The magnitude of the effect that the current can have on the conductor depends, whether it is thermal, chemical or magnetic effect of the current. That is, by adjusting the strength of the current, you can control its effect. Electric current, in turn, is the ordered movement of particles under the influence of an electric field.
Dependence of current and voltage
Obviously, the stronger the field acts on the particles, the greater the current in the circuit. The electric field is characterized by a quantity called voltage. Therefore, we conclude that the current strength depends on the voltage.
Indeed, it was possible to establish empirically that the current strength is directly proportional to the voltage. In cases where the voltage in the circuit was changed without changing all other parameters, the current increased or decreased by the same amount as the voltage was changed.
Relationship with resistance
However, any circuit or section of a circuit is characterized by another important value called resistance to electric current. Resistance is inversely related to current. If the resistance value is changed at any section of the circuit without changing the voltage at the ends of this section, the current strength will also change. Moreover, if we reduce the resistance value, then the current strength will increase by the same amount. Conversely, as the resistance increases, the current decreases proportionally.
Ohm's law formula for a chain section
Comparing these two dependencies, one can come to the same conclusion reached by the German scientist Georg Ohm in 1827. He linked together the three above physical quantities and brought forth a law which is named after him. Ohm's law for a section of a circuit reads:
The current strength in a circuit section is directly proportional to the voltage at the ends of this section and inversely proportional to its resistance.
where I is the current strength,
U - voltage,
R is resistance.
Application of Ohm's Law
Ohm's law is one of fundamental laws of physics. Its discovery at one time made it possible to make a huge leap in science. At present, it is impossible to imagine any most elementary calculation of the main electrical quantities for any circuit without using Ohm's law. The idea of this law is not the lot of exclusively electronic engineers, but a necessary part of the basic knowledge of any more or less educated person. No wonder there is a saying: "If you don't know Ohm's law, stay at home."
U=IR And R=U/I
True, it should be understood that in the assembled circuit, the resistance value of a certain section of the circuit is a constant value, therefore, when the current strength changes, only the voltage will change and vice versa. To change the resistance of a section of the circuit, the circuit must be reassembled. The calculation of the required resistance value during the design and assembly of the circuit can be made according to Ohm's law, based on the estimated values of the current and voltage that will be passed through this section of the circuit.
Lesson objectives: to comprehend the application of the studied physical quantities and the quantities connecting them.
Lesson Objectives:
- Students should learn that the amount of heat released by a conductor with current is equal to the product of the square of the current strength, the resistance of the conductor and time Q \u003d I? Rt;
- Students must learn to solve problems for finding the amount of heat in specific situations;
- Consolidation of students' skills in solving problems of settlement, qualitative
- and experimental;
- Formation of conscientious attitude to work among students, positive
- attitude to knowledge, education of discipline, aesthetic views.
During the classes
Knowledge update. Poll frontal.
1. What are the three quantities connected by Ohm's law?
I, U, R; current, voltage, resistance.
2. How is Ohm's law formulated?
The current strength in a circuit section is directly proportional to the voltage at the ends of this section and inversely proportional to its resistance.
3. How is the formula of Ohm's law written?
4. Units of measurement of physical quantities included in Ohm's law.
Amp, Volt, Ohm.
5. How to express the work of the current for some time?
6. What is called power?
To find the average power of the electric current, it is necessary to divide its work by the time P=A/t.
8. What is taken as a unit of power?
The power unit is 1 W, equal to 1 J/s, 1 W=1 J/s.
9. What connection of conductors is called serial?
10. What is the same value for all conductors connected in series?
Current strength, I \u003d I 1 \u003d I 2 \u003d I n
11. How to find the total resistance of the circuit, knowing the resistance of individual conductors, with a serial connection?
R=R 1 +R 2 +:+R n .
12. How to find the voltage of a section of a circuit consisting of series-connected conductors, knowing the voltage on each of them?
U=U 1 +U 2 +:+U n .
13. What connection of conductors is called parallel?
14. What is the same value for all conductors connected in parallel?
Voltage, U \u003d U 1 \u003d U 2 \u003d U n.
15. How to find the total resistance of the circuit, knowing the resistance of individual conductors, with a parallel connection?
R \u003d R 1 * R 2 * R n / (R 1 + R 2 + R n).
16. How to find the current strength in a circuit section with a parallel connection?
I=I 1 +I 2 +I n .
17. Electric current is called:
orderly movement of free electrons.
18. Formula for calculating the resistance of a conductor?
19. The ammeter is included in the circuit:
sequentially.
20. All consumers are under the same voltage when:
parallel connection.
21. Guess the riddle.
A very strict controller from the wall looks at point-blank range,
Looks, does not blink. All you have to do is turn on the light
Or plug in the stove -
Everything is spinning. (Electricity meter).
And what does the electric meter wind on the "mustache"?
Consumption of electrical energy.
Demonstration of the experiment.
Determining the power of a light bulb.
A=U*I*t=2.6V*1.4A*240s=873.6 J.
Q \u003d c * m * (t 2 -t 1) \u003d 4200 J / (kg * 0 C) * 0.1 kg * 2 0 C \u003d 840 J.
Exercise 27(2) from .
Question: For what purpose are the wires at the junctions not just twisted, but also soldered? Justify the answer.
The current strength in both wires is the same, since the conductors are connected in series.
If the contact point of the two conductors is not soldered, then its resistance will be quite large compared to the resistance of the conductors themselves. Then the most large quantity warmth. This will melt the contact point of the two conductors and open electrical circuit.
Formulation of the Joule-Lenz law.
The amount of heat released by a conductor with current is equal to the product of the square of the current, the resistance of the conductor and time.
Organization of independent activities of students.
I option.
1. How will the amount of heat released by a current-carrying conductor change if the current in the conductor is doubled?
A. Will increase by 2 times. B. Decrease by 2 times. B. Will increase by 4 times.
Answer. According to the Joule-Lenz law, Q=I 2 *R*t, therefore it will increase by 4 times.
B. Will increase by 4 times.
2. How much heat will a wire spiral with a resistance of 20 ohms release in 30 minutes if the current in the circuit is 2A?
A. 144000 J. B. 28800 J. W. 1440 J.
Answer. A. 144000J.
3. Copper and nichrome wires, having the same dimensions, are connected in parallel and connected to a current source. Which one will give off more heat?
A. Nichrome. B. Copper. B. Equally.
Answer. B. Copper.
II option.
1. How will the amount of heat released by a current-carrying conductor change if the current strength is reduced by 4 times?
A. It will decrease by 2 times. B. Decrease by 16 times. B. Will increase by 4 times.
Answer. According to the Joule-Lenz law, Q=I 2 *R*t, therefore, will decrease by 16 times.
B. Decrease by 16 times.
2. In an electric oven with a voltage of 220 V, a current of 30 A. How much heat will the oven release in 10 minutes?
A. 40000 J. B. 39600 J. C. 3960000 J.
Answer. 3960000 J.
3. Nickel and steel wires, having the same dimensions, are connected in series and connected to a current source. Which one will give off more heat?
A. Nickel. B. Steel. B. Equally.
Answer. Nickel.
Additional task.
Tasks from .
Answer. 500 J
Homework.
Paragraph 53, exercise 27 (1, 3) of .
Bibliography:
- Textbook "Physics", grade 8. A.V. Peryshkin.
- "Collection of problems in physics". IN AND. Lukashik.
When an external circuit is connected to a current source, the electric field propagates along the conductor at the speed of light and the free charges in them almost simultaneously come into ordered motion. There is current in the circuit.
The basic laws of direct current were established in 1826-1827 by the German scientist Georg Ohm and therefore bear his name.
Consider an inhomogeneous section of the circuit where the EMF acts. We denote the EMF in section 1-2 through ε 12, and the potential difference applied at the ends of the section - through φ 1 -φ 2. The work of forces A 12 (third-party and Coulomb) performed on current carriers, according to the law of conservation and transformation of energy, is equal to heat, prominent in the area. The work of forces performed when moving the charge in the area 1-2 is equal to
A 12 \u003d Q \u003d q 0 ε 12 + q 0 (φ 1 -φ 2) (13.12)
During the time, heat is released in the conductor
where 
(13.14)
Thus, generalized ohm's law, or Ohm's law for an inhomogeneous section of a circuit(section of the circuit containing the source of EMF), reads:
The current strength in an inhomogeneous section of the circuit is directly proportional to the sum of the EMF and the potential difference at the ends of this section and inversely proportional to its total resistance
(13.15)
where r is the internal resistance of the EMF source, R is the resistance of the external circuit.
Applying the generalized Ohm's law to one or another active section of the circuit, one should first choose the direction of bypassing this section, agreeing to consider one of its ends as the first (with a potential φ 1), and the other as the second (with a potential - φ 2). If this direction coincides with the direction of the current flowing through the section, the current strength is considered positive (I> 0), otherwise it is negative (I<0). ЭДС на рассматриваемом участке положительна тогда, когда направление обхода совпадает с направлением стороннего поля в источнике (это поле в нём направлено от отрицательного полюса к положительному); если же эти направления не совпадают, ЭДС считается отрицательной.
From the generalized Ohm's law, two other laws can be obtained.
Ohm's law for closed ( orcomplete) chain :
The current in a closed circuit is directly proportional to the EMF and inversely proportional to its total resistance.
(13.16)
Since the ends of the closed circuit are connected, and the potentials φ 1 and φ 2 on them become equal, then the potential difference φ 1 - φ 2 \u003d 0
Ohm's law for a closed circuit can be written as
ε 12 =IR+Ir (13.17)
where IR and Ir are the voltage drop, respectively, in the external and internal sections of the circuit
The connection of sources in the battery can be serial and parallel.
With a series connection, two adjacent sources are connected by opposite poles.
When connected in series, the EMF of the battery is equal to the sum of the EMF of the individual sources that make up the battery.
The current in such a circuit
(13.18)
If you connect all the positive and all negative poles of two or sources, then such a connection of energy sources is called parallel. In practice, only sources with the same EMF are always connected in parallel.
With a parallel connection of identical sources of electrical energy, the emf of the battery is equal to the emf of one source.
Then according to Ohm's law
(13.19)
Let us consider two limiting cases when the external resistance turns out to be either very large or, conversely, negligible .
R→∞ (orR >> r). A similar situation occurs when the external circuit is turned off, i.e. when the poles of the current source are open and there is an air gap between them through which no current flows. Substituting I=0 into the generalized Ohm's law, we get φ 1 - φ 2 = ε 12 . This means that the voltage at the poles of an open current source is equal to its EMF.
R→0 (orR<<r). A similar situation occurs with a short circuit. In this case, the current strength increases to a value
To 
which may exceed the value allowed for this circuit. A sharp increase in current during a short circuit can lead to a large release of heat. The field strength inside the battery then disappears. The wires may melt or become very hot and cause a fire, and the power source may be damaged. To avoid this, fuses are used.
Ohm's law for a homogeneous section of the circuit (a section that does not contain EMF) : The current in a conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor.
Value
called electrical conductivity of the conductor . The unit of conductivity is Siemens (Sm).
Ohm's law for a circuit section is a law obtained experimentally (empirically) that establishes a connection between the current strength in a circuit section and the voltage at the ends of this section and its resistance. The strict formulation of Ohm's law for a circuit section is written as follows: the current strength in the circuit is directly proportional to the voltage in its section and inversely proportional to the resistance of this section.
Ohm's law formula for a chain section is written as follows:
I - current strength in the conductor [A];
U – electrical voltage (potential difference) [V];
R is the electrical resistance (or simply resistance) of the conductor [Ohm].
Historically, the resistance R in Ohm's law for a circuit section is considered the main characteristic of a conductor, since it depends solely on the parameters of this conductor. It should be noted that Ohm's law in the mentioned form is valid for metals and solutions (melts) of electrolytes and only for those circuits where there is no real current source or the current source is ideal. An ideal current source is one that does not have its own (internal) resistance. More information about Ohm's law as applied to a circuit with a current source can be found in our article. We agree to consider the positive direction from left to right (see the figure below). Then the voltage across the section is equal to the potential difference.
φ 1 - potential at point 1 (at the beginning of the section);
φ 2 - potential at point 2 (and the end of the section).
If the condition φ 1 > φ 2 is satisfied, then the voltage U > 0. Therefore, the lines of tension in the conductor are directed from point 1 to point 2, and hence the current flows in this direction. It is this direction of the current that we will consider positive I > O.
Consider the simplest example determination of resistance in a circuit section using Ohm's law. As a result of an experiment with an electrical circuit, an ammeter (a device that shows current strength) shows, and a voltmeter. It is necessary to determine the resistance of the circuit section.
By definition of Ohm's law for a chain section
When studying Ohm's law for a section of a circuit in the 8th grade of a school, teachers often ask students the following questions to reinforce the material covered:
Between what quantities does Ohm's Law for a chain section establish a relationship?
Correct answer: between current [I], voltage [U] and resistance [R].
Why does current depend on voltage?
Correct Answer: Resistance
How does the current strength depend on the voltage of the conductor?
Correct Answer: Directly proportional
How does current depend on resistance?
Correct answer: inversely proportional.
These questions are asked so that in grade 8 students can remember Ohm's law for sections of the circuit, the definition of which says that the current strength is directly proportional to the voltage at the ends of the conductor, if the resistance of the conductor does not change.
