Capacitors (grade 10). Open lesson "capacitors" Energy of a charged capacitor application of capacitors presentation

Sections: Physics

Didactic goal

1. Give the concept of the electrical capacity of a solitary conductor and its unit; to acquaint with the device of a flat capacitor and the types of their connections.

2. Derive the formulas for the electrical capacity of a solitary conductor, a ball, a flat capacitor, a battery of capacitors connected in series and in parallel, and the energy of a charged capacitor.

3. Give a classification of capacitors depending on the type of dielectric that separates the plates, and the magnitude of the electric capacitance.

educational goal

Using the example of demonstrating the spark discharge of a capacitor or the discharge of a capacitor through an incandescent lamp, show that the electric field has energy, and therefore it is material.

Basic knowledge and skills

1. To know the physical meaning of electric capacitance, formulas for calculating the electric capacitance of a solitary conductor, a ball, a flat capacitor, a battery of parallel and series-connected capacitors and be able to apply them to solve problems.

2. Know the formula for calculating the energy of a charged capacitor and be able to apply it to solve problems.

The sequence of presentation of new material

1. Electrical capacity of the conductor. Units of electrical capacity.

2. The dependence of the electrical capacity of the conductor on its size, shape and surrounding bodies.

3. Electrical capacity of a metal ball (sphere).

4. Capacitors. Their device, purpose, charging and discharging, the role of the dielectric. Classification of capacitors.

5. Serial connection of capacitors in a battery.

6. Parallel connection of capacitors in a battery.

7. Energy of a charged capacitor. Volumetric energy density of the electric field.

Equipment

Two electrometers, four metal spheres (two diameters), an electrophore machine, two insulating stands, a demountable flat capacitor demo, a variable capacitance demo, a set of capacitors (ceramic, paper, mica, electrolytic), a flashlight, an electric lamp at 3.5 V and 0.28 A, DC source or AC powered rectifier, connection wires. Demonstrations

The dependence of the potential of a solitary conductor on the magnitude of the reported charge; the dependence of the potential of a solitary conductor on its size when the same charges are communicated; the dependence of the conductor potential on the presence of other conductors; the dependence of the electric capacitance of a flat capacitor on the area of the plate, the distance between the plates and the dielectric separating the plates; discharging a capacitor through an incandescent lamp or flash; device of various types of capacitors.

Motivation of cognitive activity of students

Nowadays, all students know about capacitors to some extent. Capacitors are widely used in radios, televisions, tape recorders and in many electronic devices. Capacitors are used to store electrical charges and electrical energy. The property of a capacitor to accumulate and store electrical charges is used in technology to obtain short-term current pulses of great strength. One example of this use of a capacitor is the electronic flash used in photography. In this case, the capacitor is discharged through a special lamp.

Lesson plan

Checking the knowledge, skills and abilities of students

1. Inform students of the results of the physical dictation performed in the last lesson; to analyze typical and blunders.

2. Orally interrogate four students on the following tasks:

Task one:

1) Explain the physical nature of electrostatic induction. Why is the voltage inside a conductor placed in an electric field zero?

2) Write the formula for the dependence of the strength and potential difference of a homogeneous electric field.

3) How much will the average kinetic energy of the chaotic motions of gas molecules change with an increase in its temperature by 100 K? Answer. ∆E k \u003d 2.07 * 10 -21 J.

Task two:

1) Explain the physical nature of the polarization of non-polar dielectrics. Why is the strength inside a dielectric placed in an electric field less than the strength of the external field?

2) Write the formula for the electric field strength of a charged plane.

3) Determine the thermal energy of 3.2 kg of oxygen at a temperature of 127°C. Answer. ∆U=831 kJ.

Task three:

1) Explain the physical nature of the polarization of polar dielectrics. Why is an uncharged paper sleeve (dielectric) attracted to a charged body?

2) Write the formula for the potential of the electric field of a charged ball. 31 How much will the internal energy of 1.2 kg of carbon change when the temperature drops by 40°C? Answer. ∆U=49.86 kJ.

Task four:

1) In which dielectrics does the polarization not depend on temperature, and in which does it? Why?

2) Why, at equilibrium, the entire excess charge of an electrified conductor is located on its surface?

3) Determine the pressure of 2 kg of oxygen in a cylinder with a capacity of 0.4 m 3 at a temperature of 27°C. Answer,

p ≈ 0.39 MPa.

3. Check homework. Additional questions for the respondents:

T. No. 958. Electrify an ebonite stick by friction. First, just touch the electroscope ball, and then run the wand over it. Was the electroscope charged in the same way in both cases? (In the second case, the electroscope will be charged more, since the charge is removed not from one, but from many points on the surface of the stick.)

Vol. No. 974. What is the field strength at the center of a uniformly charged wire ring having the shape of a circle? At the center of a uniformly charged spherical surface? (In both cases, the intensity is 0.)

T. No. 986. In order to rarefy the electroscope, it is enough to touch it with your finger. Will the electroscope discharge if there is a charged body isolated from the ground nearby (no, because the charge of the opposite sign induced by the body will remain on the electroscope.)

T. No. 987. If a needle is brought to a charged “sultan” with a point, then the leaves of the sultan gradually begin to discharge. Why? (There is a charge of the opposite sign on the needle (of the same name goes into the ground on the hand), which neutralizes the charge on the leaves.)

How is Coulomb's law read?

How is the law of conservation of charge read?

What field is called an electric field?

Frontal survey

1. What is called the magnitude of the charge?

(The excess of electric charges of the same sign in any body is called the magnitude of the charge or the amount of electricity.)

2. How is the law of conservation of charge read?

(Electric charges do not arise and do not disappear, but are only redistributed among all the bodies participating in this or that phenomenon.)

3. What are the types of electrification?

4. Why, when pouring gasoline from one tank to another, it can ignite if special precautions are not taken?

(When gasoline flows out of a pipe, it becomes so electrified that an electric spark is created that ignites it.)

5. Read Coulomb's law?

6. Why are conductors for experiments on electrostatics made hollow?

(Because static charges are located only on the outer surface of the conductor.)

7. What do we call the dielectric constant of the medium? (The value characterizing the dependence of the interaction force between charges on the environment is called e s.)

8. Why do devices for electrostatic experiments do not have sharp ends, but end with rounded surfaces?

(At the sharp ends of the conductors, the density of charges is so high that they are not retained on the conductor and "drain" from it.)

9. What field is called an electric field?

(A field that transmits the action of one fixed electric charge to another fixed charge in accordance with Coulomb's law is called an electric field.)

10. What do we call the line of tension?

(This is such a line, tangent to each point of which the field strength vectors are directed.)

11. Properties of lines of force?

12. What field is called homogeneous?

13. How to determine the sign of the charge on an electroscope, having an ebonite stick and cloth?

(The sign of the charge of the electroscope will be negative if, from the touch of an electrified, ebonite stick, the leaves disperse at a larger angle.)

14. How will the force of interaction between two point charges change if the value of each charge is increased four times, and the distance between the charges is halved?

(Increase 64 times.)

15. What do we call the field potential of a given point? (The energy characteristic of the electric field at a given point is called the potential of the field at a given point.)

16. Formula for determining φ, E?

Analyze student responses, comment and say grades.

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The presentation on the topic "Electrical capacitance and capacitors" can be downloaded absolutely free of charge on our website. Project subject: Physics. Colorful slides and illustrations will help you keep your classmates or audience interested. To view the content, use the player, or if you want to download the report, click on the appropriate text under the player. The presentation contains 13 slide(s).

Presentation slides

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Sections - Electricity

Capacitors and their types

Electric capacitance of a flat capacitor

Energy of a charged capacitor

Electric field energy

Application of capacitors

Units of electric capacity

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Electrical capacity

With any method of charging bodies - using friction, an electrostatic machine, a galvanic cell, etc. - initially neutral bodies are charged due to the fact that some of the charged particles pass from one element to another. Usually these particles are electrons. When charging two conductors, for example, from an electrostatic machine, one of them acquires a charge +|q|, and the other -|q|. An electric field appears between the conductors and a potential difference (voltage) arises. As the voltage increases, the electric field between the conductors increases. In a strong electric field (at a high voltage), a dielectric (for example, air) becomes conductive. The so-called dielectric breakdown occurs: a spark jumps between the conductors, and they are discharged. The less the voltage between the conductors increases with an increase in their charges, the more charge can be accumulated on them. Electrical capacity is a physical quantity that characterizes the ability of two conductors to accumulate an electric charge. The voltage U between two conductors is proportional to the electric charges that are on the conductors (on one +|q|, and on the other -|q|).

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Indeed, if the charges are doubled, then the electric field strength will become 2 times greater, therefore, the work done by the field when moving the charge will also increase 2 times, i.e. voltage will double. Therefore, the ratio of the charge q of one of the conductors to the potential difference between this conductor and the neighboring one does not depend on the charge. It is determined by the geometric dimensions of the conductors, their shape and mutual arrangement, as well as the electrical properties of the environment. The electrical capacity of two conductors is the ratio of the charge of one of the conductors to the potential difference between this conductor and the neighboring one:

The lower the voltage U when the conductors are charged +|q| and -|q|, the greater the electrical capacity of the conductors. Large charges can be stored on conductors without causing breakdown of the dielectric. But the capacitance itself does not depend either on the charges imparted to the conductors or on the resulting voltage.

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Units of electric capacity

The electrical capacity of two conductors is equal to one if, when they communicate charges +1 C and -1 C, a potential difference of 1 V arises between them. This unit is called farad (F); 1F=1 C/V. Due to the fact that the charge of 1 C is very large, the capacitance of 1F is very large. Therefore, in practice, fractions of this unit are often used: microfarad (μF) -10 (-6) F and picofarad (pF) - 10 (-12) F.

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Capacitors and their types

Capacitors are devices consisting of two conductors insulated from each other, located at a close distance from each other. The conductors in this case are called capacitor plates. Regardless of the shape of the conductors, they are called capacitor plates.

The simplest capacitor consists of two plane-parallel plates located at a small distance from each other. If the charges of the plates are identical in magnitude and opposite in sign, then the electric field lines begin on a positively charged plate

Capacitor and terminate on a negatively charged. Therefore, almost the entire electric field is concentrated inside the capacitor. To charge a capacitor, you need to connect its plates to the poles of a voltage source, for example, to the poles of a battery. Under the charge of the capacitor understand the absolute value of the charge of one of the plates.

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Capacitors have different devices depending on their purpose. A conventional technical paper capacitor consists of two strips of aluminum foil insulated from each other and from the metal case by paraffin-impregnated paper strips. The strips and ribbons are tightly folded into a small package. In radio engineering, capacitors of variable electrical capacity are widely used. Such a capacitor consists of two systems of metal plates,

which, when the handle is rotated, can enter one into the other. In this case, the areas of overlapping parts of the plates and, consequently, their electric capacitance change. The dielectric in these capacitors is air. An increase in electrical capacity by reducing the distance between the plates is achieved in electrolytic capacitors. The dielectric in them is a thin film of oxides,

covering one of the plates (foil strip). The second lining is paper impregnated with an electrolyte solution.

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Electric capacitance of a flat capacitor

The field created by an infinite charged conducting plate with a charge density s is equal to E \u003d s / (2 e 0).

Thus, if edge effects are neglected, the field between the plates of a flat capacitor is uniform. The accuracy of this statement is the higher, the larger the size of the plates compared to the distance between them. Using the formula U = Ed , we get:

Since | s | \u003d q / S, where S is the area of \u200b\u200bthe plate, then the field strength between the plates is:

If we bring two conducting plates closer to each other, the dimensions of which are much larger than the distance between them, and connect them to a voltage source, then we can assume that the field created by each of the plates approximately coincides with the field of an infinite plate. Then inside the resulting flat capacitor (between the plates) the field will be equal to the sum of the fields created by each plate:

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Series connection of capacitors:

Parallel connection of capacitors:

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Energy of a charged capacitor

In order to charge a capacitor, work must be done to separate the positive and negative charges. According to the law of conservation of energy, this work is equal to the energy of the capacitor. The fact that a charged capacitor has energy can be verified by discharging it through a circuit containing an incandescent lamp rated for a voltage of several volts. When the capacitor is discharged, the lamp

flashes. The energy of the capacitor is converted into other forms: heat, light. The field strength created by the charge of one of the plates is equal to E / 2, where E is the field strength in the capacitor. In a uniform field of one plate there is a charge q distributed over the surface of the other plate. Since Ed \u003d U, where U is the potential difference between the capacitor plates, then its energy is:

This energy is equal to the work that the electric field will do when the plates come close.

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Electric field energy

According to the theory of short-range action, all the energy of interaction of charged bodies is concentrated in the electric field of these bodies. This means that the energy can be expressed through the main characteristic of the field - the intensity. Since the strength of the electric field is directly proportional to the potential difference (U \u003d Ed), then according to the formula: the energy of the capacitor is directly proportional to the strength of the electric field inside it.

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Application of capacitors

The energy of the capacitor is usually not very high - no more than hundreds of joules. In addition, it is not stored due to the inevitable leakage of charge. Therefore, charged capacitors cannot replace, for example, batteries as sources of electrical energy. Capacitors can store energy for a longer or shorter time, and when charged through a low-resistance circuit, they release energy almost instantly. This property is widely used in practice. The flash lamp used in photography is powered by an electric current from the discharge of a capacitor, previously charged by a special battery. The excitation of quantum light sources - lasers is carried out using a gas discharge tube, the flash of which occurs when a battery of capacitors of large electrical capacity is discharged. However, capacitors are mainly used in radio engineering ...

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Today we have an unusual lesson. We have guests. Let's say hello to them. Please sit down.

Each of you has a lesson card on your desk. Sign your name on it. After completing each task, you will put down the number of points for completing the task in it. At the end of the lesson, we will calculate the total number of points and give the corresponding mark.

Checking the studied material.

Physical dictation.

(after completion - mutual verification).

1 option.

one). A conductor is a substance in which ... (free charges can move throughout the volume).

2). Semiconductors include ... (minerals, oxides, sulfides, germanium, silicon, selenium, fat, brine, blood, carbon).

3). The electrical capacity of a solitary conductor is calculated by the formula .... (C \u003d Q / φ).

4) ε 0 is ... (electrical constant and is equal to 8.85 * 10 -12 C 2 / N * m 2).

5) Electric capacity is measured in ... (farads).

6) The electrical capacitance of the sphere depends ... (on the radius).

7) What three groups divide all substances ... (conductor, semiconductor, dielectric).

Option 2.

1) A semiconductor is a substance in which ... (the number of free charges depends on external conditions).

2) Conductors include ... (metals. Solutions of salts, alkalis, acids, moist air, plasma, human body).

3) The electrical capacity of a solitary ball is calculated by the formula ... (C = 4π ε 0 ε R).

4) ε is ... (dielectric constant of the medium)

5) The charge is measured in ... (coulombs).

6) The electrical capacity of the sphere does not depend on ... (the charge on its surface).

7) Dielectrics include ... (gases, distilled water, benzene, oils, glass, porcelain, mica, wood, and others).

Learning new material.

(during the review, a supporting abstract is filled out).

A device called a capacitor is used to store electrical charges.

What is a capacitor? What does it consist of?

Basic outline.

A capacitor is ... (a system of two conductors separated by a dielectric layer, the thickness of which is small compared to the dimensions of the conductor).

Conductors are ... (capacitor plates).

Capable of accumulating a large charge.

Symbol:

Electric field inside a capacitor.

In order to charge .... (attach its plates to the poles of the current source).

Types of capacitors: air, mica, ceramic, paper, electrolytic, ...

(table display: Types of capacitors).

Message: first capacitor.

The main characteristic is electrical capacity.

Electric capacity is .. (a physical quantity that characterizes the ability of two conductors to accumulate an electric charge).

Show animation on the computer: "The capacity of the capacitor and its use."

C = q / φ; C = εε 0 S / d.

Unit of measure: Farad (F).

Application:

radio engineering;

camera flash;

computer keyboard;

Capacitor energy.

(Animation: "Construction and Energy of a Capacitor").

Types of connections in the diagram:

C \u003d C 1 + C 2 + ... ... ..

1 / C \u003d 1 / C 1 + 1 / C 2 + ....

Disadvantages:

Energy does not last long.

Discharges quickly.

Constant recharging is required.

Problem solving.

The capacitor has an electric capacitance C = 5pF. What charge was on each of its plates, if the potential difference between them is U = 100 V.

The charge q \u003d 6 * 10 -4 C on the plates of a flat capacitor creates a potential difference between the plates of 200 V. Determine the capacitance of the capacitor. (Kasyanov: Physics -10, p. 403, task No. 1).

Calculate the energy of the electrostatic field of a 0.1 μF capacitor charged to a potential difference of 200V. (Kasyanov: Physics - 10, p. 406, task No. 1).

Find the electric capacitance of a flat capacitor, if the area of each of its plates is 1m 2, the distance between the plates is 1.5 mm. The dielectric is mica (ε = 6).

Independent work.

(after completion, they check with each other)

1 option.

1. What is the capacitance of a capacitor if, when it is charged to a voltage of 1.4 kV, it receives a charge of 28 nC?

2. Calculate the energy of the capacitor of the starting motor at the moment of its complete discharge, if it is known that the voltage on the plates is 300 V, and the capacitance of the capacitor is 0.25 μF.

Option 2.

1. Find the capacitance of an air capacitor charged to a potential difference of 200 V. The area of each plate is 0.25 m 2, the distance between them is 1 mm. (ε = 1).

2. The capacitor says 4 uF, 100 V. What is the maximum energy it can have.

Summing up the lesson. Grading.

What have you learned? What have you learned?

Repeat the basic concepts. (capacitor, plates, capacitance, energy, application).

Homework.

Learn the baseline.

To solve a task.

A task: The area of each of the plates of a flat capacitor is 200 cm 2 , and the distance between them is 1 cm. What is the field energy if the field strength is 500 kV/m?

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Checking homework: Physical dictation. Option 1 The electrical capacity of two conductors is called ... The polarization of dielectrics is called ... The unit of electrical capacity is expressed in ... Option 2 The surfaces of equal potential are called ... The potential of the electrostatic field is called ... The unit of electric field strength is expressed in ...

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Checking homework: Physical dictation. Option 1 The electrical capacity of two conductors is the ratio of the charge of one of the conductors to the potential difference between this conductor and the neighboring one. The polarization of dielectrics is called the displacement of positive and negative bound charges in opposite directions. The unit of electrical capacity is expressed in farads (F). Option 2 Surfaces of equal potential are called equipotential. The potential of an electrostatic field is the ratio of the potential energy of a charge in the field to this charge. The unit of electric field strength is expressed in volts per meter (V / m) or in newtons per pendant (N / C).

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Lesson Objectives: To learn how to determine the energy of a charged capacitor. Develop the ability to apply physical laws in solving problems. Find out the practical significance of the capacitor.

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Capacitors. A capacitor consists of two conductors separated by a dielectric layer, the thickness of which is small compared to the dimensions of the conductors. The capacitance of a flat capacitor is determined by the formula: q C \u003d - U

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The energy of a charged capacitor. - The energy of the capacitor for the potential energy of the charge in a uniform field is: 1. W = + + + + + - - - - E - q + q 1 2 q E d 2. W = q U= CU 1 1 2 2 2 2 p p

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Application of capacitors Types of capacitors: - air, - paper, - mica, - electrostatic. Purpose: To accumulate a charge or energy for a short time to quickly change the potential. Do not pass direct current. In radio engineering - an oscillatory circuit, a rectifier. Application in photography.

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Consolidation. Theoretical material on the questions: What are capacitors for? How is a capacitor arranged? Why is the space between the plates of a capacitor filled with dielectrics? What is the energy of a charged capacitor?

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Consolidation. Problem solving: 1. What is the capacity of the capacitor. If he received a charge of 6 . 10-5 C, from a source of 120 V.

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Problem number 1. Given: q = 6 . 10-5 C U = 120 V C =? F Solution: C = q:U C = 6 . 10-5: 120= 0.5uF Answer: 0.5uF.

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