Electrical voltage. Definition, types, units of measurement. Electrical Voltage Thanks for the lesson! Good luck

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 conduct a simple experiment: first connect one current source to an 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. 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, therefore the voltmeter has such design features that a minimum current flows through it. This effect is ensured by the selection of special materials that 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 us draw, for example, an electrical circuit (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 different types of 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 dealing with direct current, which is supplied by all galvanic elements, 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 the maximum current that can be measured using a given 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 an electric current flows through an aqueous acid solution, hydrogen is released. What electric charge passes through the acid solution if, at a current of 2 A, the process of obtaining the required amount of 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.

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Physics lesson in 8th grade "Electrical voltage. Voltage units. Voltmeter"

Lesson development provided by: Yulia Vladimirovna Tolstykh, teacher of physics and computer science, I qualification category, Municipal Educational Institution Secondary School in the village of Kuzminskie Otverzhki, Lipetsk region, email: [email protected]

The purpose of the lesson:

1. Give the concept of tension and its explanation; introduce the formula and unit of voltage; study the device for measuring voltage and the rules for connecting it to the circuit.
2. Develop chain assembly skills; thinking; memory; speech; interest in the subject; the ability to apply acquired knowledge in practice.
3. Fostering a sense of responsibility, collectivism, a conscientious attitude towards completing tasks, and self-discipline.

Progress of the lesson according to the textbook by A.V. Peryshkina.

1. Checking homework.

The teacher reads the questions:

1. The current strength is indicated by…..
2. The current strength is measured…..
3. Formula for calculating current.....
4. The device is connected to the circuit.....
5. Unit of electric charge…..
6. How many Amperes are in 1 mA?

Answers: choose an option

1. A-I-R
2. Voltmeter - clock - ammeter
3. F = m a- I = q / t- q = I t
4. parallel-series-first
5. 1 sec - 1 meter - 1 pendant
6. 0.001A - 10A - 100A

Cards with tasks are distributed to weak students, and the rest work at the board and on questions

2. Explanation of new material.

1. Safety precautions when working with electrical equipment.

• Remember, guys, what is called the work of current? The work done by the electric field that creates the current is called the work done by the current.
• What kind of quantity is the work of current? What does it depend on?

It is safe to say that it depends on the strength of the current, i.e. on the electric charge flowing through the circuit at 1 s, as well as on a new value for you, which is called electric voltage.

Voltage is a physical quantity that characterizes an electric field and shows how much work the electric field does when moving a unit positive charge from one point to another. It is denoted by the letter U. To calculate the voltage, the formula is used: U = A / q. The unit of voltage is named Volt (V) in honor of the Italian scientist Alessandro Volta, who created the first galvanic cell. The unit of voltage is the electrical voltage at the ends of the conductor , in which the work of moving an electric charge of 1 C along this conductor is equal to 1 J. 1V = 1J / 1C In addition to the volt, submultiples and multiples of it are used: millivolt (mV) and kilovolt (kV). 1mV = 0.001V 1kV = 1000V To measure the voltage at the poles of a current source or at some section of the circuit, a device called a voltmeter is used. The voltmeter clamps are connected to those points in the circuit between which the voltage must be measured. This connection of the device is called parallel. Assembly of a circuit and drawing of a circuit containing a voltmeter. It explains how the device is indicated on the diagram.

Voltage

Letter U

Formula U=A/q

Unit 1 Volt

Submultiple units s 1kV = 1000V

Multiples of units 1mV = 0.001V

Device voltmeter

Connection to the circuit parallel

Demonstration of voltmeters of various types with a story and explanation of their operating principle.

3. Consolidation of acquired knowledge.

Write down 2 options on the board and call two students to work independently.

Convert these voltage values ​​to Volts:

1st option:

2nd option:

Assignments for working with the class:

Exercise 1: Draw a diagram of an electrical circuit consisting of a battery, an electric bell, a key, a voltmeter and an ammeter, which respectively measure the voltage on the bell and the current in it. The diagram will indicate the signs of the battery terminals, ammeter and voltmeter, observing the rules for connecting them. Indicate with arrows the direction of current in the circuit and the direction of movement of electrons in it.

Task 2: What work is done by the electric field when moving a charge of 4.5 C through the cross section of the filament of the lamp, if the voltage across the lamp is 3 V?

(A=Uq=3 B *4.5 Cl= 13.5 J)

Task 3: When the same amount of electricity passes through one conductor, 100 J of work is done, and 250 J of work is done in the other. Which conductor has more voltage? How many times?

(When the same amount of electricity passes through a conductor, the voltage will be greater in the case in which the work done by the current is greater. In the second case, the work done by the current is 250J/100J=2.5 times greater)

Task 4: What electrical voltage values ​​does a person encounter in everyday life? (127V, 220V)

4. Summing up the lesson.

Questions survey.

• What is the work of current called?
• How to explain the electrical voltage in a section of a circuit?
• Formula for calculating voltage.
• Submultiples and multiples of voltage units.
• The purpose of a voltmeter and the rules for connecting it to a circuit.

Well done boys! Lesson grades.

5. Homework. §39-41 Ex 16 ( A.V. Peryshkin)

Sections: Physics

Class: 8

The purpose of the lesson: to give the concept of voltage as a physical quantity characterizing the electric field that creates an electric current, to introduce the unit of voltage.

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

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?
4. How is it indicated in the diagrams?
5. What is the unit of current called? How is it designated?
6. What rules should be followed when connecting an ammeter to a circuit?
7. 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?

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.

1. 3) What electric charge flows through the ammeter in 5 minutes when the current in the circuit is 0.5 A.

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 of the electrical circuit.
a) at point M

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

Option II

1.Express 0.025 A in ammeters.

1. How many milliamps is 0.25 A?
a) 250 mA;
b)25mA;
c) 2.5mA;
d) 0.25mA;

2. How many microamps are there in 0.025mA?

2.Express 0.25mA in microamps.
a) 250 µA;
b) 25 µA;
c) 2.5 µA;
d) 0.25 µA;

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

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

In Fig. 1 shows a diagram of the electrical circuit.
a) at point M

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

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

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) The operating voltage in the lighting network of residential buildings and social facilities is 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?

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?

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

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 CD “Cyril and Methodius”. Physics.8th grade.

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 pressure

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 constant voltage source is batteries. Networks are used when there is no need to transmit energy over long distances: in all types of transport - from motorcycles to spacecraft, in military equipment, electric power and telecommunications, for 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 in the opposite 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 AC electrical networks is a time function.

The law of sinusoidal oscillations is most often used.

This happens due to what occurs in the coil of 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 a three-phase network, you can get either phase voltage or linear voltage.

Have you ever tried inflating timed balloons? One inflates quickly, while the other inflates much less during the same time. Without a doubt, the former does more work than the latter.

The same thing happens with voltage sources. To ensure the movement of particles in a conductor, work must be done. And this work is done by the source. The operation of the source is characterized by voltage. The larger it is, the more work the source does, the brighter the light bulb in the circuit will burn (under other identical conditions).

The voltage is equal to the ratio of the work of the electric field to move the charge
to the amount of charge moved in a section of the circuit.

U = A q, where \(U\) is voltage, \(A\) is the work of the electric field, \(q\) is charge.

Pay attention!

The SI unit of voltage is [\(U\)] = \(1\) B (volt).

\(1\) volt is equal to the electrical voltage in the section of the circuit where, when a charge equal to \(1\) C flows, work equal to \(1\) J is performed: \(1\) V \(= 1\) J /1 Cl.

Everyone has seen the inscription on home appliances “\(220\) V”. It means that on a section of the circuit, work \(220\) J is performed to move the charge \(1\) C.

In addition to the volt, submultiples and multiples of it are used - millivolts and kilovolts.

\(1\) mV \(= 0.001\) V, \(1\) kV \(= 1000\) V or \(1\) V \(= 1000\) mV, \(1\) V \( = 0.001\) kV.

To measure voltage, use a device called a voltmeter.

All voltmeters are designated by the Latin letter \(V\), which is applied to the instrument dial and is used in the schematic representation of the device.

In school settings, the voltmeters shown in the figure are used:

The main elements of a 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 - plus, black (blue) - minus. This was done for the purpose of knowingly correctly connecting the terminals of the device to the corresponding wires connected to the source.

Pay attention!

Unlike an ammeter, which is connected to the open circuit in series, a voltmeter is connected to the circuit in parallel.

When connecting a voltmeter to a DC circuit, polarity must be observed.

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

Voltmeters are divided into DC devices And alternating current.

If the device is intended for alternating current circuits, then it is customary to depict a wavy line on the dial. If the device is designed for DC circuits, then the line will be straight.

You can pay attention to the terminals of the device. If the polarity is indicated (“\(+\)” and “\(-\)”), then this is a device for measuring DC voltage.

Sometimes the letters \(AC/DC\) are used. Translated from English, \(AC\) (alternating current) is alternating current, and \(DC\) (direct current) is direct current.
A voltmeter is connected to the AC circuit to measure AC current. It has no polarity.

Pay attention!

You can also use a multimeter to measure voltage.

Please remember that high voltage is dangerous.

What will happen to a person who finds himself next to a fallen exposed high voltage cable?

Since the earth is a conductor of electric current, dangerous step voltages can arise around a fallen exposed cable that is energized.