Sections: Physics

Class: 8

Purpose of the lesson: to give the concept of stress as a physical quantity characterizing electric field, which creates an electric current, is the unit of voltage.

Equipment: two types of ammeters, two types of voltmeters, portrait of Alessandro Volta.

Lesson progress

I. Updating knowledge.

Checking homework. Slide 2.

1. What is current strength? What letter does it represent?
2. What is the formula for current strength?
3. What is the name of the device for measuring current? How is it indicated in the diagrams?
4. What is the unit of current called? How is it designated?
5. What rules should be followed when connecting an ammeter to a circuit?
6. What formula is used to find the electric charge passing through the cross section of a conductor if the current strength and the time of its passage are known?
7. Individual tasks:

1) 6 * 10 -19 electrons pass through the cross section of the conductor in 1 s. What is the current in the conductor? Electron charge 1.6*10 -19 C.
2) Determine the current strength in an electric lamp if an electric charge equal to 300 C passes through it in 10 minutes.
3) What electric charge flows through the ammeter in 5 minutes when the current in the circuit is 0.5 A.

1. Test work (on cards):

Option I

1. How many milliamps is 0.25 A?

a) 250 mA;
b)25mA;
c) 2.5mA;
d) 0.25mA;
d)0.025mA;

2.Express 0.25mA in microamps.

a) 250 µA;
b) 25 µA;
c) 2.5 µA;
d) 0.25 µA;
d)0.025 µA;

In Fig. 1 shows a diagram electrical circuit.

a) at point M
b) at point N

a) from point M to N
b) from point N to M

Option II

1.Express 0.025 A in ammeters.

a) 250 mA;
b)25mA;
c) 2.5mA;
d) 0.25mA;
d)0.025mA;

2. How many microamps are there in 0.025mA?

a) 250 µA;
b) 25 µA;
c) 2.5 µA;
d) 0.25 µA;
d)0.025 µA;

In Fig. 2 shows a diagram of the electrical circuit.

3. Where is the “+” sign for the ammeter in this diagram?

a) at point M
b) at point N

4. What direction does the current in the ammeter have?

a) from point M to N
b) from point N to M

9) Checking the test. Slide 3

II. Learning new material.

1. Disk Virtual School Cyril and Methodius. Physics lessons from Cyril and Methodius, 8th grade.

1) What is electric current?

Student answer: Electric current is the directed movement of charged particles.

2) What are the conditions for the existence of electric current?

Student answer: 1st condition – free charges,

Condition 2 – there must be a current source in the circuit.

3) Teacher's explanation:

The directed movement of charged particles is created by an electric field, which at the same time does work. The work that an electric current does when moving a charge of 1 C along a section of a circuit is called electrical voltage (or simply voltage).

where U – voltage (V)

A – work (J)

q – charge (C)

Voltage is measured in volts (V): 1V = 1J/C.

4) Student message: Historical information about Alessandro Volta.

VOLTA Alessandro (1745-1827), Italian naturalist, physicist, chemist and physiologist. His most important contribution to science was the invention of a fundamentally new direct current source, which played a decisive role in further studies of electrical and magnetic phenomena. The unit of electric field potential difference, the volt, is named after him.

Volta was a corresponding member of the Paris Academy of Sciences, a corresponding member of the Academy of Sciences and Letters in Padua, and a fellow of the Royal Society of London.

In 1800, Napoleon opened a university in Pavia, where Volta was appointed professor of experimental physics. At Bonaparte's suggestion, he was awarded a gold medal and the First Consul's Prize. In 1802, Volta was elected to the Academy of Bologna, a year later - a corresponding member of the Institute of France and received an invitation to the St. Petersburg Academy of Sciences (elected in 1819). The Pope grants him a pension, and in France he is awarded the Order of the Legion of Honor. In 1809 Volta became a senator of the Kingdom of Italy, and the following year he was given the title of count. In 1812, Napoleon from his headquarters in Moscow appointed him president of the electoral college.

Since 1814, Volta has been dean of the Faculty of Philosophy in Pavia. The Austrian authorities even grant him the right to act as dean without attending services and confirm the legality of paying him the pensions of an honorary professor and ex-senator.

5) Submultiples and multiples:

1 mV = 0.001 V;
1 µV = 0.000001 V;
1 kV = 1,000 V.

6) Working with the textbook.

Working with table No. 7 in the textbook on page 93.

7) Operating voltage in the lighting network of residential buildings and social facilities - 127 and 220 V.

Hazard due to high voltage current.

Safety rules when working with electricity and electrical appliances. Slide 4.

8) The device for measuring voltage is called voltmeter.

In the diagrams it is represented by the sign:

Rules for connecting a voltmeter to a circuit find it in the textbook.

1. The voltmeter clamps are connected to those points in the circuit between which the voltage must be measured (parallel to the corresponding section of the circuit).

2. The voltmeter terminal with the “+” sign should be connected to the point of the circuit that is connected to the positive pole of the current source, and the terminal with the “–” sign to the point that is connected to the negative pole of the current source.

Demonstration of two types of voltmeters.

The difference between a voltmeter and an ammeter is in appearance.

Determination of the division price of a demonstration voltmeter, laboratory voltmeter.

9) Working with the textbook:(task according to options)

Find in the textbook (§ 41) answers to the questions:

A) How to use a voltmeter to measure the voltage at the poles of a current source?

B) How much should the current passing through the voltmeter compare to the current in the circuit?

III. Consolidation of the studied material.

1. Express the voltage in volts equal to:

A) U =2,000 mV =
B) U = 100 mV =
B) U = 55 mV =
D) U = 3 kV =
D) U = 0.5 kV =
E) U = 1.3 kV =

2. Express the voltage in mV equal to:

A) U = 0.5 V =
B) U = 1.3 V =
B) U = 0.1 V =
D) U = 1 V =
D) U = 1 kV =
E) U = 0.9 kV =

3. Let's solve problems: Slide 7.(work at the board)

A) On a section of the circuit, when an electric charge of 25 C passes through, 500 J of work is done. What is the voltage in this section?

B) The voltage at the ends of the conductor is 220 V. What work will be done when an electric charge equal to 10 C passes through the conductor?

4. Questions for consolidation:

1) What does voltage in an electrical circuit indicate?
2) In what units is voltage measured?
3) Who is Alessandro Volta?
4) What is the name of the device for measuring voltage?
5) What are the rules for turning on a voltmeter to measure the voltage on a section of a circuit?

IV. Homework.

§ 39 – 41. Ex.16. Prepare for laboratory work No. 4 (p. 172).

V. Lesson summary.

Literature:

1. Peryshkin A.V. Physics. 8th grade: textbook. for general education textbook establishments. – M.: Bustard, 2007.
2. Shevtsov V.A. Physics. 8th grade: lesson plans based on the textbook by A.V. Peryshkin. - Volgograd: Teacher, 2007. - 136 p.
3. Maron A.E. Physics. 8th grade: educational and methodological manual / A.E. Maron, E.A. Maron. - 6th ed., stereotype. – M.: Bustard, 2008.-125 p.: ill.-(Didactic materials)
4. Educational disk “Cyril and Methodius”. Physics.8th grade.

Attention! The site administration is not responsible for the content methodological developments, as well as for compliance with the development of the Federal State Educational Standard.

This open lesson was filmed on January 21, 2016 at the Municipal Budgetary Educational Institution Secondary School No. 21 in Nizhnekamsk, Nizhnekamsk District of the Republic of Tatarstan as part of the municipal professional skills competition “Teacher of the Year - 2016”.

In the class at the time of filming there were 20 8th grade students from MBOU Secondary School No. 21, 16 jury members - methodologists of the Nizhnekamsk Education Department, as well as teachers and contestants, a film crew of operators.

The students with whom I worked were unfamiliar to me, therefore, when constructing the lesson, I took into account and considered, as far as possible, various situations.

This video contains material where I share my own experience of using and testing my methodological topic“Application of information and communication technologies in teaching activities.” It takes a lot of work to captivate and surprise modern children, who are accustomed to gadgets from birth. Especially if the physics classroom has not been updated since the opening of the school. There is no interactive whiteboard in the office, despite this, with the help of a laptop and a projector I overcome this “obstacle” and find a solution to teach the lesson in a more accessible and interesting way.

Lesson objectives:	Subject: Form the concept of “tension”. Enter voltage units. Introduce students to the rules of measuring with a voltmeter. Improve practical skills in assembling an electrical circuit, reading and drawing diagrams, and measuring voltage.
Lesson objectives:	a) formation of ideas about tension, organization of the assimilation of basic concepts on this topic, formation of the scientific worldview of students (subject result). b) developing the ability to generate ideas, identify cause-and-effect relationships, work in a group, use alternative sources of information, develop the ability to analyze facts when observing and explaining phenomena, when working with textbook text (metasubject result). c) formation of skills to manage one’s educational activities, formation of interest in physics when analyzing physical phenomena, formation of motivation by setting cognitive tasks, revealing the connection between theory and experience, development of attention, memory, logical and creative thinking (personal result).
Teaching methods:	reproductive, problematic, heuristic.
Forms of organization cognitive activity students:	collective, individual, group.
Learning Tools:	textbook, laboratory equipment, reflection cards, multi-level didactic material, laptop, projector, internet.
Lesson type	Learning new material
	Job electric current. Voltage, voltage unit – 1 Volt. Voltmeter. Voltage measurement.
Equipment	Voltmeter, current source, light bulb, connecting wires, key, handouts. Exhibition of drawings on the topic “Electricity” The physical newspaper is an addition to the headings “Everyone should know this”, “Historical information”, “Facts”...
Demonstrations	Voltage measurement on various areas in a chain

Lesson Plan	Lesson progress	Fallback position
1) Organizational part:	Good afternoon, guys! My name is Akhmetova Aizarya Zanifovna. Everyone good mood and good luck. Have a seat.
1 slide	Lesson motto:“I hear - I forget, I see - I remember, I do - I understand” (Chinese proverb)
2) Knowledge test: 2 slide	Look carefully at the screen. What do we see in the picture? (field). In life we ​​see this field, but from the point of view of electricity does the field exist? (yes, electric)
3-4 slide; 3 animation "El.Tok"	Now what are we seeing? (water flow in a pipe). What can happen in electricity? (electric current)
Poem (work in pairs, given to 3 students) 1 lesson working at the board 2 – work in pairs. (simultaneously)	“How is current strength calculated?”- poem It’s not in vain that I praise myself, I tell everyone and everywhere, That I love physics That I'm studying physics. Once upon a time I came up with an idea Solve the problem. Without a doubt I knew here Time, exactly 5 minutes. But there's one thing I don't understand What is q? I need to find the current strength Not difficult at all: You need to divide the charge for a while, And it will be wonderful!
Answer. 8 A 5 slide At the same time, play “believe it or not” with the rest of the class.	What is the current in the circuit if the charge is 2.4 kC? Guys, you have green and red signal cards on your desk. I read out the sentences, and within 3 seconds you must raise a red card if you do not agree with the statement, a green card if you agree. The ordered movement of charged particles is an electric field (email current). cr. card The current strength is indicated by the letter I. green card Unit of current Cl. (A) cr. Card A device for measuring current strength is an electroscope. (Ammeter) cr. Card Bodies that conduct electric current are called conductors. green card.
Examination. Self-esteem. Checking the task at the board. 6 slide Self-esteem.	Now let's check. Who answered all the questions correctly? Give yourself 3 points; if you made a mistake 1-2 times, we put 2 points; if you made 3 or more points, we give 1 point. Make notes in the margins of your notebooks or on your own pieces of paper.
Demonstration of load movement with a dynamometer 7 slide	From the 7th grade course you are familiar with the term mechanical work. What causes body movement? (applied force) Power does...? (A) What causes the movement of charge in a circuit? (EP) The electric field does work
Animation 2	– Shall we create a similar situation with electricity? Those. If in mechanics there is mechanical work, then in electricity...? (There is current work) - And this work is done... (EP) What conclusion can we come to from the above?
Definition	The work done by the electric field forces that create an electric current is called current work. In the process of such work, the energy of the electric field is converted into another type of energy - WHAT? (mechanical, internal, etc.) WHAT DOES THE WORK OF CURRENT DEPEND ON? (on the strength of the current, i.e. the electric charge flowing through the circuit in 1 s) - you were convinced of this in previous lessons and when performing L/R.
Study Divided into 2 groups, students close the collected circuits. Compare ammeter readings in Fig. 63 and 64 Animation 3 (Lamp demonstration)	And now, guys, each of you has a piece of paper on your desk that says “Research.” The word is written on the back. (Written on the pieces of paper Dielectrics: distilled water, glass, plastics, benzene, oils, mica, porcelain, air, rubber, various resins, wood; Conductors: salt solutions, acid solutions, silver, copper, aluminum, gold, water, graphite, copper,) On 2 desks there are inscriptions DIELECTRIC and CONDUCTOR. The slide shows a group of dielectrics and conductors. Everyone finds their word in the group and goes to the table where there is a card with this name. The guys share this. into 2 groups and conduct a study: they close the assembled circuit.. Guys, look at the ammeter readings. Voice each group. (Each group names). If the ammeter shows less current in the circuit with the lighting lamp, and more current passes through the circuit with the lamp from the flashlight, then why do the brightness of the light bulbs differ? (guys' answers) Idealized, special case when the ammeter readings are the same. This means that the work of current depends not only on the current strength, but also from another value... (which is called electrical voltage or just tension)
3) Studying new material:	So, today we will learn what voltage is, learn how to measure it, and get acquainted with its main characteristics.
Write down the date and topic of the lesson in your notebook (On the board)	Topic of our lesson: “Electrical voltage. Voltage units"
Plan on the board and 1 one on the desk	When getting acquainted with a new quantity, we will use what is already known to us plan. Guys, find the definition in the textbook; whoever found it, read it to the class. (p. 91)
Definition	Electrical voltage – this is a physical quantity characterizing the electric field
Definition	Conclusion: Voltage shows how much work the electrical system does. field when a unit positive charge moves from one point to another.
Designation I write on the board, simultaneously with the slide	- Voltage is indicated U; - Job A; - letter charge q;
	Based on the definition of voltage: knowing the work of the current in a given section of the circuit and the entire electrical circuit. charge passing through this section, we can draw up an equation, i.e., the work of the current when moving a unit electric. charge:	Task card.
Formula for calculation Animation 4	U = A/ q →A= Uq; q = A/U
(Mini message) Give to one child in advance	- Is this portrait familiar to you? (Yes, Alessandro Volta) What do you think the unit of electrical voltage is called? (Volt) . Voltage designation IN. Let's turn to the voltage formula and try to derive the unit of measurement. U = A/ q; 1 V = 1 J/C
Units of measurement	The unit of voltage is taken to be the following: voltage at the ends of the conductor at which the work of moving an electric charge of 1 C along this conductor is equal to 1 J: 1 V = 1 J/C In the next lesson you will get acquainted with a device for measuring voltage - a voltmeter. It is connected to the circuit in parallel, try to assemble the circuit using a voltmeter.
Slide 12 Measuring device Designation on the diagram Connection rules	- How to understand the meaning of tension? Electric current is similar to the flow of water in rivers and waterfalls, i.e. similar to the flow of water from a higher level to a lower one. Charge q corresponds to the mass of water, and voltage corresponds to the difference in levels, the pressure of water in the river.
Slide 13	The work done by falling water depends on its mass and height of fall, and therefore depends on potential energy. The greater the difference in water levels, the more work the water does. The work done by the current depends on the electrical charge and voltage on that conductor. The higher the voltage on a section of the circuit, the greater the work done by the current for the same amount of charge. In grade 10, we will express the work of an electric field through the difference in potential energy. If there is no voltage in the circuit, then there is no electric current in it (just as there is no flow in a lake or pond if there is no difference in levels in the relief).
4) Homework: written on the board in advance	§39-40, take a test using a telephone on the topic “Electrical Voltage”.
Creative project. Slide 14	The class is working on a creative project. Let's try to paint our own picture? It will need to be explained from an electrical point of view.	If I don't have time, then Homework
Reflection(green and red circles are attached to the light bulb drawing using magnets). (1 min) Attached to the board is a Whatman paper with a light bulb drawn on it. Before leaving.

PLAN(on the board)

1. Definition
2. Designation
3. Units of measurement
4. Formula for calculation
5. What device is used to measure
6. Designation on the diagram
7. Rules for connecting in a circuit

Mini message

VOLTA Alessandro - Italian naturalist, physicist, chemist and physiologist. His most important contribution to science was the invention of a direct current source, which played a decisive role in further studies of electrical and magnetic phenomena.

Individual card for strong students

1. Determine the voltage on a section of the circuit if, when a charge of 15 C passes through it with a current, 6 kJ of work is done?
2. When transferring 60 C of electricity from one point of an electrical circuit to another, 900 J of work is done in 12 minutes. Determine the voltage and current in the circuit.

Lesson topic: Electrical voltage. Voltmeter

Lesson type: studying and primary consolidation of new knowledge and methods of activity

Electrical voltage. Voltmeter

Lesson objectives: organize activities for perception, comprehension and primary memorization of new knowledge and methods of activity on the topic: “Electrical voltage. Voltmeter".

Lesson objectives:

Ensure that students are familiar with the concept of voltage and its units of measurement;

Create conditions for nurturing motives for learning, a positive attitude towards knowledge, and discipline;

Ensure the formation of the skills to highlight the main thing, draw up a plan, take notes, observe, develop the skills of partial - search activity, putting forward a hypothesis and solving it.

Lesson progress:

1. Organizational stage

Greeting, recording absentees, checking students’ preparedness for the lesson, disclosing the goals of the lesson and its plan.

2. Checking homework

Testing 2 options of 6 tasks

Test on the topic: “Current strength. Units of current. Ammeter. Current measurement"

1. Current strength is a physical quantity equal to...

a) ... the electric charge passed through the electrical circuit during its operation.

b) ... the electric charge passing through the cross-section of the conductor in the circuit.

c) ... the electric charge passed in the circuit through the cross-section of the conductor in 1 s.

d) ...electric charge moved in 1 s from the positive pole of the current source to the negative.

2. What is the name of the unit of current?

a) Joule (J). b) Watt (W). c) Coulomb (Cl). d) Ampere (A).

3. Convert current values ​​equal to 0.05 A and 500 μA into milliamps.

a) 50 mA and 0.5 mA. b) 500 mA and 5 mA.

c) 500 mA and 0.5 mA. d) 50 mA and 5 mA.

4. What is the current strength in the circuit if a charge of 120 C passes through its cross section within 4 minutes?

a) 30 A. b) 0.5 A. c) 5 A. d) 3 A.

5. The current strength is measured...

a) ...galvanometer. b) ...a galvanic cell.

c) ...ammeter. d) ...an electrometer.

6. According to the reading of ammeter No. 2, the current in the circuit is 0.5 mA. What current strength will be recorded by ammeters No. 1 and No. 3?

a) No. 1 - less than 0.5 mA, No. 3 - more than 0.5 mA.

b) No. 1 - more than 0.5 mA, No. 3 - less than 0.5 mA.

c) No. 1 and No. 3, like No. 2, - 0.5 mA.

1. What formula is used to determine current strength?

a) N = A/t. b) I = q/t. c) m = Q/λ. d) m = Q/L.

2. Express the currents equal to 0.3 A and 0.03 kA in milliamps?

a) 30 mA and 3000 mA. b) 300 mA and 30,000 mA.

c) 300 mA and 3000 mA. d) 30 mA and 30,000 mA.

3. What are the current values ​​of 800 µA and 0.2 kA in amperes?

a) 0.008 A and 200 A. b) 0.0008 A and 20 A.

c) 0.0008 A and 200 A. d) 0.008 A and 20 A.

4. In a conductor connected to the circuit for 2 minutes, the current was 700 mA. How much electricity passed through its cross section during this time?

a) 8.4 Kl. b) 14 Kl. c) 1.4 Kl. d) 84 Kl.

5. The current strength in which lamp is indicated by an ammeter connected to this circuit?

d) In each of them.

6. At what part of the circuit in which the electric lamp and the bell operate should the ammeter be turned on to find out the current strength in the bell?

a) Before the bell (in the direction of the electric current).

b) After the call.

c) Near the positive pole of the current source.

d) On any part of this chain.

Answers

3. Updating the subjective experience of students

1. The current strength can be judged by the ammeter readings, or by the effect of the current (the more hot the filament is, the greater the current strength)

Question: What does the current strength depend on?

Demonstration: increasing ammeter readings as the number of current sources increases.

Answer: The strength of the current depends on some quantity associated with the current source.

2. The current source creates an electric field by performing work to separate electric charges.

4. Learning new knowledge and ways of doing things

The work done by the electric field that creates the current is called the work done by the current.

A-current work

The stronger the electric field, the greater the speed of movement of charged particles, the greater the transferred charge, the greater the electric current.

An electric field is characterized by a quantity called electric field voltage.

Electric field voltage is a physical quantity that characterizes the effect of an electric field on charged particles.

U is the electric field voltage.

U = A/q - voltage shows how much work the electric field does to move a unit of charge in a given section of the circuit.

Safe voltage 42 V.

A voltmeter is a device for measuring voltage.

The voltmeter is connected to those points in the circuit between which the voltage must be measured (in parallel), plus to plus, and minus to minus.

5. Initial check of understanding of what has been learned

Questions:

1. What is the work of current called? (The work of the electric field that creates the current)

2. What is called electrical voltage? (Physical quantity characterizing the effect of an electric field on charged particles)

3. Designation and units of voltage. (U, volts)

3. What is the name of the device for measuring voltage? (Voltmeter)

4. How is a voltmeter connected to a circuit? (Connect to those points of the circuit between which the voltage must be measured (in parallel), plus to plus, and minus to minus)

6. Stage of consolidation of what has been learned

Work on a collection of problems in physics (V.I. Lukashik, E.V. Ivanova) No. 1265, 1266-orally.

7. Stage of generalization and systematization

Solve problems:

1. Determine the voltage on a section of the circuit if, when a charge of 15 C passes through it, a current of 6 kJ is performed.

U = A/q = 6000 J/15 C = 400 V.

2. When transferring 60 C of electricity from one point of an electrical circuit to another in 12 minutes, 900 J of work is done. Determine the voltage and current in the circuit.

U = A/q = 900 J/60 C = 15 V

I = q/t = 60 C/720 s = 0.08 A.

8. Results, homework p.39-41

9. Reflection

Reflection. (Draw the arrows to those statements that correspond to your state at the end of the lesson).

List of used literature

1. Peryshkin A.V. Physics. 8th grade. - M.: Bustard, 2009.

2. Lukashik V.I., Ivanova E.V. Collection of problems in physics grades 7-9 - M.: Prosveshchenie, 2008.

3. Chebotareva V.A. Physics tests. 8th grade - Publishing house "Exam", 2009.

Electrical voltage refers to the work done by an electric field to move a charge of 1 C (coulomb) from one point of a conductor to another.

How does tension arise?

All substances consist of atoms, which are a positively charged nucleus around which smaller negative electrons circle at high speed. In general, atoms are neutral because the number of electrons matches the number of protons in the nucleus.

However, if a certain number of electrons are taken away from the atoms, they will tend to attract the same number, forming a positive field around themselves. If you add electrons, then an excess of them will appear, and a negative field will appear. Potentials are formed - positive and negative.

When they interact, mutual attraction will arise.

The greater the difference - the potential difference - the stronger the electrons from the material with their excess content will be drawn to the material with their deficiency. The stronger the electric field and its voltage will be.

If you connect potentials with different charges of conductors, then electric will arise - a directed movement of charge carriers, seeking to eliminate the difference in potentials. To move charges along a conductor, the electric field forces perform work, which is characterized by the concept of electric voltage.

What is it measured in?

Temperatures;

Types of voltage

Constant voltage

The voltage in the electrical network is constant when there is always a positive potential on one side and a negative potential on the other. Electric in this case has one direction and is constant.

The voltage in a direct current circuit is defined as the potential difference at its ends.

When connecting a load to a DC circuit, it is important not to mix up the contacts, otherwise the device may fail. A classic example of a source DC voltage are batteries. Networks are used when there is no need to transmit energy over long distances: in all types of transport - from motorcycles to spacecraft, V military equipment, electric power industry and telecommunications, during emergency power supply, in industry (electrolysis, smelting in electric arc furnaces, etc.).

AC voltage

If you periodically change the polarity of the potentials, or move them in space, then the electric one will rush into reverse direction. The number of such changes in direction over a certain time is shown by a characteristic called frequency. For example, standard 50 means that the polarity of the voltage in the network changes 50 times per second.

Voltage in electrical networks AC is a time function.

The law of sinusoidal oscillations is most often used.

This happens due to what appears in the coil asynchronous motors due to the rotation of an electromagnet around it. If you expand the rotation in time, you get a sinusoid.

Consists of four wires - three phase and one neutral. the voltage between the neutral and phase wires is 220 V and is called phase. Between phase voltages also exist, called linear and equal to 380 V (potential difference between two phase wires). Depending on the type of connection in three-phase network You can get either phase voltage or linear voltage.

27.01.2016

Lesson 35 (8th grade)

Subject. Electrical voltage. Voltmeter

1. Electrical voltage, unit of measurement, calculation formula

In previous lessons, we learned about what current strength is and that this value characterizes the action of electric current. We have already considered several factors on which it depends, now we will consider other parameters that influence it. To do this, it is enough to carry out a simple experiment: first connect one current source to the electrical circuit, then two identical ones in series, and then three identical sources, each time measuring the current strength in the circuit. As a result of the measurements, a simple relationship will be visible: the current strength increases in proportion to the number of connected sources. Why does this happen? The function of a current source is to create an electric field in a circuit; accordingly, the more sources connected in series to the circuit, the stronger the electric field they create. From this we can conclude that the electric field affects the current strength in the circuit. In this case, when charges move along a conductor, work is done by the electric current, which indicates that the work of the electric field determines the strength of the current in the circuit.

On the other hand, we can recall the analogy between the flow of electric current in a conductor and water in a pipe. When talking about the mass of water flowing through a cross-section of a pipe, this can be compared with the amount of charge that passed through the conductor. And the height difference in the pipe, which forms the pressure and flow of water, can be compared to such a concept as electrical voltage.

To characterize the operation of the electric field in moving a charge, a quantity such as electric voltage has been introduced.

Electric voltage is a physical quantity that is equal to the work of an electric field to move a unit charge from one point to another.

Designation. voltage

Unit of measurement. volt

The unit of voltage measurement is named after the Italian scientist Alessanro Volta (1745–1827) (Fig. 1).

If we give a standard example about the meaning of the well-known inscription on any household appliances “220 V”, then it means that 220 J of work is done on a section of the circuit to move a charge of 1 C.

Rice. 1. Alessanro Volta

Formula for calculating voltage:

Electric field work on charge transfer, J;

Charge, Cl.

Therefore, the voltage unit can be represented as follows:

There is a relationship between the formulas for calculating voltage and current that you should pay attention to: and. Both formulas contain the value of electric charge, which may be useful in solving some problems.

2. Voltmeter

To measure voltage, a device called voltmeter(Fig. 2).

Rice. 2. Voltmeter

There are various voltmeters according to the features of their application, but the principle of their operation is based on the electromagnetic effect of current. All voltmeters are designated by a Latin letter, which is applied to the instrument dial and is used in a schematic representation of the device.

In school settings, for example, voltmeters are used, shown in Figure 3. They are used to measure voltage in electrical circuits during laboratory work.

The main elements of a demonstration voltmeter are the body, scale, pointer and terminals. The terminals are usually labeled plus or minus and are highlighted in different colors for clarity: red for plus, black (blue) for minus. This was done in order to ensure that the terminals of the device are obviously correctly connected to the corresponding wires connected to the source. Unlike an ammeter, which is connected to the open circuit in series, a voltmeter is connected to the circuit in parallel.

Of course, any electrical measuring device should have minimal influence on the circuit under study, which is why the voltmeter has such design features, that minimal current flows through it. This effect is achieved by selecting special materials, which contribute to minimal charge flow through the device.

3. Voltmeter in electrical circuits

Schematic representation of a voltmeter (Fig. 4):

Rice. 4.

The circuit contains an almost minimal set of elements: a current source, an incandescent lamp, a switch, an ammeter connected in series, and a voltmeter connected in parallel to the light bulb.

Let's depict for example electrical diagram(Fig. 5), in which it is connected voltmeter.

Comment. It is better to start assembling an electrical circuit with all elements except the voltmeter, and connect it at the end.

When connecting a voltmeter to a circuit, the following rules must be observed:
1) the voltmeter clamps are connected to those points of the circuit between which the voltage must be measured (parallel to the corresponding section of the circuit);
2) the voltmeter terminal with the “+” sign should be connected to that point in the circuit section that is connected to the positive pole of the current source, and the terminal with the “-” sign should be connected to the point that is connected to the negative pole of the current source.
If you need to measure the voltage at a current source, then a voltmeter is connected directly to its terminals (Fig. 31).

In other cases, for example when measuring the voltage across a lamp, this is done as shown in Figure 32.

4. Types of voltmeters

There are many various types voltmeters with different scales. Therefore, the question of calculating the price of the device in this case is very relevant. Microammeters, milliammeters, simply ammeters, etc. are very common. Their names make it clear with what frequency the measurements are taken.

In addition, voltmeters are divided into direct current and alternating current devices. Although there is alternating current in the city network, at this stage of studying physics we are studying DC which everyone serves galvanic cells, so we will be interested in the corresponding voltmeters. The fact that the device is intended for alternating current circuits is usually depicted on the dial as a wavy line (Fig. 6).

Rice. 6. AC voltmeter

Comment. If we talk about voltage values, then, for example, a voltage of 1 V is a small value. Industry uses much higher voltages, measured in hundreds of volts, kilovolts and even megavolts. In everyday life, a voltage of 220 V or less is used.

Consolidation. Solving typical problems:
Problem 1

The tile is included in the lighting network. How much electricity flows through it in 10 minutes if the current in the supply cord is 5A?

Time in SI system 10 minutes = 600s,
By definition, current is equal to the ratio of charge to time.
I=q/t
Hence, the charge is equal to the product of current and time.
q = I t = 5A 600 s = 3000 C

Problem 2

How many electrons pass through the filament of an incandescent lamp in 1 s when the current in the lamp is 1.6 A?

The charge of the electron is e= 1.6 10 -19 C,
The entire charge can be calculated using the formula:
q = I t – charge is equal to the product of current and time.
The number of electrons is equal to the ratio of the total charge to the charge of one electron:
N=q/ e
This implies N = I t / e= 1.6A 1s/1.6 10 -19 Cl = 10 19

Problem 3

Determine the voltage on a section of the circuit if, when a charge passes through it,

In a 15 C current, work of 6 kJ was performed.

U = A/q = 6000 J/15 C = 400 V.

Problem 4

When transferring 60 C of electricity from one point in an electrical circuit to

the other completes 900 J of work in 12 minutes. Determine the voltage and current

U = A/q = 900 J/60 C = 15 V

I = q/t = 60 C/720 s = 0.08 A.

Homework:

1. V.V.Belaga, I.A.Lomachenkov, Yu.A.Panebrattsev. Physics. 8th grade, Moscow, “Enlightenment”, 2016. Read § 34 (p.82-83).

2. Answer questions (orally).

2.1. The student claims that an ammeter connected to the circuit in front of the light bulb will show a greater current strength than one connected after it. Is the student right?

2.2. How to determine maximum strength current that can be measured using this ammeter?

3. Solve problems:

3.1. At what current strength does 32 C pass through the cross-section of the conductor in 4 s?

3.2. Calculate the current strength in the conductor through which a charge of 24 C passed in 96 s.

3.3. When electric current flows through aqueous solution acid releases hydrogen. What electric charge passes through the acid solution if, at a current strength of 2 A, the process of obtaining required quantity hydrogen lasts 5 hours?

4. Solve problems:

4.1. Calculate how much charge passed through the conductor if, at a voltage of 36 V, the electric field performed 72 J of work.

4.2. Determine the division price of the device.