Ohm's law for a complete circuit. Presentation on the topic: Electromotive force

Ohm's law for a closed circuit. Current sources. To receive in an electrical circuit direct current the charges must be subject to some force other than the (Coulomb) forces of the electrostatic field. Such forces are called third-party forces. A characteristic of the action of external forces is electromotive force (EMF), which is numerically equal to the work of external forces to move a single positive (test) charge along a closed circuit or, in other words, is determined by the work of external forces to move a charge along a closed circuit, related to the value of this charge, EMF is measured in volts. The section of the circuit where there is an emf is called a non-uniform section of the circuit. Inside the source, charges move against Coulomb forces under the influence of external forces, and throughout the rest of the circuit they are driven by an electric field. Such sources can be galvanic cells, batteries, DC electric generators. The emf of the current source is equal to the electrical voltage at its terminals when the circuit is open. From the law of conservation of energy it follows that the work of external forces is equal to the amount of heat released in the circuit Q = I2? R0? ?t where R0 = R + r is the total resistance of the circuit, and R is the resistance of the external circuit, r is the internal resistance of the source. Then? ? I? ?t = I2? (R + r) ?t.

“Georg Ohm” - In particular, he became the best billiard player and speed skater at the university, and became interested in dancing. Georg Ohm was born on March 16, 1787 in Erlang in the family of a hereditary mechanic. Om plunged into sports with passion. Since 1825, Ohm began to study galvanism. Tube rheostat. Ohm's law for a section of a circuit.

“Current in a circuit” - From which pole of the current source and to which is the direction of the current usually considered? What parts does an electrical circuit consist of? What experiment shows the dependence of current on voltage? How does the current in a conductor depend on the voltage at the ends of the conductor? What needs to be created in a conductor for a current to arise and exist in it?

“Kirchhoff's Law” - Idle mode of the energy source (XX). Balance of voltages in any circuit circuit. Kirchhoff's first law. Nominal operating mode of the energy source. Calculation of power transmitted to the load. Analytical expression of Kirchhoff's second law. Energy source short circuit mode. Kirchhoff's laws and operating modes of energy sources.

“Om Tok” - Worked as a teacher in Gottstadt (Switzerland). As the resistance of the conductor increases, the current decreases. German physicist. Dependence of current on voltage I (U) Dependence of current on resistance I(R). Lesson abstracts on the topic “Ohm’s Law for a section of a circuit.” Last years Om devoted his life to research in the field of acoustics.

“Current Characteristics” - Series connection of conductors. Voltage measurement. Electromotive force. Dependence of current on voltage and resistance. Conditions for the existence of current. Resistance of metals. Current strength. Current work. Characteristics of electric current. Current strength is a physical quantity. Parallel connection of conductors.

“Ohm's law for a section of a circuit” - The power released is maximum. Ohm's law in differential form. Work and current power. Kirchhoff's rules for branched chains. Kirchhoff's second rule (generalization of Ohm's law for a branched chain). Ohm's law. Ohm's law in differential form. Efficiency of the current source. Dividing work by time, we obtain an expression for power.

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Third-party forces Electromotive force External part of the circuit Internal part of the circuit Current source Concepts and quantities:

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Laws: Ohm for a closed circuit

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Short circuit current Electrical safety rules in various rooms Fuses Aspects of human life:

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Electromotive force. Ohm's law for a closed circuit. Current sources. To obtain direct current in an electrical circuit, the charges must be subject to some forces other than the (Coulomb) forces of the electrostatic field. Such forces are called third-party forces. A characteristic of the action of external forces is electromotive force (EMF), which is numerically equal to the work of external forces to move a single positive (test) charge along a closed circuit or, in other words, is determined by the work of external forces to move a charge along a closed circuit, related to the value of this charge, EMF is measured in volts. The section of the circuit where there is an emf is called a non-uniform section of the circuit. Inside the source, charges move against Coulomb forces under the influence of external forces, and throughout the rest of the circuit they are driven by an electric field. Such sources can be galvanic cells, batteries, DC electric generators. The emf of the current source is equal to the electrical voltage at its terminals when the circuit is open. From the law of conservation of energy it follows that the work of external forces is equal to the amount of heat released in the circuit Q = I2 ∙ R0 ∙ ∆t where R0 = R + r is the total resistance of the circuit, and R is the resistance of the external circuit, r is the internal resistance of the source. Then ε ∙ I ∙ ∆t = I2 ∙ (R + r) ∆t

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From here we obtain Ohm's law for a complete circuit: The current strength in a complete circuit is equal to the electromotive force of the source divided by the sum of the resistances of the external and internal sections of the circuit. In the case when the resistance of the external circuit tends to zero, a short circuit current appears in the circuit - the maximum possible current in a given source. Short circuit current - the maximum current that can be obtained from a given source with electromotive force and internal resistance r. For sources with low internal resistance, the short circuit current can be very high and cause destruction of the electrical circuit or source. For example, lead-acid batteries used in automobiles can have short-circuit currents of several hundred amperes. Short circuits in lighting networks powered from substations (thousands of amperes) are especially dangerous. To avoid the destructive effects of such large currents, fuses or special circuit breakers are included in the circuit. Galvanic cells have a small short-circuit current and therefore are not very dangerous for them.

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Hello!!! I hope you are in a great mood.

Lesson objectives Educational: to promote students' knowledge of Ohm's law for a complete chain. Introduce the concept of electromotive force, explain the content of Ohm's law for a complete closed circuit. To promote the development of logical thinking, independence, the ability to draw conclusions, analyze, generalize. 3. Ensure sanitary and hygienic standards during the lesson, preventing fatigue through changing student activities. Educational: practicing methods of educational and cognitive activity for students; developing the ability to apply acquired knowledge in mathematics and physics lessons when solving standard problems and explaining theoretical material; Developmental: developing students’ independence in solving applied problems and in experimental research; development of students’ creative abilities and cognitive interest;

Lesson objectives: Educational: formation key competencies students using modern pedagogical technologies (technology of student-centered learning, ICT, technology of differentiated learning, problem-search technology, project method) and the introduction of a competent approach to educational process Developmental: development of independent critical thinking and communication skills of students when working in shift groups Educational: providing pedagogical assistance in choosing the direction of further education

Georg Ohm Yes, electricity is my soulmate, It will warm you, entertain you, add light. Experiments conducted by Ohm showed that current, voltage and resistance are interconnected quantities.

Repetition

Electric current is created Unit of current strength Unit of voltage Unit of resistance Formula of Ohm's Law for a section of a circuit Current strength is measured using the formula Device for measuring current strength Device for measuring voltage Device whose resistance can be adjusted An ammeter is included in the circuit Formula for finding resistance The direction of the current is taken to be the direction of movement moving charged particles Ampere Volt Ohm I=U/R I = q/ t Ammeter Voltmeter Rheostat in series R= ρ l/S positively charged particles

When the conductors are connected in series, the total resistance of the circuit is equal to the Sum of all resistances When the conductors are connected in parallel, the current strength in the circuit... Is equal to the sum of the currents When the conductors are connected in parallel, the voltage in the circuit... Is the same on each conductor With a change in voltage or current in the circuit, the resistance... Does not change

Calculate the current strength in the spiral of an electric stove connected to a network with a voltage of 220V, if the resistance of the spiral is 100 Ohms. 2. The current passing through the lamp filament is 0.3 A, the lamp voltage is 6 V. What is the electrical resistance of the lamp filament? 3. The current in the circuit is 2 A, the resistor resistance is 110 Ohms. What is the voltage in the circuit? 2.2 A 20 Ohm 220 V

Updating knowledge. 1. Why did the extension cord work properly before, but then suddenly it caught fire? 2. What phenomenon occurred? 3. What law needs to be studied for a theoretical explanation of this phenomenon?

Conclusion 1: Ohm's law for a section of a circuit: the current strength in a section of a circuit is directly proportional to the voltage at the ends of this section and inversely proportional to its resistance.

Current-voltage characteristic of a conductor A graph expressing the dependence of current on voltage is called the current-voltage characteristic of a conductor.

Conclusion 2: Ohm's law for a complete circuit: Ohm's law for a circuit section considers only this section of the circuit, and Ohm's law for a complete circuit considers the total resistance of the entire circuit. Both Ohm's laws show the dependence of current strength on resistance - the greater the resistance, the less current strength and vice versa.

I took pieces of cylindrical wire of arbitrary length from various materials and placed them alternately in a circuit... Georg Ohm... Ohm's discovery was received with skepticism in scientific circles. This was reflected both in the development of science - for example, the laws of current distribution in branched circuits were derived by G. Kirchhoff only twenty years later - and in Ohm’s scientific career

Question Ohm's law for a section of a chain Ohm's law for a complete chain 1. What quantities are connected by Ohm's law? 2. How is Ohm's law formulated? 3. Write the formula Ohm's law 4. Write the units of measurement 5. Conclusion

Any non-electrostatic forces acting on charged particles are usually called external forces. That. In addition to Coulomb forces, external forces act on the charges inside the source and carry out the transfer of charged particles against the Coulomb forces.

E F k → F st → e F k → A B Forces of electrostatic origin cannot create and maintain a constant potential difference at the ends of the conductor (electrostatic forces are conservative forces). A current source is required in which forces of non-electrostatic origin act, capable of maintaining the potential difference at the ends of the conductor

Ohm's law for a complete circuit The current strength in a circuit is directly proportional to the electromotive force of the current source and inversely proportional to the sum of the electrical resistances of the external and internal sections of the circuit. Current strength (A) EMF-electromotive force of the current source (V) Load resistance (Ohm) Internal resistance of the current source (Ohm)

If there is no EMF acting on a section of the circuit (there is no current source) U = φ 1 - φ 2 If the ends of the section containing a current source are connected, then their potential will become the same U = ε In a closed circuit, the voltage on its external and internal sections is equal to the EMF of the source current ε = U ext + U int

Short circuit When short circuit R → 0, current

Calculate the short circuit currents Current source ε, V r, Ohm I short circuit, A Galvanic cell 1.5 1 Battery 6 0.01 Lighting networks 100 0.001 1.5 600 100 000

Types of fuses Fusible Automatic Surge filters Automatic switchboards Automatic switchboard

Problem solution: No. 1 A galvanic cell with an emf E = 5.0 V and an internal resistance r = 0.2 Ohm is connected to a conductor with a resistance R = 40.0 Ohm. What is the voltage U on this conductor? No. 2 A light bulb with resistance R =100 Ohm is connected to a battery with an emf and internal resistance r = 0.5 Ohm. Determine the current strength in the circuit. No. 3 Determine the EMF of a current source with an internal resistance r = 0.3 Ohm, if when connecting resistors R 1 = 10 Ohm and R 2 = 6 Ohm in parallel to the terminals of the current source, the current strength in the circuit is: I = 3 A. IN

Problem solution: No. 1 A galvanic cell with an emf E = 5.0 V and an internal resistance r = 0.2 Ohm is connected to a conductor with a resistance R = 40.0 Ohm. What is the voltage U on this conductor? Answer: U = 4.97 V. No. 2 A light bulb with resistance R = 100 Ohm is connected to a battery with EMF and internal resistance r = 0.5 Ohm. Determine the current strength in the circuit. No. 3 Determine the EMF of a current source with an internal resistance r = 0.3 Ohm, if when connecting resistors R 1 = 10 Ohm and R 2 = 6 Ohm in parallel to the terminals of the current source, the current strength in the circuit is: I = 3 A. B Answer: 0.119 A Answer: 12.15 V

Make an analogy

Test 1 The formula expressing Ohm's law for a closed circuit is written as: a) I = U / R b) c) d)

Test 2. Short circuit current can be calculated using the formula: a) b) c) d)

Test (preparing for the Unified State Exam) 3. The emf of a battery with internal resistance r = 0.2 Ohm, when a resistance R = 5 Ohm is connected to it is equal to... Current I = 1.5 A flows through the circuit. A) 3 V B) 12 V C) 7.8 V D) 12.2 V

Test (preparing for the Unified State Exam) 4. What internal resistance does a current source with emf B have if, when it is closed in parallel by resistors Ohm and Ohm, current I = 2 A flows in the circuit. A) 26 Ohm B) 1.45 Ohm C) 12 Ohm D) 2.45 Ohm

Answers to the test: No. 1 No. 2 No. 3 No. 4 D C C B

Reflection A. I liked everything. I understood everything B. I liked it, but I didn’t understand everything C. Everything was the same as always, nothing unusual D. I didn’t like it

Read homework § 107-108, exercise 19 No. 5,6. Problem (at home): When connecting a light bulb to a battery of elements with an emf of 4.5 V, the voltmeter showed a voltage across the light bulb of 4 V, and the ammeter showed a current of 0.25 A. What is the internal resistance of the battery? Thank you for the lesson!

Current source characteristics

The role of the current source To ensure that the electric current in the conductor does not stop, it is necessary to use a device that would transfer charges from one body to another in the direction opposite to the one in which the charges are transferred electric field. A current source is used as such a device.

A current source is a device in which any type of energy is converted into electrical energy. Exist different kinds current sources: Mechanical current source - mechanical energy is converted into electrical energy. These include: an electrophore machine (the disks of the machine are driven into rotation in opposite directions. As a result of the friction of the brushes on the disks, charges of the opposite sign accumulate on the conductors of the machine), a dynamo, and generators. Thermal current source - internal energy is converted into electrical energy. For example, a thermoelement - two wires made of different metals need to be soldered at one end, then the junction is heated, then a voltage will appear between the other ends of these wires. Used in temperature sensors and geothermal power plants.

Light current source - light energy is converted into electrical energy. For example, a photocell - when certain semiconductors are illuminated, light energy is converted into electrical energy. Solar batteries are made from photocells. They are used in solar batteries, light sensors, calculators, and video cameras. Chemical current source - as a result of chemical reactions, internal energy is converted into electrical energy. For example, a galvanic cell - a carbon rod is inserted into a zinc vessel. The rod is placed in a linen bag filled with a mixture of manganese oxide and carbon. The element uses a flour paste with ammonia solution. When ammonia interacts with zinc, the zinc acquires a negative charge, and the carbon rod acquires a positive charge. An electric field arises between the charged rod and the zinc vessel. In such a current source, the carbon is the positive electrode, and the zinc vessel is the negative electrode. A battery can be made from several galvanic cells. Current sources based on galvanic cells are used in household autonomous electrical appliances and uninterruptible power supplies. Batteries - in cars, electric vehicles, cell phones.

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1. A voltage of 10 V is applied to the ends of the circuit. Determine the current in each resistor if R1 = R2 = 2 Ohm, R3 = 9 Ohm. 2. An electric kettle with a power of 150 W is connected to a 220V network. Determine the current strength in its spiral and the resistance of the spiral. Option 2 A conductor 200 m long and with a cross section of 2 mm 2 is connected to a circuit with a voltage of 12 V. What is the current strength in the circuit? Two resistors with a resistance of 10 and 50 Ohms are connected in parallel in the circuit. In the unbranched part of the circuit, the current is 6 A. Determine the voltage across each resistor and the current flowing in each conductor.

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Current source. An electric current can occur if oppositely charged balls or capacitor plates are connected with a metal wire. However, such an electric current turns out to be short-lived: since the shortage and excess of electrons on the plates are compensated by moving electrons, the electric field driving the charges weakens to zero.

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To further maintain current in the conductors, a device called a current source is used. Inside the current source, a redistribution of positive and negative charges occurs, so that an excess of positive and negative charges appears at the two terminals of the current source (the “+” terminal and the “–” terminal). Forces of a non-electrostatic nature that carry out such separation of charges are called extraneous forces. When a metal conductor comes into contact with the terminals of a current source, such a charge distribution is very quickly established on the surface of the wire that a constant electric field of intensity directed along its axis appears inside the conductor. The current strength in the entire conductor becomes constant, charges move along a closed circuit.

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Any current source is usually characterized by the work of external forces Ast, which they perform during such a movement of charge q inside the source. The ratio is called the electromotive force (EMF) of the current source. The EMF is expressed in volts (1 V = 1 J/1 C), as is the potential difference.

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Table "Types of current sources and the principle of their operation" Electrophoric machine Mechanical rotation of non-conducting disks with applied conductive sections, part of which on one of the disks is electrified by friction, leads to the accumulation of charges in a special device called a Leyden jar. Currently used mainly for demonstration experiments requiring controlled generation of large (up to tens of thousands of volts) voltages Galvanic cell Two different materials immersed in a solution or other conductive medium. Due to irreversible chemical reactions occurring at the “solution-solid” boundary, electrons or charged ions accumulate on the electrodes. In galvanic cells, the energy of chemical bonds accumulated during the synthesis of these substances is irreversibly converted into the energy of separated charges

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Solar cell When some semiconductor materials come into contact with metals, light transfers electrons from the metal to the semiconductor. Piezoelectric element When some crystals (for example, quartz) are mechanically deformed, electrons move from one area of ​​the crystal to another

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