Electrical voltage. Voltage units

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.

During the classes:

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 the current equal to 0.05 A and 500 μA to 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 section 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-work of current

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

The 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.

Today we will get acquainted with another physical quantity, but first answer my question: when the light bulbs dim, what do we say?

(Voltage drops)

Topic: Electric voltage. Voltmeter. Voltage measurement.

Let's repeat and remember:

  • what is electric current;
  • what is an electric field;
  • what does an electrical circuit consist of?

We will find out:

  • what is tension?
  • units of voltage;

    • network voltage;

  • how to connect a voltmeter to a circuit.
HOMEWORK Yes - clap no - stomp

Clash of the Titans of Physics

Name the electrical appliances Find the symbol

What is electric current? Let us recall the conditions for the existence of electric current.

What particles carry electric charge in metals?

What makes these particles move?

The current strength can be judged by the ammeter readings, or by the effect of the current (the more heated the filament, the greater the current strength). Question: what does the current strength depend on?

Answer: The strength of the current depends on some quantity associated with the current source. The current source creates an electric field by performing work to separate electric charges.

Regular light bulb and battery

Flashlight bulb and battery

Let's find out what the work of current depends on

Electrical voltage characterizes electric field created by current... Voltage (U) shows which work (A) performs electrical field when moving a single positive charge (q) from one point to another.

Voltage =

SI unit of voltage:

U = 1V “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:

Convert to SI system:

  • 200 mV =
  • 6 kV =
  • 0.02 kA =
  • 270 mA =
  • 20 minutes. =
  • 2.1 MV =

2,100,000 V

Games with tension have a sad outcome

– Electric current doesn’t like to joke!

SAVE YOURSELF WHO YOU CAN!

  • The voltage considered safe for humans in a dry room is up to 36 V.
  • For a damp room this value drops to 12 V.
  • When a person touches a wire energized above 240 V, the current penetrates the skin. If a current flows through a wire, the magnitude of which is not yet lethal, but is sufficient to cause involuntary contraction of the muscles of the hand (the hand seems to “stick” to the wire), then the resistance of the skin gradually decreases, and eventually the current reaches a value lethal for a person at 0.1 A. A person who finds himself in such a dangerous situation needs to be helped as soon as possible, trying to “tear” him away from the wire without endangering himself.
Voltmeter:
  • Calibration "0"
  • "+" to "+" "-" to "-"
  • Connected in parallel
  • Symbol

Measuring voltage

Determine the division price of the device:

  • 2 V/div
  • 0.5 V/div

Electrical circuit assembly and voltage measurement

1. build a diagram of an electrical circuit in your notebook and determine the direction of the current

2. Assemble the electrical circuit, the key should be open

2. Find the “+” and “-” on the battery.

3. Consider the voltmeter, determine the division value

Find “0” on the voltmeter, remember how the voltmeter is connected

4. Call a teacher to check the electrical circuit

5. Only after the teacher’s permission, lock the key

and determine the voltmeter readings

6. Write down the voltmeter readings in your notebook

Problems 1. When an electric charge equal to 5 C passes through a conductor, 200 J of work is done. What is the voltage at the ends of this conductor? A) 1000 V B) 40 V C) 40 A D) 0.025 V

2. The voltage on a car light bulb is 12 V. What charge passed through the filament of the light bulb if 1200 J of work was done? A) 0.01 Kl B) 100 Kl C) 14400 Kl D) 10 V

3. Determine the work done when a charge of 80 C passes through the spiral of an electric stove, if it is connected to a network with a voltage of 220 V A) 0.36 J B) 2.75 J C) 17600 J D) 0.36 V

5. Determine the Voltmeter division value

A) 1 V B) 1.5 V C) 3 V D) 15 V

4. It is necessary to measure the current in the lamp and the voltage across it. How should the ammeter and voltmeter be connected in relation to the lamp?

Lesson summary:

We learned?

  • what is tension?
  • units of voltage?
  • what device is used to measure

    • network voltage?

  • How should a voltmeter be connected to a circuit?

Have you learned?

Homework

§39-41 Ex. 6 (2.3) Additionally (for evaluation): 1264.1265 - Lukashik.

Lightning When lightning strikes, for example a tree. It heats up, the moisture evaporates from it, and the pressure of the resulting steam and heated gases lead to destruction. To protect buildings from lightning discharges, lightning rods are used, which are a metal rod that rises above the protected object. Lightning. In deciduous trees, the current passes inside the trunk through the core, where there is a lot of sap, which boils under the influence of the current and the vapors tear the tree apart. The reason is that there is no voltage difference between the cable and the bird that lands on it. After all, she sits on it without touching the ground, and besides, she sits on only one cable. Thus, the voltages of the cable and the bird are absolutely the same. But if suddenly, flapping its wings, the same bird accidentally touches the neighboring cable, but with a different voltage, then the infernal machine will work... The reason is that there is no voltage difference between the cable and the bird landing on it. After all, she sits on it without touching the ground, and besides, she sits on only one cable. Thus, the voltages of the cable and the bird are absolutely the same. But if suddenly, flapping its wings, the same bird accidentally touches a neighboring cable, but with a different voltage, then the infernal machine will work... Fortunately, the cables are usually located at a considerable distance from each other, which makes their contact almost impossible. That is why the threat to the lives of birds is negligible. But God forbid you test this statement in practice.

Why do birds land on high-voltage transmission wires with impunity?

Why do electrified people's hair rise up?
  • The hair is electrified with the same charge. As you know, like charges repel each other, so hair, like the leaves of a paper plume, diverges in all directions. If any conducting body, including a human body, is isolated from the ground, then it can be charged to a high potential. Thus, with the help of an electrostatic machine, the human body can be charged to a potential of tens of thousands of volts.
- Electricity not only plays an important role in human life, but also in his health. By contracting, the muscle cells of the heart produce electricity. It is thanks to these impulses that the electrocardiogram measures the rhythm of the heart. - Electricity not only plays an important role in human life, but also in his health. By contracting, the muscle cells of the heart produce electricity. It is thanks to these impulses that the electrocardiogram measures the rhythm of the heart. Find physical errors:

Thank you for the lesson! Good luck!

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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: select 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)

The lesson is devoted to the concept of electrical voltage, its designation and units of measurement. The second part of the lesson is devoted primarily to demonstrating voltage measuring devices on a section of a circuit and their features.

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.

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.

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.

() () ()

Rice. 3. Voltmeters

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 - plus, black (blue) - 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.

Schematic representation of a voltmeter (Fig. 4):

Rice. 4.

Let us draw, for example, an electrical circuit (Fig. 5) in which a voltmeter is connected.

Rice. 5.

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.

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

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. Microvoltmeters, millivoltmeters, simply voltmeters, etc. are very common. Their names make it clear with what frequency the measurements are made.

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.

In the next lesson we will learn what the electrical resistance of a conductor is.

Bibliography

  1. Gendenshtein L. E., Kaidalov A. B., Kozhevnikov V. B. Physics 8 / Ed. Orlova V. A., Roizena I. I. - M.: Mnemosyne.
  2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
  3. Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M.: Education.

Additional precommended links to Internet resources

  1. Cool physics ().
  2. YouTube().
  3. YouTube().

Homework

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.

DC voltmeter

AC voltmeter

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.