductor section.

The force acting on a moving charged particle from the magnetic field is called the Lorentz force. This force can be found using Ampere's law.

The modulus of the Lorentz force is equal to the ratio of the modulus of the force F acting on a section of a conductor of length êl to the number N of charged particles, the ordering of which moves in this section of the conductor:

Direction using the left-hand rule: If the left hand is positioned so that the component of magnetic induction B, perpendicular to the speed of the charge, enters the palm, and the four fingers are directed along the movement of the positive charge (against the movement of the negative), then the thumb bent 90° will show direction of the Lorentz force acting on the charge.

Since the Lorentz force is perpendicular to the particle velocity, then. it doesn't do work.

Ampere force is used in loudspeakers and speakers.

Operating principle: An alternating electric current flows through the coil with a frequency equal to the audio frequency from a microphone or from the output of a radio receiver. Under the action of the Ampere force, the coil oscillates along the axis of the loudspeaker in time with the current fluctuations. These vibrations are transmitted to the diaphragm, and the surface of the diaphragm emits sound waves.

The Lorentz force is used in televisions and mass spectrographs.

Operating principle: The vacuum chamber of the device is placed in a magnetic field. Charged particles (electrons or ions) accelerated by an electric field, having described an arc, fall on a photographic plate, where they leave a trace that makes it possible to measure the radius of the trajectory with great accuracy. This radius determines the specific charge of the ion. Knowing the charge of an ion, it is easy to determine its mass.

Ticket number 15

Experimental proof of the existence of free electrons in metals. Experimental proof that the conductivity of metals is due to the movement of free electrons was given in the experiments of L. I. Mandelstam and N. D. Papaleksi.

A wire is wound onto a coil, the ends of which are soldered to two metal disks isolated from each other. A galvanometer is attached to the ends of the disks using sliding contacts.

The reel is set into rapid motion and then abruptly stopped. After a sudden stop of the coil, free charged particles move relative to the conductor by inertia for some time, and, consequently, an electric current will arise in the coil. The current exists for a short time, since due to the resistance of the conductor, charged particles are slowed down and the ordered movement of particles that forms the current stops.

The direction of the current indicates that it is created by the movement of negatively charged particles.

If you pass current from the battery through a steel coil and then start heating it in the burner flame, the ammeter will show a decrease in current. This means that as the temperature changes, the resistance of the conductor changes.

If at a temperature equal to 0 ° C, the resistance of the conductor is equal to Ro, and at a temperature t it is equal to R, then the relative change in resistance, as experience shows, is directly proportional to the change in temperature t:

The proportionality coefficient α is called the temperature coefficient of resistance. It characterizes the dependence of the resistance of a substance on temperature. The temperature coefficient of resistance is numerically equal to the relative change in the resistance of the conductor when heated by 1 K. For all metal conductors α>0 and changes slightly with temperature. In pure metals.

For electrolyte solutions, the resistance does not increase with increasing temperature, but decreases. Dependence of resistivity on temperature:

In 1911, the Dutch physicist Kamerlingh Onnes discovered a remarkable phenomenon - superconductivity. He discovered that when mercury is cooled in liquid helium, its resistance first changes gradually, and then very sharply drops to zero at a temperature of 4.1 K. This phenomenon was called superconductivity.

Superconductivity occurs at very low temperatures—about 25 K.

If a current is created in a ring conductor that is in a superconducting state, and then the source of the electric current is eliminated, then the strength of this current does not change for any length of time. In an ordinary non-superconducting conductor, the electric current stops in this case.

Superconductors are widely used. Thus, powerful electromagnets with a superconducting winding are built, which create a magnetic field over long periods of time without consuming energy. After all, no heat is released in the superconducting winding.

However, it is impossible to obtain an arbitrarily strong magnetic field using a superconducting magnet. A very strong magnetic field destroys the superconducting state. Such a field can be created by current in the superconductor itself. Therefore, for each conductor in a superconducting state, there is a critical current value, which cannot be exceeded without violating the superconducting state.

Ticket No. 16

Liquids, like solids, can be dielectrics, conductors and semiconductors. Dielectrics include distilled water, conductors include solutions and melts of electrolytes: acids, alkalis and salts. Liquid semiconductors are molten selenium, molten sulfides, etc.

Electrolytic dissociation.

When electrolytes dissolve under the influence of the electric field of polar water molecules, the electrolyte molecules disintegrate into ions. This process is called electrolytic dissociation.

The degree of dissociation, i.e. the proportion of molecules in a solute that have broken up into ions, depends on the temperature, concentration of the solution and the dielectric constant of the solvent. With increasing temperature, the degree of dissociation increases and, consequently, the concentration of positively and negatively charged ions increases.

When ions of different signs meet, they can again unite into neutral molecules - recombine. Under constant conditions, a dynamic equilibrium is established in the solution, in which the number of molecules that disintegrate into ions per second is equal to the number of pairs of ions that, at the same time, recombine into neutral molecules.

Ionic conductivity. Charge carriers in aqueous solutions or melts of electrolytes are positively and negatively charged ions.

If a vessel with an electrolyte solution is connected to an electrical circuit, then negative ions will begin to move towards the positive electrode - the anode, and positive ions - towards the negative - cathode. As a result, an electric current will be established. Since charge transfer in aqueous solutions or electrolyte melts is carried out by ions, such conductivity is called ionic.

Electrolysis. In ionic conduction, the passage of current is associated with the transfer of matter. At the electrodes, substances that make up the electrolytes are released. At the anode, negative ions give up their extra electrons, and at the cathode, positive ions receive the missing electrons. The process of releasing a substance at the electrode associated with redox reactions is called electrolysis.

Obviously, the mass of the released substance is equal to the product of the mass of one ion m0j by the number of ions Nj that reached the electrode during the time Δt: m= m0j Nj. Ion mass

where M is the molar (or atomic) mass of the substance, and

The number of ions reaching the electrode is equal to:

Faraday's law of electrolysis. the mass of the substance released on the electrode during the time Δt during the passage of an electric current is proportional to the current strength and time.

Applications of electrolysis.

The surface of one metal is electrolytically coated with a thin layer of another (nickel plating, chrome plating, copper plating, etc.). This durable coating protects the surface from corrosion.

In the printing industry, such copies (stereotypes) are obtained from matrices (type imprint on a plastic material), for which thick layers of iron or other material are deposited on the matrices. This allows you to reproduce the set in the required number of copies.

Using electrolysis, metals are purified from impurities. Thus, the crude copper obtained from the ore is cast into the form of thick sheets, which are then placed in a bath as anodes. During electrolysis, the copper of the anode dissolves, impurities containing valuable and rare metals fall to the bottom, and pure copper settles on the cathode.

Ticket No. 17

Semiconductors differ most clearly from conductors in the nature of the dependence of electrical conductivity on temperature. Measurements show that for a number of elements (silicon, germanium, selenium, etc.) the resistivity does not increase with increasing temperature, as with metals, but, on the contrary, decreases extremely sharply. Such substances are called semiconductors.

Hole conductivity. When a bond is broken, a vacant site with a missing electron is formed. It's called a hole. The hole has an excess positive charge compared to the others. One of the electrons that provides communication

atoms, jumps to the place of the formed hole and restores the pair-electronic bond here, and where this electron jumped from, a new hole is formed. Thus, the hole can move throughout the crystal.

Semiconductors have not only electronic but also hole conductivity -

intrinsic conductivity of semiconductors.

The intrinsic conductivity of semiconductors is usually low, since the number of free electrons is small. The number of free electrons is approximately one ten billionth of the total number of atoms.

An essential feature of semiconductors is that in the presence of impurities, along with their own conductivity, an additional conductivity appears - impurity conductivity. By changing the impurity concentration, you can significantly change the number of charge carriers of one or another sign. Thanks to this, it is possible to create semiconductors with a predominant concentration of either negatively or positively charged carriers.

Application:

Semiconductor diode - used to rectify electric current in radio circuits. In a pn junction, charge carriers are formed when an acceptor or donor impurity is introduced into the crystal. Here there is no need to use an energy source to obtain free charge carriers. The energy savings are significant. Semiconductor rectifiers are smaller than vacuum tubes. Semiconductor radio devices are much more compact. Semiconductor elements are used on artificial Earth satellites, spacecraft, and electronic computers. Semiconductor diodes are made from germanium, silicon, selenium and other substances. They are highly reliable and have a long service life, but are limited to a temperature range from –70 to 125 degrees C.

Transistors. They replace vacuum tubes in many electrical circuits of scientific, industrial and household equipment. Portable radios using such devices are commonly called transistors. Advantage: the absence of a heated cathode, which consumes significant power and requires time to warm up. Transistors are tens and hundreds of times smaller in size and weight than vacuum tubes. Operate at lower voltages. The disadvantages are the same as those of semiconductor diodes.

Thermistors. One of the simplest semiconductor devices. Available in the form of rods, tubes, disks, washers and beads ranging in size from micrometers to several centimeters. Thermistors are used for remote temperature measurement, fire alarms, etc. The measured temperature range of most thermistors is from 170 to 570 K. There are thermistors for measuring very high temperatures up to 1300 and very low temperatures 4-80 K.

Photoresistors (photoresistors). The electrical conductivity of semiconductors increases not only when heated, but also when illuminated. This effect is also observed at constant temperature. Photoresistors are devices that use the photoelectric effect in semiconductors. The miniature size and high sensitivity of photoresistors make it possible to use them in a wide variety of fields of science and technology for recording and measuring weak light fluxes. Photoresistors are used to determine the quality of surfaces, control the dimensions of products, etc.

Ticket#18

By pumping gas out of a vessel, it is possible to reach a concentration at which the molecules have time to fly from one wall of the vessel to the other without ever colliding with each other. This state of gas in the tube is called vacuum.

Conductivity of the interelectron gap in a vacuum can only be ensured by introducing a source of charged particles into the tube.

Thermionic emission. Most often, the effect of such a source of charged particles is based on the property of bodies heated to a high temperature to emit electrons. This process is called thermionic emission. It can be considered as the evaporation of electrons from the surface of the metal. For many solids, thermionic emission begins at temperatures at which evaporation of the substance itself does not yet occur. Such substances are used to make cathodes.

One-way conduction. The phenomenon of thermionic emission leads to the fact that a heated metal electrode, unlike a cold one, continuously emits electrons. The electrons form an electron cloud around the electrode. In this case, the electrode becomes positively charged, and under the influence of the electric field of the charged cloud, electrons from the cloud are partially returned to the electrode.

In the equilibrium state, the number of electrons leaving the electrode per second is equal to the number of electrons returning to the electrode during this time. The higher the temperature of the metal, the higher the density of the electron cloud.

The difference between hot and cold electrodes, sealed into a vessel from which air has been evacuated, leads to one-way conduction of electric current between them.

When the electrodes are connected to a current source, an electric field arises between them. If the positive pole of the current source is connected to a cold electrode (anode), and the negative pole to a heated one (cathode), then the electric field strength is directed towards the heated electrode. Under the influence of this field, electrons partially leave the electron cloud and move towards the cold electrode. The electrical circuit is closed and an electric current is established in it. When the source is turned on in the opposite direction, the field strength is directed from the heated electrode to the cold one. The electric field pushes the cloud's electrons back toward the heated electrode. The circuit appears to be open.

Diode. One-way conductivity is used in electronic devices with two electrodes - vacuum diodes.

The structure of a modern vacuum diode (electron tube) is as follows. Inside a glass or metal-ceramic cylinder, from which air has been pumped out to a pressure of 10~6-10~7 mmHg. Art., two electrodes are placed (Fig. 173, a). One of them, the cathode, has the form of a vertical metal cylinder, usually coated with a layer of oxides of alkaline earth metals, such as barium, strontium, and calcium. This cathode is called an oxide cathode.

When heated, the surface of an oxide cathode releases many more electrons than the surface of a pure metal cathode. Inside the cathode there is an insulated conductor heated by alternating current. The heated cathode emits electrons that reach the anode if the anode is at a higher potential than the cathode.

Properties of electron beams and their applications.

When fast electrons hitting a substance are decelerated, X-rays are produced. Some substances (glass, zinc and cadmium sulfides) bombarded* with electrons glow. To the present. Nowadays, materials of this type (luminophores) are those in which up to 25% of the energy of the electron beam is converted into light energy.

Electron beams are deflected by an electric field. For example, passing between the plates of a capacitor, electrons are deflected from a negatively charged plate to a positively charged one (Fig. 177).

The electron beam is also deflected in a magnetic field. Flying over the north pole of the magnet, electrons are deflected to the left, and flying over the south pole, they are deflected to the right (Fig. 178). The deviation of electron flows coming from the Sun in the Earth's magnetic field leads to the fact that the glow of gases in the upper layers of the atmosphere (auroras) is observed only at the poles.

The ability to control an electron beam using electric or magnetic fields and the glow of a phosphor-coated screen under the action of the beam is used in a cathode ray tube.

Cathode-ray tube. A cathode ray tube is the main element of a television and an oscilloscope * - a device for studying rapidly changing processes in electrical circuits (Fig. 179).

The structure of a cathode ray tube is shown in Figure 180. The tube is a vacuum cylinder, one of the walls of which serves as a screen. At the narrow end of the tube there is a source of fast electrons - an electron gun (Fig. 181). It consists of a cathode, a control electrode and an anode (usually several anodes are located one behind the other). Electrons are emitted by the heated oxide layer from the end of the cylindrical cathode C, surrounded by a heat shield //. They then pass through a hole in the cylindrical control electrode B (it regulates the number of electrons in the beam). Each anode ai and L 2 consists of disks with small holes. These discs are inserted into metal cylinders. A potential difference of hundreds and even thousands of volts is created between the first anode and the cathode. A strong electric field accelerates the electrons and they acquire greater speed. The shape, location and potentials of the anodes are chosen so that, along with the acceleration of electrons, the electron beam is also focused, i.e., the cross-sectional area of ​​the beam on the screen is reduced almost to a point.

On its way to the screen, the beam sequentially passes between two pairs of control plates, similar to the plates of a parallel-plate capacitor (see Fig. 180). If there is no electric field between the plates, then the beam is not deflected and the luminous point is located in the center of the screen. When a potential difference is imparted to vertically located plates, the beam is displaced in the horizontal direction, and when a potential difference is communicated to horizontal plates, it is displaced in the vertical direction.

The simultaneous use of two pairs of plates allows you to move the luminous point across the screen in any direction. Since the mass of electrons is very small, they react almost instantly to changes in the potential difference of the control plates.

In a cathode ray tube used in a television (the so-called kinescope), the beam created by the electron gun is controlled using a magnetic field. This field is created by coils placed on the neck of the tube (Fig. 182).

Ticket No. 19

Electric discharge in gas.

Let's take an electrometer with the disks of a flat capacitor attached to it and charge it. At room temperature, if the air is dry enough, the capacitor does not noticeably discharge. This shows that the electric current caused by the potential difference in the air between the disks is very small. Consequently, the electrical conductivity of air at room temperature is low and it can be considered a dielectric.

Now let's heat the air between the disks with a burning match. Note that the electrometer needle is quickly approaching zero, which means the capacitor is discharging. Consequently, the heated gas is a conductor and an electric current is established in it.

The process of electric current flowing through a gas is called a gas discharge.

Ionization of gases. We have seen that at room temperature air is a very poor conductor. At

When heated, air conductivity increases. An increase in air conductivity can be caused by other means, for example, by exposure to radiation: ultraviolet, x-ray, radioactive, etc.

Under ordinary conditions, gases consist almost entirely of neutral atoms or molecules and are therefore dielectrics. Due to heating or exposure to radiation, some of the atoms are ionized - they break up into positively charged ions and electrons. Negative ions can also form in the gas, which appear due to the addition of electrons to neutral atoms.

The ionization of gases when heated is explained by the fact that as they heat up, the molecules move faster. At the same time, some molecules begin to move so quickly that some of them disintegrate during collisions, turning into ions. The higher the temperature, the more ions are formed.

Conductivity of gases. The mechanism of conductivity of gases is similar to the mechanism of conductivity of solutions and melts of electrolytes. The difference is that the negative charge is carried mainly not by negative ions, as in aqueous solutions or electrolyte melts, but by electrons.

Non-independent discharge. To study a discharge in a gas at various pressures, it is convenient to use a glass tube with two electrodes.

Let a certain number of pairs of charged particles be formed per second in a gas using some ionizer: positive ions and electrons.

When there is a small potential difference between the electrodes of the tube, positively charged ions move to the negative electrode, and electrons and negatively charged ions move to the positive electrode. As a result, an electric current arises in the tube, i.e., a gas discharge occurs.

Not all ions formed reach the electrodes; some of them reunite with electrons, forming neutral gas molecules. As the potential difference between the electrodes of the tube increases, the proportion of charged particles reaching the electrodes increases. The current in the circuit also increases. Finally, a moment comes at which all the charged particles formed in the gas per second reach the electrodes during this time. In this case, no further increase in current occurs. The current is said to reach saturation. If the action of the ionizer is stopped, the discharge will also stop, since there are no other sources of ions. For this reason, the discharge is called a non-self-sustaining discharge.

Independent discharge.

Experience shows that in gases, as the potential difference between the electrodes increases, starting from a certain value, the current increases again. This means that additional ions appear in the gas beyond those formed due to the action of the ionizer. The current strength can increase by hundreds and thousands

times, and the number of ions generated during the discharge process can become so large that an external ionizer will no longer be needed to maintain the discharge. If you remove the external ionizer, the discharge will not stop. Since the discharge does not require an external ionizer to maintain it, it is called an independent discharge.

Glow discharge. At low temperatures, a glow discharge is observed in the tube. To excite a glow discharge, a voltage between the electrodes of several hundred volts is sufficient. During a glow discharge, almost the entire tube, with the exception of a small area near the cathode, is filled with a uniform glow, called a positive column.

Glow discharge is used in advertising tubes. The positive column in argon has a bluish-greenish color.

Electric arc. When two carbon rods come into contact

at the point of their contact, due to the high resistance, a large amount of heat is released. The temperature rises so much that thermionic emission begins. As a result, when the carbon electrodes move apart, a discharge begins between them. A column of brightly glowing gas appears between the coals—an electric arc (Fig. 193). The conductivity of the gas in this case is significant even at atmospheric pressure, since the number of electrons emitted by the negative electrode is very large.

If you increase the current during a glow discharge, the temperature of the cathode due to ion bombardment will increase so much that an arc discharge will begin. Thus, for an arc discharge to occur, it is not necessary that the electrodes be brought closer together beforehand.

An arc discharge is a powerful light source; it is used in spotlights.

Other types of self-discharge. At atmospheric pressure, near pointed sections of a conductor carrying a large electric charge, a gas discharge is observed, the luminous region of which resembles a crown. This discharge, called corona, is caused by a high (about 3 * 106 V/m) electric field strength near the charged tip.

At very low temperatures, all substances are in a solid state. Heating causes a substance to change from a solid to a liquid state. A further increase in temperature leads to the transformation of liquid into gas.

At sufficiently high temperatures, gas ionization begins due to collisions of rapidly moving atoms or molecules. The substance goes into a new state,

called plasma. Plasma is a partially or fully ionized gas in which the densities of positive and negative charges are almost identical.

Properties of plasma.

1. Due to their high mobility, charged plasma particles easily move under the influence of electric and magnetic fields.

2. Coulomb forces act between plasma particles and decrease relatively slowly with distance.

3. Each particle interacts with a large number of surrounding particles at once. Plasma particles can participate in ordered movements.

4. Plasma conductivity increases as the degree of ionization increases. At high temperatures, plasma conductivity approaches that of superconductors.

Ticket No. 20

1 Magnetic permeability. Permanent magnets can be made from only a few substances, but all substances placed in a magnetic field become magnetized, that is, they themselves create a magnetic field. Due to this, the magnetic induction vector B in a homogeneous medium differs from the vector B at the same point in space in a vacuum.

The relationship characterizing the magnetic properties of the medium is called magnetic

permeability of the environment.

In a homogeneous medium, the magnetic induction is equal to: where m is the magnetic permeability of the given medium.

The magnetic properties of any body are determined by closed electric currents inside it.

Paramagnetic substances are substances that create a weak magnetic field in the same direction as the external field. The magnetic permeability of the strongest paramagnetic substances differs little from unity: 1.00036 for platinum and 1.00034 for liquid oxygen. Diamagnets are substances that create a field that weakens an external magnetic field. Silver, lead, and quartz have diamagnetic properties. The magnetic permeability of diamagnetic materials differs from unity by no more than ten thousandths.

Ferromagnets and their applications. By inserting an iron or steel core into a coil, you can increase the magnetic field it creates many times over without increasing the current in the coil. This saves energy. The cores of transformers, generators, electric motors, etc. are made of ferromagnets.

When the external magnetic field is turned off, the ferromagnet remains magnetized, i.e., it creates a magnetic field in the surrounding space. The ordered orientation of elementary currents does not disappear when the external magnetic field is turned off. This is why permanent magnets exist.

Permanent magnets are widely used in electrical measuring instruments, loudspeakers and telephones, sound recording devices, magnetic compasses, etc.

Ferrites are widely used - ferromagnetic materials that do not conduct electric current. They are chemical compounds of iron oxides with oxides of other substances. The first ferromagnetic material known to people—magnetic iron ore—is ferrite.

I Curie temperature. At a temperature higher than a certain one defined for a given ferromagnet, its ferromagnetic properties disappear. This temperature is called the Curie temperature. If you heat a magnetized nail too much, it will lose its ability to attract iron objects. The Curie temperature for iron is 753 °C, for nickel 365 °C, and for cobalt 1000 °C. There are ferromagnetic alloys with a Curie temperature of less than 100°C.

F=1 C/V

Information about the work “Tickets in Physics”

Section: Physics Number of characters with spaces: 70190 Number of tables: 0 Number of images: 0

Similar works

Sample exam papers in physics (grade 11)

5021

0

0

… . The effect of a magnetic field on an electric charge and experiments confirming this effect. Magnetic field induction. 2) Laboratory work “Measuring air humidity”. Ticket No. 16 1) Semiconductors. Intrinsic and impurity conductivity of semiconductors. Semiconductor devices. 2) The task of using isoprocess graphs. Ticket No. 17 1) Electromagnetic induction. Magnetic flux. ...

In-depth exam papers in physics and answers (grade 11)

83041

0

0

... are called semiconductors. They did not attract attention for a long time. One of the first to start researching semiconductors was the outstanding Soviet physicist Abram Fedorovich Ioffe. Semiconductors turned out to be not just “bad conductors,” but a special class with many remarkable physical properties that distinguish them from both metals and dielectrics. To understand the properties of semiconductors,...

Physics tickets with solutions 11th grade

10419

17

38

... heat will be released in the outer part of the circuit in 1 s. Given: Solution: r=4 Ohm ε=12 V R=8 Ohm t=1 c Q=I2Rt Q=12х8х1=8 J Q — ? Answer: 8 J. 2. 3. Ticket No. 20 Laboratory work “Measuring the power of an incandescent lamp” Equipment: ...

Answers to exam papers in physics, grade 11 (answers to 29 tickets)

71210

0

40

... current and voltage is equal to the product of the effective values ​​of current and voltage: P = IU. P = I2R ; R = P/I2(active resistance). Um = ImLω; Xl = Um/Im = Lω Im = UmωC; Xc = Um/Im = 1/ωC ​​Ticket No. 11 1. Newton’s second law establishes a connection between the kinematic characteristic of movement - acceleration, and the dynamic characteristics of interaction - forces. , or in …

Conductors and dielectrics

Some divide the world into black and white, while we divide it into conductors and dielectrics.

• Conductors are materials through which electric current passes. The best conductors are metals.
• Dielectrics are materials through which current does not pass. Easy!

Just because a dielectric doesn't conduct electricity doesn't mean it can't store charge. The accumulation of charge does not depend on the ability to transfer it.

Current direction

Previously, in physics textbooks they wrote this: once upon a time they decided that the current was directed from plus to minus, and then they found out that electrons flow through the wires. But these electrons are negative, which means they cannot go to minus. But since we’ve already agreed on the direction, let’s leave it as it is. The question then arose for everyone: why can’t the direction of the current be changed? But no one received an answer.

Now they write it a little differently: positive particles flow along the conductor from plus to minus, and the current is directed there. No one has any questions here.

In what materials does current occur?

The processes of electric current formation in various environments have their own characteristics:

1. In metals, charge is transported by free negatively charged particles - electrons. The transfer of the substance itself does not occur - the metal ions remain in their nodes of the crystal lattice. When heated, the chaotic vibrations of ions near the equilibrium position intensify, which interferes with the ordered movement of electrons—the conductivity of the metal decreases.
2. In liquids (electrolytes) charge carriers are ions - charged atoms and disintegrated molecules, the formation of which is caused by electrolytic dissociation. Ordered movement in this case represents their movement towards oppositely charged electrodes, on which they are neutralized and deposited.
   

   Cations (positive ions) move towards the cathode (negative electrode), anions (negative ions) move towards the anode (positive electrode). As the temperature rises, the conductivity of the electrolyte increases, as the number of molecules decomposed into ions increases.

3. Plasma is formed in gases Charged particles are ions, positive and negative, and free electrons formed under the influence of an ionizer.
4. In a vacuum, electric current exists in the form of a stream of electrons that move from the cathode to the anode.
5. In semiconductors, directional movement involves electrons moving from one atom to another, and the resulting vacancies - holes, which are conventionally considered positive.

Abrahamyan Evgeniy Pavlovich

Associate Professor, Department of Electrical Engineering, St. Petersburg State Polytechnic University

Ask a Question

At low temperatures, semiconductors have properties similar to insulators, since electrons are occupied by covalent bonds of atoms in the crystal lattice. As the temperature increases, the valence electrons receive enough energy to break bonds and become free. Accordingly, the higher the temperature, the better the conductivity of the semiconductor.

Watch the video below for a detailed explanation of electric current:

The occurrence of current in various materials

Current source

The water in the hose comes from a water supply, a spring with water in the ground - in general, not from nowhere. Electric current also has its own source.

The source can be, for example, a galvanic cell (a conventional battery). A battery works based on chemical reactions inside it. These reactions release energy, which is then transferred to the electrical circuit.

Any source necessarily has poles - “plus” and “minus”. The poles are its extreme positions. Essentially the terminals to which an electrical circuit is connected. Actually, the current flows from “+” to “-”.

Designations of direct and alternating current: AC and DC current

Electrical energy accompanies us every step of the way. Without it, the life of any person is unthinkable. Throughout our lives, we encounter manifestations of electricity to one degree or another. This happens more often, as a rule, when electrical appliances break down. And in order to understand their structure and circuits, it is useful to know that alternating and direct current are designated as AC and DC current.

• Sources of electrical energy
• Designations on diagrams and devices
• Application areas of DC voltage

Sources of electrical energy

Initially, the sources of electricity were only disposable chemical galvanic cells. Later, reusable batteries appeared. It is noteworthy that the polarity of chemical sources is not able to change on its own. In order to obtain constant voltage on an industrial scale, generators and sometimes solar panels are used. Electronic equipment, in turn, is powered by an alternating voltage network, and power supplies are used to obtain constant voltage. The alternating current is reduced to the required values ​​using transformers and subsequently rectified. At the same time, the ripple frequency is reduced by smoothing filters, stabilizers and voltage regulators.

Switching power supplies are common in the modern world. In them, the pulsation frequency of the output electricity is smoothed out by integrating elements. They concentrate electrical energy and transfer it to the load. The result is the required constant voltage.

Electrolytic capacitors can also condense electrical energy. When such a capacitor is discharged, an alternating current appears in the external circuit. If it is discharged through a resistor, then a gradually decreasing (unidirectional) alternating current occurs. When using an induction coil, a bidirectional alternating current is generated in the circuit. Electrolytic capacitors can have enormous capacitances , reaching hundreds of microfarads. When such capacitors are discharged through a large resistance, the electricity decreases more slowly and a constant voltage flows in the external circuit.

There are also combinations of capacitors and chemical sources - ionistors. They have the ability to accumulate and release significant amounts of electricity. A typical example is electric cars.

Designations on diagrams and devices

It is generally accepted that the direction of electricity goes from a contact with a plus sign to a contact with a minus sign.
Places with high potentials are called “positive pole” and are indicated by a + (plus) sign. Points with lower potentials, accordingly, are called the “negative pole” and are designated by the sign - (minus).

Initially, it was accepted that the electrical insulation of positive wires is red, while wires with a minus sign are painted blue or black.

Symbols on electrical appliances: - or =. Unidirectional electricity (including constant electricity) is denoted by the Latin alphabet DC, or the Unicode symbol is used - U+2393.

The abbreviations AC and DC are firmly rooted in everyday use and are used along with the usual names “variable” and “constant”:

• designation of direct voltage (—) or DC (Direct Current);
• AC sign (

) or AC (Alternating Current) - designation of alternating current.

Application areas of DC voltage

The use of constant voltage allows the electrical energy to be transferred to be increased and then transferred between power systems that use alternating current of different frequencies (for example, 50 and 60 hertz).
Direct current is also actively used in transport. Constantly excited electric motors are used in various mechanisms:

• electric locomotives;
• electric trains;
• trams;
• trolleybuses;
• lifts, etc.

There was also constant tension in other areas of science and technology. It is widely used in this way:

• In almost all electronic circuits as power supply.
• Galvanic cells and batteries charge electronic devices: flashlights, toys, batteries in tools, and others.
• In industrial electrolytic plants, for example, aluminum, magnesium, potassium, and chlorine are obtained from solutions and molten salts.
• In galvanization and galvanoplasty.
• For electric arc and electric gas welding.
• In on-board networks of cars.
• On some types of vessels - icebreakers, submarines, diesel-electric ships.
• In medicine. For example, electrophoresis is the introduction of drugs into the body using electricity.

Electricity accompanies us everywhere: at work and at home. It’s scary to even imagine for a minute what will happen to humanity if it suddenly loses electrical energy.

Ammeter

We know where the current is directed, how the current is measured, how to calculate it, knowing the charge and the time during which this charge has passed. All that remains is to measure.

A device for measuring current is called ammeter . It is included in an electrical circuit in series with the conductor in which the current is measured .

Ammeters come in very different operating principles: electromagnetic, magnetoelectric, electrodynamic, thermal and induction - and these are just the most common.

We will only consider the principle of operation of a thermal ammeter, because to understand the principle of operation of other devices it is necessary to know what a magnetic field and coils are.

The thermal ammeter is based on the property of current to heat wires. It is designed like this: a thin wire is attached to two fixed clamps. This thin wire is pulled down by a silk thread connected to a spring. Along the way, this thread loops around the fixed axis on which the arrow is attached. The measured current is supplied to the fixed clamps and passes through the wire (arrows in the figure show the current path).

Under the influence of current, the wire will heat up slightly, causing it to elongate, as a result of which the silk thread attached to the wire will be pulled back by a spring. The movement of the thread will turn the axis, and therefore the arrow. The arrow will indicate the measurement value.