Constant and variable current presentation. Multimedia presentation AC

Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

The presentation was compiled by a physics teacher from MKOU VSOSH No. 2 at the IK village. Chuguevka by Murzagildina Lyudmila Borisovna 2016 Lesson objectives: 1. Continue to develop ideas about harmonic electromagnetic oscillations, forced electromagnetic oscillations and types of resistance in an alternating current circuit. 2. Develop students’ cognitive interests on this topic through various information resources: textbook, presentation, tables. 3. Learn to find useful and necessary things in the material being studied. Updating knowledge. 1. What oscillations are called harmonic? Oscillations occurring according to the law of sine or cosine. 2. Give the definition of electromagnetic oscillations. Processes in electrical circuits in which charge, current, voltage and emf change periodically. 3. Why do free electromagnetic oscillations damp out? Free electromagnetic vibrations are damped due to resistance. 4. State the formula for the period of electromagnetic oscillations. 5. Name the system in which electromagnetic oscillations occur. Solving problems on the topic “Electromagnetic oscillations”. 1. The charge q on the plates of the capacitor of the oscillatory circuit changes over time in accordance with the equation q = 5٠10-4cos 103πt. What is the amplitude of charge oscillations, the phase of oscillation and the initial phase of the charge? Amplitude - 5٠10- 4 Charge oscillation phase - 103πt Initial phase =0 Solving problems on the topic “Electromagnetic oscillations”. 2.Which of the listed devices is necessarily included in the direct current circuit and the oscillatory circuit? Match the position of the first column with the desired position from the second. Write down the resulting numbers in the table under the appropriate letters. A) DC circuit 1. Ammeter B) Oscillating circuit 2. Current source A 3. Capacitor 4. Magnet B Answer to the problem: A B 2 3 Studying the new topic of our lesson “Alternating current.” Resistance in an alternating current circuit" An electric current that changes its magnitude and direction over time is called alternating current. Our task is to check during the lesson: - alternating current is forced oscillations; - that over time the current changes its direction and magnitude. “The current runs through the wires and is never visible. He lights the light bulbs and brings the appliances to life.” Yakov Byl “War of Currents” There was a period in history that is known under the code name “war of currents.” The main characters at that time were the well-known Nikola Tesla and Thomas Edison. Nikola Tesla saw the potential and convenience of alternating current. And Edison insisted that constant electricity should be used (a point of view that was generally accepted at that time). Edison even held public demonstrations, which were quite brutal. The fact is that alternating current, despite its advantages, poses a great danger to living beings. Thomas Edison used this fact to create fear and distrust of Tesla's ideas among the people: he publicly killed animals using alternating current. Once they even performed a demonstration on an elephant: a couple of seconds - and the mighty animal fell dead. From history The first source of electricity in our era was the electrostatic generator, invented in 1663 by the mayor of Magdeburg, Otto von Guericke. So what is alternating current? Current strength and voltage change according to a harmonic law, and the oscillation frequency is determined by the frequency of the current source connected to the circuit. (50 Hz) How to create alternating voltage and alternating current? Alternating voltage and current in the network are created by alternating current generators at the power plant. Alternating current generator The standard frequency of industrial current is 50 Hz - this means that in 1 second the current changes its direction 50 times. What happens in an alternator? We have established that 1. The magnetic flux F penetrating the coil circuit changes in magnitude and direction. Ф = V S cos ωt 2. The current induced in the coil changes in magnitude and direction. i = Im sin (ωt+φ₀) 3. Voltage and current fluctuations differ in the phase of the oscillation (φ₀). u = Um cos ωt What role do resistances play in an alternating current circuit? Electrical resistances can be included in an alternating current circuit - resistors, inductive and capacitive reactance (oscillatory circuit). Resistors have a resistance R (active resistance), an inductor with an inductance L - X L (inductive reactance), and a capacitor with a capacitance C - X C (capacitive reactance). Active resistance in an alternating current circuit. So we found out that the current and voltage in an alternating current circuit with active resistance fluctuate in U one phase, and active resistance R = m I m Capacitance in an alternating current circuit We found out that: 1. Direct current does not pass through the capacitor. 2. The capacitor provides resistance to alternating current. Formula for capacitive reactance Inductance in an alternating current circuit We have found out that: 1. At direct current, the coil has a small active resistance (i.e. it is a resistor) and a change in its inductance does not affect its resistance. 2. With alternating current, the greater the inductance of the coil, the greater the inductive reactance. 3. Inductive reactance So we know that if an alternating current circuit contains active resistance R = 1 X C = C and inductive reactance X = ωL, then L is a capacitive reactance, we can find the total resistance of the alternating current circuit Z: , Lesson summary: 1. We learned what alternating current is and its characteristics, which vary according to the harmonic law: Ф = BS cos ωt; i= Imsin (ωt+φ₀) ; u = Um cos ωt. 2. An alternating current circuit can contain three types of resistance: L 1 R – active; X = - capacitive; С С Х L = ωL – inductive. 3. We have learned the formula by which the total resistance in an alternating current circuit is calculated: Z = √ R² + (X L- X C)² Reinforcing the lesson learned: 1. Why do not they use alternating current with a frequency of 10 - 15 Hz for lighting? The lights will flash. The eye perceives a frequency of 10 Hz as flickering. 2. A coil is connected to the electrical circuit, through which a direct current is first passed, then an alternating current of the same voltage is passed. In which case will the coil heat up more? In the first. The coil for alternating current will also have reactance. Therefore, in the second case, the current is less, and, accordingly, the heat generation is less. 3. How will the glow of the lamp change if the capacitor is broken and the circuit is closed in this place? Each capacitor has a resistance, if we remove this resistance, the lamp will increase in intensity. 4. The AC circuit includes a resistor with R = 5 Ohms, a capacitor with resistance XC = 6 Ohms and an inductor with resistance XL = 18 Ohms. Find the total resistance of the circuit. Given: Solution: R=5Ohm Z= √R²+(XL -Xc)² XC=6Ohm Z=√25Ohm²+(18Ohm-6Ohm)² XL=18Ohm =√25Ohm²+144Ohm² ________ =13 Ohm. Z-? Performing Independent work (test) on the topic “Alternating Current”. time 5-7 min. Reflection: 1. Today I learned that ... 2. I was surprised by the given facts about ... 3. I was interested in learning that ... 4. It was difficult for me to understand ... 5. I liked the lesson ...

Electricity. This presentation covers the topic “direct and alternating electric current.” The presentation is intended for secondary students secondary schools. The presentation is intended for students of secondary schools. 1 Basic laws of electricity.

9 If the current strength in a circuit changes in magnitude and direction over time (the speed and direction of movement of free charges changes), then such an electric current is called alternating. Alternating electric current In Russia, the industrial frequency of alternating current is 50 Hertz (USA - 60 Hz) - this means that 50 (60) complete current oscillations occur in one second, so we do not notice the blinking of light bulbs

Based on their ability to conduct electric current, substances are divided into 1. Conductors, in which there are free charged particles; 2. Non-conductors, in which all charged particles are bound; 3. Semiconductors are substances that, when heated or illuminated, free charged particles appear. eleven

For an electric current to arise, it is necessary: ​​1. The presence of a conductor, that is, free charged particles (electrons, ions); 2. The presence of a current source, inside which the charges are separated and accumulated at the poles of the current source; 3. The electrical circuit must be closed. 12

There are different current sources, but in each of them there is a separation of positively charged and negatively charged particles, which accumulate at the poles. 13 Batteries and galvanic cells. The separation of charges occurs due to chemical reactions Thermocouple - if you heat the junction of two different metals, an electric current is created. Application in sensors. Photocells and solar batteries. Charge separation occurs under the influence of light. The main element is semiconductors. Application in calculators and household appliances, in spacecraft.

There are different current sources, but in each of them there is a separation of positively charged and negatively charged particles, which accumulate at the poles. 14 Alternating current generators, the main part of power plants. In a wire winding wound on a drum (armature), rotating in a magnetic field, an alternating electric current is created, which is removed through slip rings. An electromagnet is usually used to create a magnetic field. In powerful generators, an electromagnet rotates inside a stationary coil. The rotating part is called the rotor, the stationary part is the stator. DC generators. In a wire winding wound on a drum (armature), rotating in a magnetic field, an alternating electric current is created, which is removed through commutator brushes. The collector is a ring cut into halves. Each half of the ring is attached to different ends of the armature coil. If the brushes are installed correctly, they will always remove current in only one direction. DC generators are needed, for example, to charge the battery.

Power plants (induction) Wind power plants The main element is an induction alternating current generator. The engine is a wind turbine. The coil is connected to a turbine (a wheel with impellers) and rotates inside a magnet. The coil and magnets extend beyond the plane of the slide Magnet N turbine S Magnet Wind Wind Wind Note: In high-power generators, an electromagnet rotates inside a stationary coil.

Power plants (induction) Hydroelectric power plants The main element is an induction alternating current generator. The engine is a hydraulic turbine. The coil is connected to a turbine (a wheel with impellers) and rotates inside a magnet. The coil and magnets extend beyond the plane of the slide Magnet N turbine S Magnet Water Water Note: in powerful generators, an electromagnet rotates inside a stationary coil.

Power plants (induction) Thermal and nuclear power plants, combined heat and power plants The main element is an induction alternating current generator. Engine - steam turbine. The coil is connected to a turbine (a wheel with impellers) and rotates inside a magnet. The coil and magnets extend beyond the plane of the slide Magnet N turbine S Magnet Hot steam Note: In powerful generators, an electromagnet rotates inside a stationary coil.

19 Designation - U Designation - U Device – voltmeter Unit of measurement - 1 volt (V) 1kV=1000V=10 3 V; 1MV= V=10 6 V Electrical voltage is the ratio of the field work when moving a charge to the amount of charge transferred

20 Designation - R Device – ohmmeter Unit of measurement - 1 Ohm (Ω) 1kOhm=1000 Ohm=10 3 Ohm; 1 MΩ = Ohm = 10 6 Ohm The electrical resistance of a conductor characterizes the ability of a conductor to conduct electric current. If the resistance of the conductor is greater, then the conductor conducts current less well.

21 Conductor resistivity - the resistance of a conductor 1 meter long and area cross section 1 mm 2 The unit of measurement (Ohm*mm 2)/m is the table value. Formula ρ = (R*S)/l Length of the conductor in meters Cross-sectional area of ​​the conductor in mm 2 If the cross-section is circular, then S=π*r 2 Formula for calculating the resistance of the conductor (Ohm) Conversion of area cm 2 to mm 2 1 cm = 10 mm; 1cm 2 =(10mm) 2 =100mm 2

Ohm's law for a complete circuit The current in a circuit is directly proportional to the electromotive force of the current source and inversely proportional to the sum electrical resistance 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)

24 Series connection of conductors In a series connection, the current strength in any part of the circuit is the same I = I 1 = I 2 The total resistance of the circuit in a series connection is equal to the sum of the resistances of the individual conductors R = R 1 + R 2 The total voltage in the circuit in a series connection, or the voltage at the poles of the current source is equal to the sum of the voltages at individual sections of the circuit: U = U 1 + U 2 R1R1 R2R2

25 Parallel connection of conductors The voltage at the section of the circuit and at the ends of all parallel-connected conductors is the same U = U 1 = U 2 The current in the unbranched part of the circuit is equal to the sum of the currents in the individual parallel-connected conductors I = I 1 + I 2 R1R1 R2R2

Description of the presentation by individual slides:

1 slide

Slide description:

2 slide

Slide description:

Answer the questions: What are called electromagnetic oscillations? What is the difference between free and forced electrical vibrations? How are the amplitudes of charge and current oscillations related when a capacitor is discharged through a coil? What formula is used to determine the natural cyclic frequency of free electrical oscillations? What formula is used to determine the period of free electrical oscillations? How will the period of free electrical oscillations in the circuit change if the capacitance of the capacitor in it is doubled or halved? What is the energy of the circuit at an arbitrary moment in time?

3 slide

Slide description:

Independent work 633, 636 1.var No. 5. The oscillatory circuit consists of a capacitor with a capacity of 10 μF and a coil with an inductance of 10 mH. Find the amplitude of voltage fluctuations if the amplitude of current fluctuations is 0.1 A. 2.var.No.8. The inductance of the oscillating circuit coil is 0.5 mH. It is required to configure this circuit to a frequency of 1 MHz. What should be the capacitance of the capacitor in this circuit? 3. General problem No. 948 The capacitance of the oscillating circuit capacitor is 1 μF, the inductance of the coil is 0.04 H, the amplitude of voltage fluctuations is 100V. At a given moment in time, the voltage on the capacitor is 80 V. Find the maximum current, Total energy, electric field energy, magnetic field energy. Instantaneous current value.

4 slide

Slide description:

Alternating electric current is undamped forced electrical oscillations. An electric current that changes over time is called alternating. Alternating current found wide application: in the lighting network of an apartment, in factories and factories, etc., the current strength and voltage change over time according to a harmonic law. Voltage fluctuations can be detected using an oscilloscope.

5 slide

Slide description:

The frequency of alternating current is the number of oscillations in 1 s. In Russia and other countries, the standard frequency of industrial alternating current is 50 Hz (within 1 second, the current flows 50 times in one direction and 50 times in the opposite direction). In the USA, Canada, Japan, the frequency of industrial alternating current is 60 Hz. Alternating current with a frequency of 400 Hz is used in the on-board network of aircraft.

6 slide

Slide description:

Alternating voltage in the sockets of the lighting network is created by generators at power plants. The frame rotates in a magnetic field. Since the magnetic flux penetrating the frame changes over time, an induced variable EMF arises in it: , e = – dФ/dt = -B∙S∙(cos ωt) = B∙S∙ω∙sin ωt = = εm∙ sin ωt, where εm = B∙S∙ω – amplitude of the induced emf. ω is the angular velocity of rotation of the frame, plays the role of cyclic frequency.

Slide 7

Slide description:

The voltage at the ends of the circuit changes according to a harmonic law, and the electric field strength inside the conductors will also change harmoniously. These harmonic changes in field strength, in turn, cause harmonic oscillations in the speed of the ordered movement of charged particles, i.e., harmonic oscillations of current strength.

8 slide

Slide description:

An alternating current generator is a device designed to convert mechanical energy into alternating current energy. The operation of the generator is based on the phenomenon of electromagnetic induction.

Slide 9

Slide description:

The current in the circuit flows in one direction for half a revolution of the frame, and then changes direction to the opposite. The main parts of an alternating current generator are: inductor, armature, commutator, stator, rotor.

10 slide

Slide description:

We will further study forced electrical oscillations that occur in circuits under the influence of voltage changing with a cyclic frequency ω according to the law of sine or cosine: u = Um ∙ sin ωt or u = Um cos ωt Um is the voltage amplitude, ω is the cyclic frequency of voltage and force current in the circuit. i= Im∙sin (ωt + φc) current strength і at any time. The current fluctuations are out of phase with the voltage fluctuations. Im is the amplitude of the current, φc is the phase difference (shift) between the fluctuations of current and voltage.

11 slide

Slide description:

Active resistance. Effective value of current and voltage. R is called active resistance because in the presence of a load that has this resistance, the circuit absorbs the energy coming from the generator. This energy turns into internal energy of the conductors - they heat up. Instantaneous current value according to Ohm's law:

12 slide

Slide description:

The effective (effective) value of alternating current is the strength of such direct current, which, passing through the circuit, would release the same amount of heat as the given alternating current. I0,U0, - amplitude of current and voltage. Id., Ud., - effective value of current and voltage. The average AC power in the section of the circuit containing the resistor is:

Slide 13

Slide description:

Resonance in an alternating current circuit (voltage resonance) is the phenomenon of a sharp increase in the amplitude of alternating current in a circuit. The frequency at which resonance occurs is called the resonant frequency. The resonant frequency is equal to the frequency of free oscillations of the circuit.

Slide 14