Study of series and parallel connection of conductors.
Lesson topic: Laboratory work No. 5 "Study serial connection conductors."
Lesson objectives:
educational: continue to develop skills in working with an ammeter,
a voltmeter, the ability to make observations and present experimental results in the form of tables;
educational: ensure that students understand that fairness
physical laws can be verified not only by direct experience, but also indirectly - by checking the consequences arising from these laws;
value: conscience (sub-values: justice);
development of thinking: experiment is a criterion of truth; provide understanding that
Objective data can only be obtained if the measurements are correct, ensuring the necessary accuracy.
Main material
Laboratory work.
During the classes
No amount of experimentation can prove a theory; but one experiment is enough to refute it (Albert Einstein)
Lesson Test 17 35.5
//. Solve a problem exercise 20(3)
/17. Laboratory work No. 5
The work is performed according to the description in the textbook (part 1 only).
IV. Homework assignment
*
Rep. §39-41 Exercise 19°(1,2), exercise 20(1)
*
Lesson 37.7 J
i
Lesson topic: Work and current power. Thermal effect of current and its practical application.
Lesson objectives:
educational: form the concepts of work and power electric current,
short circuit; teach how to calculate the cost of spent electrical energy by meter; ensure the assimilation of the Joule-Lenz law, the structure and operation of an incandescent lamp and other heating devices, fuses;
educational: continue the formation of a scientific worldview by increasing
the basis of ideas about the structure of matter, the composition of molecules and atoms, their movement and interaction when studying the mechanism of heating conductors;
spiritual and moral education: value: inner freedom of a person; true
(sub-values: self-comprehension; introspection);
development of thinking: develop the ability to apply the law of conservation of energy to
case of heating of stationary conductors.
Main material
^ Formula for current work (output). Current power. Joule-Lenz law. Construction of electric heating devices. Causes of network overload and short circuit. Circuit breakers.
During the classes
We slam the door on mistakes. The truth is in confusion: “How will I enter now?” (R. Tagore)
I. Checking homework. ! Frontal survey.
How does the resistance of a metal conductor depend on temperature?
What is superconductivity?
Current, voltage and resistance in series connection.
Voltage, current and resistance in parallel connection.! Independent work № 9 for lessons 35.5, 36.6
P. New material.
Formulas for current work.
Current work - this is the work of the electric field forces that create an electric current.
A =Aqu = IuAt = I 2 RAt; 1B-1A-1s = 1 J,
where A is the work of the current,R- resistance of the circuit section,At- current passage time.
Current power - this is the ratio of the work done by the current to the time during which this work is done.
R= - = Iu =rR = -; = 1 W.
* At R lc
A = pAt. 1J= 1 Bt-1 cThen 1 kWh = 1000 W-3600 s = 3600000 J = 3.6-10 6 J.Demonstrations.
Measuring the work of electric current. *
Show a single-phase electric energy meter with with the lid removed. Draw students' attention to the connection between the disk and the counting mechanism. Record the initial meter readings on the board with an accuracy of 0.1 kWh. Collect electrical circuit, turn on the load (lamp rheostat), record the readings of the counting mechanism.
tg
Cost = A-tariff(4.6).
kWh
A - work in kWh.
Measuring the power of an electrical energy consumer. *
* The wattmeter is set to 30 W and
bulb additional resistance 30 Ohm. An A-20 lamp is used as a load. The chain is connected to the BC-24 clamps. When the circuit is closed, the readings of the device are recorded and the power consumed by the lamp is determined.
Joule-Lenz law.
When an electric current passes, the conductor heats up.Demonstrations. The experience described on p. 153 textbooks.
In this case, the work done by the electric current is equal to the amount of heat released by the current-carrying conductor:
Q = I 2 RAt
Construction of electric heating devices.
Demonstrations. Incandescent lamp: device and switching on - mains lamp and socket for it.
Causes of overload and short circuit. Circuit breakers. Fuse blown due to overload or short circuit.
The installation contains four A-20 automotive light bulbs. A long thin copper conductor is stretched at the bottom of the installation. He is the fuse. If the rated voltage is supplied to the light bulbs from the VS-24 rectifier, the fuse will blow. If such an experiment does not work, then the electrodes of one of the parallel-connected lamps should be short-circuited, fuses
“And the electric current is strong” by A.A. Block
It is necessary to show ("traffic jams") and circuit breakers, which open the circuit in the event of an overload or short circuit as a result of the operation of a thermobimetallic or electromagnetic release.
///. Solve problems: control .2 1(1-3) IV Homework assignment
§42-44
Control 21°(4.5), control 22(1.4)
Study of resistor resistances in series and parallel connections.
Goal of the work: Using experience, check the patterns of an electrical circuit with series and parallel connections of resistors.
1.Explanation of work
Brief theoretical information
A series connection of resistances is a connection in which the end of the first resistance is connected to the beginning of the second, the end of the second to the beginning of the third, etc.
The total resistance of series-connected resistors is equal to the sum of their resistances.
Rtot. =R 1 +R 2 +R 3
R total = 5ohm+10ohm+25ohm=40ohm
Current value in series circuit
Since there is no current branch in this circuit, it is obvious that the amount of electricity flowing through the cross section of the conductor per unit time. at any point in the chain will be the same.
Therefore, at all points in a series circuit the current value is the same.
These four ammeters will show the same current values. Therefore, with a series connection to measure current, it is enough to turn on one ammeter at any part of the circuit.
Voltage distribution in a series circuit
The voltage of the current source applied to the external section of the circuit is distributed over the sections of the circuit in direct proportion to the resistance of these sections. The voltage applied to each of these resistors is determined by the formula:
Since the current in a series circuit is the same everywhere, it means that the voltage in its sections really depends on the resistance; the greater the resistance, the greater the voltage applied to this section.
The sum of the voltages in the sections of the series circuit is equal to the voltage of the current source
A parallel connection of resistances is a connection in which the beginnings of the resistances are connected to one terminal of the source, and the ends to the other terminal.
The total resistance of parallel-connected resistances is determined by the formula:
The total resistance of resistances connected in parallel is always less than the smallest resistance included in a given connection.
From the above figure we can immediately say that total resistance will be less than 10 ohms.
First special case
If only two resistors are connected in parallel, then their total resistance can be determined by the formula:
Second special case
If any number of resistors of the same resistance are connected in parallel, then their total resistance can be determined by dividing the resistance of one resistor by the number of resistors.
Distribution of currents and voltages in parallel branches
Since the beginnings of all resistances are brought to one common point, and the ends to another, it is obvious that the potential difference at the ends of any parallel-connected resistance is equal to the potential difference between the common points.
So, when connecting resistances in parallel, the voltages across them are equal to each other.
If the branching is connected directly to the terminals of the current source, then the voltage at each of the resistances is equal to the voltage at the source terminals.
The second property of a parallel circuit is that the electric current is distributed among the parallel branches in inverse proportion to their resistance.
This means that the greater the resistance, the less current will flow through it.
Considering the branching point A, we notice that current I flows into it, and currents I1, I2, I3 flow out of it. Since moving electric charges do not accumulate at a point, it is obvious that the total charge flowing to the branching point is equal to the total charge flowing away from it:
Therefore, the third property parallel connection can be formulated like this:
The magnitude of the current in the unbranched part of the circuit is equal to the sum of the currents in the parallel branches.
2.Technical specifications
2.1.Assemble an electrical circuit of series connection of resistors (Figure 1)
Figure 1. Electrical circuit diagram.
2.2.Assemble an electrical circuit for parallel connection of resistors (Figure 2)
Figure 2. Electrical circuit diagram.
2.3. Take instrument readings and write them down in the table
2.4.Make calculations
2.5. Build graphs
2.6.Reply to Control questions
2.7. Draw a conclusion
3.Work in the laboratory
3.1. Study of series connection of resistors
3.1.1 Assemble the circuit (Figure 3).
Figure 3. Study design.
3.1.2 Set the values R1=100 Ohm + N, R2=100 Ohm + 2N and R3=130 Ohm + 4N on the diagram,where N is the student’s number according to the magazine (resistor power 1 W).
3.1.3. Turn on the source and set the voltage U=15 V, 24 V.
3.1.4. Measure the amount of current flowing in the circuit and enter the value in Table 1.
3.1.5. Measure the voltage across each resistor and record it in Table 1.
3.1.6. Measure the resistance of each resistor and record it in Table 1.
3.1.7. Disable the scheme.
3.1.8. Calculate the resistance of resistors using the formulas:
Table 1 - Measured parameters
|
Measurement no. |
By measuring |
By calculation |
|||||||||||
3.2. Study of parallel connection of resistors
3.2.1. Assemble the circuit (Figure 4).
Figure 4. Study design.
3.2.2. Set on the diagram the values R 1 = 70 Ohm + N, R 2 = 100 Ohm + N and R 3 = 150 Ohm + N,
where N is the student’s number according to the magazine (resistor power is more than 1 W).
3.2.3. Turn on the source and set the voltage U=15 V, 24 V.
3.2.4. Measure the amount of current flowing in the entire circuit and enter the value in Table 2.
3.2.5. Measure the amount of current flowing in each resistor and record it in Table 2.
3.2.6. Calculate the conductivity of each resistor and write it in Table 2 (by setting):
3.2.8. Disable the scheme.
T table 2 - Measured parameters
|
Meas. no. |
By measuring |
Installation |
By calculation |
||||||||||||
|
g e |
R e |
g e |
R e |
||||||||||||
|
Cm |
Cm |
Cm |
Cm |
Ohm |
Cm |
Cm |
Cm |
Cm |
Ohm |
||||||
5.Security questions
5.1. Which connection of resistors is called series?
5.2. How to determine the total resistance of resistors in a series connection?
5.3. What is conductivity called and in what units is it measured?
5.4. What is equal to total current circuits and voltage in sections with a series connection?
5.5. How is the power determined in sections of the circuit and the entire circuit when connected in series?
5.6. What connection of resistors is called parallel?
5.7. How to determine the total resistance of resistors in a parallel connection?
5.8. What is the total current of the circuit and the voltage in the sections with a parallel connection?
5.9. How is the power determined in sections of the circuit and the entire circuit in a parallel connection?
Laboratory work
Study of series and parallel connection of conductors
Homemade exercise:
To study the distribution of currents and voltages when connecting conductors in series, the experimenter assembled the electrical circuit shown in Figure 1 and obtained the voltage distribution shown in Figure 2.
Using the laws of electric current to connect conductors in series, determine the total resistance and voltage of the circuit, as well as the strength of the electric current in the circuit.
Resistor resistance | Resistor voltage | Current strength I in the circuit |
||||||
Using the laws of electric current to connect conductors in parallel, determine the total resistance and strength of the electric current, as well as the voltage across the resistors.
Write the results of measurements and calculations in the table
Resistor resistance | Voltage U across the resistor |
|||||||
Performing laboratory work
Goal of the work: check the validity of the laws of electric current for series and parallel connections of conductors.
Equipment: current source, two wirewound resistor, ammeter, voltmeter, rheostat.
Theory:
Laws of electric current for series connection of conductors:
Laws of electric current for parallel connection of conductors:
Conducting the experiment and processing the results:
rice. 5
Electric current strength I in the circuit | Resistor voltage | Resistor resistance |
||||
Assemble an electrical circuit (Fig. 6) and use a rheostat to set the voltmeter needle to a certain scale division.
Use an ammeter to measure the electric current in the general circuit and in the circuits of individual consumers.
rice. 6
Write down the results of measurements and calculations in the table:
Voltage U across the resistor | Electric current strength in the circuit | Resistor resistance |
||||
Carry out calculations based on the results of the experiment.
Based on the experiments performed, draw a conclusion about whether the laws of electric current are satisfied for series and parallel connections of conductors.
Report on laboratory work must contain
Topic of work
Goal of the work
List of equipment used
Theory (completed tables)
Description of the work progress
Wiring diagrams
Tables with measurement and calculation results