Strength and direction of electric current presentation. Presentation on the topic "direction of electric current"

Basic questions 1. Electric current. Current strength. Direction of current 2. Resistance of conductors. Resistivity 3. Ohm's Law for a section of a circuit direct current. Current-voltage characteristics of conductors. 4. Methods of connecting conductors 5. Current sources. Outside forces. EMF of a current source 6. Ohm's law for a closed circuit with a current source. 7.Work and current power 8.Measurement of current and voltage. Shunts and additional resistances

Andre Ampère () Introduced the concept of “electric current” into physics

Electric current is the ordered (directed) movement of charged particles. To obtain electric current in a conductor, it is necessary to create an electric field in it. In order for an electric current to exist in a conductor for a long time, it is necessary to maintain an electric field in it all this time. This is done using current sources. For direction electric current the direction of movement of positive charges is not accepted. Therefore, the electric current in the external circuit is directed from the positive to the negative pole of the current source.

For the occurrence and maintenance of electric current, the following conditions are necessary: ​​1) the presence of free current carriers (free charges); 2) the presence of an electric field that creates the ordered movement of free charges; 3) free charges, in addition to Coulomb forces, must be acted upon by external forces of a non-electrical nature; these forces are created by various current sources (galvanic cells, batteries, electric generators, etc.) 4) the electric current circuit must be closed.

Ampere Andre Marie. Years of life: French physicist and mathematician. He created the first theory that expressed the connection between electrical and magnetic phenomena. Ampere came up with a hypothesis about the nature of magnetism; he introduced the concept of “electric current” into physics.

The unit of current is taken to be the current at which sections of parallel conductors 1 m long interact with a force of 2 * 10 -7 N (0, N) 1 AMPERE

When an electrical circuit is closed, an electric current occurs. Free electrons, under the influence of electric field forces, move along the conductor. In their movement, electrons collide with the atoms of the conductor and give them a supply of their kinetic energy. The speed of electron movement continuously changes: when electrons collide with atoms, molecules and other electrons, it decreases, then under the influence of an electric field it increases and decreases again during a new collision. As a result, a uniform flow of electrons is established in the conductor at a speed of several fractions of a centimeter per second. Consequently, electrons passing through a conductor always encounter resistance to their movement from its side.

Substance Resistivity, Ohm*mm2/m Silver 0.016 Copper 0.017 Gold 0.024 Aluminum 0.028 Iron 0.10 Tin 0.12 Constantan 0.5 Nichrome 1.1 Electrical resistance of the conductor depends on: 1) the length of the conductor, 2) the cross-section of the conductor , 3) conductor material, 4) conductor temperature. The resistance of a conductor 1 m long, with a cross section of 1 mm 2 is called resistivity

Om Georg Years of life(). German physicist. He discovered theoretically and confirmed experimentally a law expressing the relationship between current strength in a circuit, voltage and resistance.

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Devices using which create an electric field inside conductors are called current sources. The current source consists of two conductors, one conductor maintains a constant positive potential, the other a constant negative potential. In the external circuit, positive electrical charges move under the influence of Coulomb forces. To maintain a constant positive potential on the left conductor, positive charges inside the current source must move against Coulomb forces, this is only possible when they are acted upon by forces of non-electrical origin - external forces. The external forces must be greater than the Coulomb forces and directed in the opposite direction.

Extraneous forces arise as a result of friction Until the end of the 18th century, all technical sources of current were based on electrification by friction. The most effective of these sources has become the electrophore machine (the machine's disks are rotated in opposite directions. As a result of the friction of the brushes on the disks, charges of the opposite sign accumulate on the machine's conductors) Electrophore machine

The first electric battery appeared in 1799. It was invented by the Italian physicist Alessandro Volta () Italian physicist, chemist and physiologist, inventor of a source of direct electric current. His first current source, the “voltaic column,” was built in strict accordance with his theory of “metallic” electricity. Volta alternately placed several dozen small zinc and silver circles on top of each other, placing paper moistened with salted water between them.

The zinc atom gives up two electrons, becomes a positive zinc ion and goes into solution. Electrons leave the zinc electrode through the wires, thereby removing the negative charge from it, which could prevent further dissolution of the electrode. The electrons fall on the copper electrode, where a copper ion fits and, having received two electrons, settles as a neutral atom on the copper electrode.

Third-party forces arise under the influence of light Solar battery When certain substances are illuminated with light, a current appears in them, light energy is converted into electrical energy. In this device, charges are separated under the influence of light. Solar batteries are made from photocells. Used in solar powered, light sensors, calculators, video cameras. Photocell

Third-party forces arise under the influence of heat Thermocouple Thermocouple (thermocouple) - two wires from different metals must be soldered at one end, then the place of the junction is heated, then a current arises in them. The charges are separated when the junction is heated. Thermal elements are used in temperature sensors and in geothermal power plants as a temperature sensor. Thermocouple

When charges move along a direct current circuit, external forces acting inside the sources do work. A physical quantity equal to the ratio of the work of external forces to move a charge from the negative pole of a current source to the positive pole to the value of this charge is called the electromotive force (EMF) of the source
The EMF of the current source is spent on overcoming the resistance of the internal and external circuits with electric current. That part of the EMF that is spent on overcoming the resistance of the external circuit is called the voltage on the external resistance, the part of the EMF that is spent on overcoming the resistance inside the current source is called the voltage on the internal resistance.

Each pole of the intermediate source is connected to one pole of the previous and subsequent sources. E.m.f. battery is equal to the algebraic sum of emf. individual sources. i The sign is determined arbitrarily according to the selected direction of traversal of the contour (see figure). If, during the bypass, we move from the negative pole to the positive one, then For example, in the figure shown, the internal resistance of the battery is r = r + r r n

Magnetoelectric, electromagnetic, electrodynamic and electrostatic voltmeters are measuring VOLTAGE MEASUREMENTS ON A PART OF AN ELECTRIC CIRCUIT To measure voltage, there is a special measuring device voltmeter. Symbol for a voltmeter on an electrical diagram: When connecting a voltmeter to an electrical circuit, two rules must be observed: 1. The voltmeter is connected in parallel to the section of the circuit on which the voltage will be measured; 2. We observe polarity: the “+” of the voltmeter is connected to the “+” of the current source, and the “minus” of the voltmeter is connected to the “minus” of the current source. ___ To measure the voltage of a power source, a voltmeter is connected directly to its terminals.

Shunts and additional resistances. A shunt is a resistance connected in parallel to an ammeter (galvanometer) to expand its scale when measuring current. If the ammeter is designed for current I 0, and with it it is necessary to measure a current strength n times greater than the permissible value, then the resistance of the connected shunt must satisfy the following condition: Additional resistance - resistance connected in series with the voltmeter (galvanometer) for expansion its scale when measuring voltage. If the voltmeter is designed for voltage U 0, and it is necessary to measure a voltage that exceeds n times the permissible value, then the additional resistance must satisfy the following condition:

1. Formula for determining current strength? AI=qt BI=t/q BI=q/t GI=qt 2 2. What is the name of the device for measuring the magnitude of current? AAmmeter BVoltmeter VDynamometer GGalvanometer 3. What formula can you use to determine voltage? АU=A/I БУ=A/q ВU=q/A ГU=Aq 4. Voltage unit? AAmpere Bohm VCoulomb GVolt 5. A device used to change the resistance in a circuit? AREsistor BKey VReostat GS Among the answers there is no correct answer 6. Which formula determines the resistance of a conductor? AR=рl/s BR=sр/l VR=s/рl GR=l/рs Answer the test questions.

Electricity. Current strength

When charged particles move in a conductor, transfer occurs electric charge from one place to another. However, if charged particles undergo random thermal motion, such as free electrons in a metal, then charge transfer does not occur. An electric charge moves through the cross section of a conductor only if, in addition to the random movement, the electrons participate in an ordered movement. In this case, they say that an electric current is established in the conductor.

Electric current is the ordered (directed) movement of charged particles.
Electric current arises from the ordered movement of free electrons and ions. If you move a generally neutral body, then, despite the ordered movement of a huge number of electrons and atomic nuclei, no electric current arises. The total charge transferred through any cross section of the conductor will be equal to zero, since charges of different signs move with the same average speed.

Electric current has a certain direction. The direction of current is taken to be the direction of movement of positively charged particles. If the current is formed by the movement of negatively charged particles, then the direction of the current is considered opposite to the direction of movement of the particles. (This choice of current direction is not very successful, since in most cases the current represents the movement of electrons - negatively charged particles. The choice of current direction was made at a time when nothing was known about free electrons in metals.

We do not directly see the movement of particles in a conductor. The presence of electric current must be judged by the actions or phenomena that accompany it. Firstly, the conductor through which the current flows heats up. Secondly, electric current can change the chemical composition of the conductor, for example, releasing its chemical components (copper from a solution of copper sulfate, etc.). Thirdly, the current exerts a force on neighboring currents and magnetized bodies. This action of current is called magnetic. Thus, a magnetic needle near a current-carrying conductor rotates. The magnetic effect of current, in contrast to chemical and thermal, is the main one, since it manifests itself in all conductors without exception. The chemical effect of current is observed only in solutions and melts of electrolytes, and heating is absent in superconductors.
Effect of current

If an electric current is established in a circuit, this means that an electric charge is constantly being transferred through the cross-section of the conductor. The charge transferred per unit time serves as the main quantitative characteristic of current, called current strength. If a charge ∆q is transferred through the cross section of a conductor during a time ∆t, then the current strength is equal to:
Current strength

Thus, the current strength is equal to the ratio of the charge ∆q transferred through the cross section of the conductor during the time interval ∆t to this time interval. If the current strength does not change over time, then the current is called constant. Current strength, like charge, is a scalar quantity. It can be both positive and negative. The sign of the current depends on which direction along the conductor is taken as positive. Current strength I > 0 if the direction of the current coincides with the conditionally chosen positive direction along the conductor. Otherwise I

Ammeter. Current measurement

Prezentacii.com

• establish experimentally that the current strength is the same in all parts of the circuit

• get acquainted with the new ammeter device
• develop experimental skills (put forward and

• justify a hypothesis, plan an experiment on it
• verification)

Goals and objectives

• What do we call electric current?
• Current sources
• Current strength
• What is the formula for current strength?
• Current units
• Wonderful Scientists

Electricity

Electricity- ordered (directed) movement of charged particles.

Conditions for the existence of current:

Availability of free charge carriers;

Presence of an electric field.

Current direction :

The direction of the current is taken to be the direction of the ordered movement of positively charged particles. The direction of the current coincides with the direction of the electric field strength causing this current.

Direct electric current

Constant electric current is a current whose strength does not change over time.

Direct current is widely used in electrical diagrams cars, as well as in microelectronics, etc.

A current source is a device that separates positive and negative charges.

battery, batteries, generator...

Current sources

Current strength

The current strength at a given moment in time is a scalar physical quantity equal to the limit of the ratio of the electric charge passing through the cross section of the conductor to the time interval of its origin.

I – current strength, (A)

q – charge, (C)

t – time, (s)

I = q:t

Wonderful Scientists

Ampere Andre Marie. Years of life: 1775-1836. French physicist and mathematician. He created the first theory that expressed the connection between electrical and magnetic phenomena. Ampere came up with a hypothesis about the nature of magnetism; he introduced the concept of “electric current” into physics.

Current measuring device- Ammeter. The circuit is connected in series.

• Formulate a goal;
• Put forward and justify a hypothesis;
• Draw up a plan for conducting an experiment;
• Select the necessary equipment;
• To conduct an experiment;
• Analyze the results;
• Draw conclusions.

Science experiment

Purpose: to determine the current strength in different parts of the circuit.

Hypothesis: do you think the ammeter readings in all parts of the circuit will be the same?

• Determine the current flowing through the cross section of the lamp spiral
• Instruments: battery, ammeter, conductor, wires, key, lamp.

Practical work

Work in groups.

Assembling the electrical circuit in the following sequence:

1 group - battery, key, ammeter, lamp.

Group 2 - battery, ammeter, key, lamp.

Group 3 - battery, lamp, ammeter, key.

Study results table

№ groups

Prezentacii.com

• The current strength in all parts of the circuit is the same when connected in series.

• Primary consolidation.
• 1. Determine by results practical work, how much charge passes through the cross section of the conductor in 2 minutes.
• 2. How many electrons pass through in 2 minutes?

Independent work

• 1. Guys, what questions do you have for me about the lesson?
• 2. What new did you learn in class today?
• 3. What aroused your special interest in the lesson?
• 4. Where does the acquired knowledge find practical application?
• 5. How do you evaluate your activities in class?

Reflection of activity.

• 38. UPR14(2); exr15(2); No. 1277(L) - optional.
• Additional material on the topic “The use of electrical measuring instruments in the basis of agricultural machinery.

Homework