Ohm's law for a complete circuit presentation for a lesson in physics (grade 10) on the topic. Presentation on the topic "electromotive force" Ohm's law for a complete circuit presentation
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Slide captions:
Hello!!! I hope you are in a great mood.
Lesson Objectives Educational: To contribute to the formation of students' knowledge of Ohm's Law for a complete circuit. Introduce the concept of electromotive force, explain the content of Ohm's law for a complete closed circuit. Promote the development of logical thinking, independence, the ability to draw conclusions, analyze, generalize. 3. Provide sanitary and hygienic standards during the lesson, prevention of fatigue through a change in the activities of students. Educational: practicing the techniques of educational and cognitive activity among students; the formation of skills to apply the knowledge gained in mathematics and physics lessons when solving standard problems and explaining theoretical material; Developing: development of students' independence in the course of solving applied problems and in experimental search; development of students' creativity and cognitive interest;
Lesson objectives: Educational: the formation of key competencies of students by means of modern pedagogical technologies (technology of student-centered learning, ICT, differentiated learning technology, problem-search technology, project method) and the introduction of a competent approach in the educational process Developing: the development of independent critical thinking and communication skills students when working in shift teams Educational: providing pedagogical assistance in choosing the direction of further education
Georg Ohm Yes, electricity is my soul mate, It will warm, entertain, add light. The experiments carried out by Ohm showed that current strength, voltage and resistance are quantities related to each other.
Repetition
Electric current is created Unit of current unit Voltage unit Resistance unit Ohm's law formula for a section of a circuit Current is measured by the formula A device for measuring current strength A device for measuring voltage A device whose resistance can be adjusted Ammeter is included in the circuit Formula for finding resistance The direction of current is taken as the direction of movement moving charged particles Ampere Volt Ohm I = U / RI = q / t Ammeter Voltmeter Rheostat in series R = ρ l / S of 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 in the circuit is ... Equal to the sum of currents.
Calculate the current strength in the spiral of an electric hotplate connected to a 220V network, 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 resistance of the resistor is 110 Ohm. What is the voltage in the circuit? 2.2 A 20 Ohm 220 V
Knowledge update. 1. Why did the extension cord work properly before, but then suddenly caught fire? 2. What phenomenon happened? 3. What law should be investigated for a theoretical explanation of this phenomenon?
Conclusion 1: Ohm's law for a section of a circuit: the current 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 The 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 section of a circuit considers only a given section of a circuit, and Ohm's Law for a complete circuit considers the total impedance of the entire circuit. Both Ohm's laws show the dependence of current strength on resistance - the greater the resistance, the lower the current strength and vice versa.
I took pieces of cylindrical wire of arbitrary length from various materials and placed them in turn in a chain ... Georg Ohm ... Ohm's discovery was skeptical in scientific circles. This was reflected in the development of science - for example, the laws of distribution of currents in branched circuits were deduced by G. Kirchhoff only twenty years later - and on 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 5. Conclusion
Any non-electrostatic forces acting on charged particles are usually called external forces. That. the charges inside the source, in addition to the Coulomb ones, are acted upon by external forces and carry out the transfer of charged particles against the Coulomb ones.
E F to → F st → e F to → 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 needed in which forces of non-electrostatic origin act, capable of maintaining a potential difference at the ends of the conductor
Ohm's law for a complete circuit The current in a circuit is directly proportional to the electromotive force of the current source and is 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 the EMF does not act on the circuit section (there is no current source) U = φ 1 - φ 2 If the ends of the section containing the 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 In case of 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 protectors Automatic shields Automatic shields
Problem solving: No. 1 Galvanic cell with EMF E = 5.0 V and internal resistance r = 0.2 Ohm is closed to a conductor with resistance R = 40.0 Ohm. What is the voltage U on this conductor? No. 2 To the battery with EMF and internal resistance r = 0.5 Ohm, a light bulb with resistance R = 100 Ohm was connected. Determine the current in the circuit. No. 3 Determine the EMF of the current source with an internal resistance r = 0.3 Ohm, if, when connected to the terminals of the current source, resistors R 1 = 10 Ohm and R 2 = 6 Ohm connected in parallel, the current in the circuit is: I = 3 A. V
Problem solving: No. 1 Galvanic cell with EMF E = 5.0 V and internal resistance r = 0.2 Ohm is closed to a conductor with resistance R = 40.0 Ohm. What is the voltage U on this conductor? Answer: U = 4.97 V. No. 2 To the battery with EMF and internal resistance r = 0.5 Ohm, a light bulb with resistance R = 100 Ohm was connected. Determine the current in the circuit. No. 3 Determine the EMF of the current source with an internal resistance r = 0.3 Ohm, if, when connected to the terminals of the current source, resistors R 1 = 10 Ohm and R 2 = 6 Ohm connected in parallel, the current in the circuit is: I = 3 A. V 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 The short-circuit current can be calculated by the formula: a) b) c) d)
Test (preparing for the Unified State Exam) 3. The EMF of a battery with an internal resistance r = 0.2 Ohm, when connected to it, the resistance R = 5 Ohm is equal to ... A current I = 1.5 A flows through the circuit. A) 3 C B) 12V C) 7.8 C D) 12.2 C
Test (preparing for the exam) 4. What is the internal resistance of the current source with EMF V, if, when it is closed in parallel by resistors Ohm and Ohm connected in parallel, a 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 did not understand everything C. Everything is as usual, nothing unusual D. I didn’t like it
Homework § 107-108 read, exercise 19 No. 5.6. Task (at home): When a light bulb was connected to a battery of cells 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!
Power source characteristics
The role of the current source In order for the electric current in the conductor not to stop, it is necessary to use a device that would transfer charges from one body to another in the direction opposite to that in which the charges are transferred by the electric field. A current source is used as such a device.
A current source is a device that converts some type of energy into electrical energy. There are different types of power sources: Mechanical power source - mechanical energy is converted into electrical energy. These include: an electrophoretic machine (the disks of the machine are driven in 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, generators. Thermal current source - internal energy is converted into electrical energy. For example, a thermoelement - two wires of different metals must be soldered from one edge, then heat the junction, then a voltage will appear between the other ends of these wires. They are used in thermal sensors and in geothermal power plants.
Light source of current - light energy is converted into electrical energy. For example, a photocell - when some semiconductors are illuminated, light energy is converted into electrical energy. Solar cells are composed of photocells. They are used in solar batteries, light sensors, calculators, video cameras. Chemical source of current - 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 on a solution of ammonia. When ammonia interacts with zinc, zinc acquires a negative charge, and a carbon rod - 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 up of several galvanic cells. Current sources based on galvanic cells are used in self-contained household appliances, uninterruptible power supplies. Batteries - in cars, electric vehicles, cell phones.
Slide 2
External forces Electromotive force Outer part of the circuit Inner part of the circuit Current source Concepts and quantities:
Slide 3
Laws: Ohm for a closed circuit
Slide 4
Short-circuit current Electrical safety rules in various rooms Fuses Aspects of human activity:
Slide 5
Electromotive force. Ohm's law for a closed circuit. Sources of current. To obtain a direct current in an electric circuit, the charges must be acted upon by forces other than the (Coulomb) forces of the electrostatic field. Such forces are called outside forces. The characteristic of the action of external forces is the 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 loop, referred to the value of this charge, EMF is measured in volts. The section of the circuit on which there is an EMF is called an inhomogeneous section of the circuit. Inside the source, charges move against the Coulomb forces under the action of external forces, and in the rest of the circuit they are set in motion by an electric field. Such sources can be galvanic cells, accumulators, direct current electric generators. The EMF of the current source is equal to the electric voltage at its terminals when the circuit is open. It follows from the law of conservation of energy 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
Slide 6
From here we get Ohm's law for a complete circuit: The current 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 is 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 are especially dangerous in lighting networks powered by substations (thousands of amperes). To avoid the destructive effect of such high currents, fuses or special circuit breakers are included in the circuit. In galvanic cells, the short-circuit current is small and therefore not very dangerous for them.
"Georg Om" - He became, in particular, the best billiard player and skater at the university, became interested in dancing. Georg Ohm was born on March 16, 1787 in Erlang in the family of a hereditary locksmith. Om enthusiastically plunged into sports. Since 1825 Ohm begins to study galvanism. Lamp rheostat. Ohm's law for a section of a chain.
"Current in the circuit" - From which pole of the current source and to which is the current direction considered? What parts does an electrical circuit consist of? What experience shows the dependence of current strength on voltage? How does the current strength 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). Equilibrium of voltages in any circuit of the circuit. Kirchhoff's first law. The nominal operating mode of the energy source. Calculation of the power transmitted to the load. Analytical expression of Kirchhoff's second law. Power source short circuit mode. Kirchhoff's laws and modes of operation of energy sources.
Om Tok - Worked as a teacher in Gotstadt (Switzerland). With an increase in the resistance of the conductor, the current decreases. German physicist. Dependence of current strength on voltage I (U) Dependence of current strength on resistance I (R). Lesson theses on the topic "Ohm's Law for a section of a chain." Ohm devoted the last years of his life to research in the field of acoustics.
"Current characteristics" - Series connection of conductors. Voltage measurement. Electromotive force. Dependence of current strength on voltage and resistance. Conditions for the existence of a current. Resistance to metals. Current strength. Current work. Electric current characteristics. The strength of the current is a physical quantity. Parallel connection of conductors.
"Ohm's Law for a section of the circuit" - The power dissipated is maximum. Ohm's law in differential form. Work and power of current. Kirchhoff 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. Current source efficiency. Dividing the work by time, we get an expression for the power.
Ohm's law for a complete circuit
Physics teacher BOU secondary school № 37 stanitsa Staromyshastovskaya T.A. Pelipenko
Let's repeat the basic concepts
Electricity
directional movement of charged particles
a physical quantity that shows what kind of charge passes through the cross section of the conductor per unit of time: 𝐼 = 𝑞 / 𝑡
Current strength
The unit of current measurement is ampere
The area of the figure under the current graph is numerically equal to the charge (q = It)
Let's repeat the basic concepts
Ohm's law for a section of a chain
Electrical resistance of metallic conductors
Conditions for the existence of electric current
The presence of free charges in matter
The presence of an external electric field (current source)
Power source is a device in which some type of energy is converted into electrical energy
Let's repeat the basic concepts
There are different types of power sources:
Mechanical power sources
Thermal current sources
Chemical power sources
Light sources of current
The distribution of charges inside DC sources occurs due to forces of non-electrical origin (electromagnetic, chemical, mechanical forces, etc.), which are called outside forces
In any current source, work on the separation of positively and negatively charged particles that accumulate at the poles of the source
Forces of non-electrical origin (mechanical, chemical, electromagnetic, etc.) cause the charges inside the current source to be redistributed between its poles
The ratio of the work of external forces to move charges inside the current source to the value of the moved charge is called electromotive force (EMF) this current source
Unit of measurement of EMF in SI - volt
[ε] = 1V
With an open circuit, the voltmeter shows EMF
Any DC source
has a certain internal
resistance
r - internal resistance of the current source
[r] = 1 ohm
Ohm's law for a complete circuit
I - current in the circuit
R - resistance of the external section of the circuit
r - internal resistance of the current source
– EMF of the current source
A short closure
Converting Ohm's Law
for a complete chain,
we get the following expression
Potential difference inside
current source
ε = IR + Ir
External voltage
section of the chain
Exercise 1
The EMF of the battery is 2 V. With a current in the circuit of 2 A, the voltage at the battery terminals is 1.8 V. Find the internal resistance of the battery and the resistance of the external circuit
Let's check the solution to the problem
Answer: R = 0.9 ohm; r = 0.1 ohm.
Answer: R = 0.9 ohm; r = 0.1 ohm.
ε = U + Ir, r =
r = = 0.1 ohm
Let's check the solution to the problem
Given:
R = 20 Ohm
Solution
Ɛ = 5V
Since the current sources are connected in series,
Answer: U = 4 V.
r = 2.5 ohm
Ɛ = U + 2 Ir
U = Ɛ - 2 Ir
U = 5V - 2 0.2 A 2.5 ohm = 4V
Homework:
§ 107, § 108, ex. 19 (tasks 6, 7, 8)
thanks
Electromotive force. Ohm's law for a complete circuit.
Lesson 10 grade
Let us connect with a conductor two metal balls carrying charges of opposite signs.
Under the influence of the electric field of these charges, an electric current arises in the conductor.
But this current will be very short-lived.
The charges are quickly neutralized, the potentials of the balls will become the same, and the electric field will disappear.
Outside forces
In order for the current to be constant, it is necessary to maintain a constant voltage between the balls.
This requires a device (current source) that would move charges from one ball to another in the direction opposite to the direction of the forces acting on these charges from the electric field of the balls.
In such a device, in addition to electrical forces, the charges must be acted upon by forces of non-electrical origin.
The electric field of charged particles (the Coulomb field) alone is not capable of maintaining a constant current in the circuit.
External forces set in motion charged particles inside all current sources: in generators in power plants,
in galvanic cells,
batteries, etc.
Alternator, Russia
Battery, Tyumen
Galvanic cells, USSR
When a circuit is closed, an electric field is created in all conductors of the circuit.
Inside the current source, charges move under the action of external forces against the Coulomb forces (electrons from a positive charged electrode to a negative one), and in the rest of the circuit they are set in motion by an electric field.
The nature of outside forces
Sources of current
Outside power
Power plant generator
Force acting from a magnetic field on electrons in a moving conductor
Galvanic cell
(Volta element)
Chemical forces dissolving zinc in sulfuric acid solution
Electromotive force
The action of external forces is characterized by an important physical quantity called electromotive force (abbreviated EMF).
The electromotive force in a closed loop is the ratio of the work of external forces when the charge moves along the loop to the charge:
EMF is expressed in volts: [Ɛ] = J / Kl = V
Consider the simplest complete (closed) circuit consisting of a current source and a resistor with resistance R.
Ɛ - EMF of the current source,
r - internal resistance of the current source,
R - external resistance of the circuit,
R + r Is the total resistance of the circuit.
Ohm's law for a closed circuit connects the current in the circuit, EMF and impedance R + r chains.
Let us establish this connection theoretically using the laws of conservation of energy and Joule - Lenz.
Let an electric charge pass through the cross-section of the conductor in a time.
When performing this work, an amount of heat is released on the inner and outer sections of the chain, equal according to the Joule-Lenz law:
Q = I² ∙ R ∙ ∆t + I² ∙ r ∙ ∆t
The current in the complete circuit is equal to the ratio of the EMF of the circuit to its impedance .
0. For this circuit: Ɛ = Ɛ₁ - Ɛ₂ + Ɛ₃ and Rп = R + r₁ + r₂ + r₃ If Ɛ 0, then I 0 → the direction of the current coincides with the direction of the loop bypass. "width =" 640 " If the circuit contains several elements connected in series with EMF Ɛ₁, Ɛ₂, Ɛ₃, etc., then the total EMF of the circuit is equal to the algebraic sum of the EMF of individual elements.
To determine the sign of the EMF, we choose the positive direction of bypassing the contour.
If, when bypassing the circuit, they pass from "-" poles to "+", then the EMF Ɛ 0.
For a given chain: Ɛ = Ɛ₁ - Ɛ₂ + Ɛ₃ and Rп = R + r₁ + r₂ + r₃
If Ɛ 0 , then I 0 →
the direction of the current coincides with the direction of the loop bypass.
Solving problems
- What is the voltage at the terminals of a galvanic cell with an EMF equal to Ε, if the circuit is open?
- What is the current strength during a short circuit of a battery with an EMF Ε = 12 V and an internal resistance r = 0.01 Ohm?
- The flashlight battery is shorted to a variable resistor. With a resistance of 1.65 ohms, the voltage across it is 3.30 V, and with a resistance of 3.50 ohms, the voltage is 3.50 V. Determine the EMF and internal resistance of the battery.
- Current sources with EMF 4.50 V and 1.50 V and internal resistances 1.50 Ohm and 0.50 Ohm, connected as shown in figure (15.13), feed the lamp from a flashlight. What power does the lamp consume if it is known that the resistance of its filament in a heated state is 23 ohms?
Bibliography:
- G.Ya. Myakishev, B.B. Bukhovtsev "Physics" Grade 10, "ENLIGHTENING", Moscow 2001.
