Chain work definition physics. Electromotive force
>>Physics: Work and power DC
Electric current has received such widespread use because it carries with it energy. This energy can be converted into any form.
With the ordered movement of charged particles in a conductor electric field does work. It is commonly called current work. Now we will recall information about operation and current power.
Current work. Let's consider an arbitrary section of the chain. This can be a homogeneous conductor, for example, the filament of an incandescent lamp, the winding of an electric motor, etc. Let a charge pass through the cross section of the conductor over time. The electric field will do work 
(U- voltage between the ends of the conductor section).
Since the current strength 
, then this work is equal to:
According to the law of conservation of energy, this work must be equal to the change in the energy of the section of the circuit under consideration. Therefore, the energy released in a given section of the circuit during time is equal to the work of the current (see formula (15.12)).
If no mechanical work is performed on a section of the circuit and the current does not produce chemical effects, then only heating of the conductor occurs. A heated conductor gives off heat to surrounding bodies.
The conductor is heated as follows. An electric field accelerates electrons. After colliding with ions of the crystal lattice, they transfer their energy to the ions. As a result, the energy of random motion of ions near equilibrium positions increases. This means an increase in internal energy. At the same time, the temperature of the conductor rises, and it begins to transfer heat to surrounding bodies. Some time after the circuit is closed, the process is established, and the temperature stops changing over time. Due to the work of the electric field, energy is continuously supplied to the conductor. But its internal energy remains unchanged, since the conductor transfers to surrounding bodies an amount of heat equal to the work done by the current. Thus, formula (15.12) for the work of the current determines the amount of heat transferred by the conductor to other bodies.
If in formula (15.12) we express either voltage in terms of current, or current in terms of voltage using Ohm’s law for a section of the circuit, we obtain three equivalent formulas:
Formula 
It is convenient to use in the case of a series connection of conductors, since the current strength in this case is the same in all conductors. For a parallel connection, the following formula is convenient: 
since the voltage on all conductors is the same.
Joule-Lenz law. The law that determines the amount of heat released by a conductor carrying current environment, was first established experimentally by the English scientist D. Joule (1818-1889) and the Russian scientist E. X. Lenz (1804-1865). Joule-Lenz law
is formulated as follows: the amount of heat generated by a conductor carrying current is equal to the product of the square of the current, the resistance of the conductor and the time the current passes through the conductor:
We obtained this law using reasoning based on the law of conservation of energy. Formula (15.14) allows you to calculate the amount of heat generated in any section of the circuit containing any conductors.
Current power. Any electrical appliance(lamp, electric motor, etc.) is designed to consume a certain energy per unit of time. Therefore, along with the work of the current, very important has a concept current power. The current power is equal to the ratio of the work done by the current to the time the current passes.
According to this definition, current power
From this formula it is obvious that the current power is expressed in watts(W).
This expression for current power can be rewritten in several equivalent forms using Ohm's law for a section of the circuit:
Most appliances indicate their power consumption.
The passage of electric current through a conductor is accompanied by the release of energy in it. This energy is determined by the work of the current - the product of the transferred charge and voltage at the ends of the conductor.
???
1. What is the work of current called?
2. What is current power?
3. In what units is current power expressed?
G.Ya.Myakishev, B.B. Bukhovtsev, N.N. Sotsky, Physics 10th grade
Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for a year methodological recommendations discussion programs Integrated LessonsIf you have corrections or suggestions for this lesson,
Current work- this is the work of the electric field to transfer electric charges along the conductor;
The work done by the current on a section of the circuit is equal to the product of the current, voltage and time during which the work was performed.
Using the formula of Ohm's law for a section of a circuit, you can write several versions of the formula for calculating the work of the current:
According to the law of conservation of energy:
work is equal to the change in the energy of a section of the circuit, therefore the energy released by the conductor is equal to the work of the current.
In the SI system:
JOULE-LENZ LAW
When current passes through a conductor, the conductor heats up and heat exchange occurs with the environment, i.e. the conductor gives off heat to the bodies surrounding it
The amount of heat released by a current-carrying conductor into the environment is equal to the product of the square of the current strength, the resistance of the conductor and the time the current passes through the conductor.
According to the law of conservation of energy, the amount of heat released by a conductor is numerically equal to the work done by the current flowing through the conductor during the same time.
In the SI system:
[Q] = 1 J
DC POWER
The ratio of the work done by the current during time t to this time interval.
In the SI system:
Electrostatics and the laws of direct current - Cool physics
For the curious
Footprints in the sand
If you've ever walked along a beach at low tide, you've probably noticed that as soon as you step on the wet, hard sand, it immediately dries out and turns white around your footprint. This is usually explained by the fact that under the weight of the body, water is “squeezed” out of the sand. However, this is not the case because sand does not behave like a washcloth. Why does the sand turn white? Will the sand stay white the entire time you stand still?
Turns out...
The whitening of beach sand was first explained by Reynolds in 1885. He showed that the volume of sand increases when it is stepped on. Before this, the grains of sand were “packed” in the most dense way. Under the influence of shear deformation that occurs under the sole of the shoe, the volume occupied by grains of sand can only increase. While the sand level rises sharply, the water level can only rise as a result of capillary action, and this takes time. Therefore, at the bottom of the footprint, the sand appears for some time above the water level - it is dry and white.
From the formula for determining voltage (), it is easy to obtain an expression for calculating the work of electric charge transfer; since the current strength is related to the charge by the ratio , then the work done by the current is: , or .
Power by definition is therefore .
The Russian scientist H. Lenz and the English scientist D. Joule experimentally in the middle of the 19th century. independently established a law called Joule-Lenz law and it reads like this: when current passes through a conductor, the amount of heat released in the conductor is directly proportional to the square of the current strength, the resistance of the conductor and the time the current passes:
A complete closed circuit is an electrical circuit that includes external resistances and a current source (Fig. 17). As one of the sections of the circuit, the current source has a resistance, which is called internal.
In order for the current to flow through a closed circuit, it is necessary that additional energy be imparted to the charges in the current source; it appears due to the work of moving the charges, which is produced by forces of non-electric origin (external forces) against the forces of the electric field. The current source is characterized by an energy characteristic called EMF - electromotive force of the source. EMF is measured the ratio of the work of external forces to move along a closed chain of a positive charge to the magnitude of this charge.
Let an electric charge pass through the cross section of the conductor over time. Then the work of external forces when moving a charge can be written as follows: . According to the definition of current strength, , therefore . When this work is performed, a certain amount of heat is released on the internal and external sections of the circuit, the resistance of which is and . According to the Joule-Lenz law, it is equal to: 
. According to the law of conservation of energy, . Hence, . The product of the current and the resistance of a section of a circuit is often called the voltage drop across that section. Thus, the EMF is equal to the sum of the voltage drops in the internal and external sections of the closed circuit. Typically this expression is written like this: 
. This dependence was experimentally obtained by Georg Ohm, it is called Ohm's law for complete chain
and it reads like this: the current strength in a complete circuit is directly proportional EMF source current and is inversely proportional to the circuit impedance. When the circuit is open, the emf is equal to the voltage at the source terminals and, therefore, can be measured with a voltmeter.
Work and power electrical circuit.
Job is a measure of the conversion of one type of energy into another.
A watt-second is the work done by an electric current of magnitude 1A under tension 1B for 1s.
1 Watt ∙ hour [W ∙ h] = 3600 W ∙ h = 3600 J
1 kW ∙ h = 1000 W ∙ h = 3600 000 J
The work done by electric current in one second is called electric current power, it characterizes the intensity of work performed by the current. The unit of power is Watt [W].
;
from Ohm's law
,
Watt– the power that is developed at
1 kW = 1000 W 1 MW = 1000,000 W
Full is called the power developed by the current source , A useful– power consumed in the external circuit by the consumer .
Ratio of useful power to full power, developed by a current source, is called coefficient useful action(efficiency), denoted by “this”.
; 
Thermal effect of current. When an electric current passes through a conductor, the conductor heats up as a result of the collision of electrons with its atoms.
Joule-Lenz law.The amount of heat generated is directly proportional to the square of the current, the resistance of the conductor and the time it takes the current to pass through the conductor.
This relationship was established in 1841 by the English physicist Joule and somewhat later (in 1844) by the Russian academician Lenz.
The thermal effect of current is used: incandescent lamps, heating devices, electric welding, thermal relays (bimetallic plates).
Each conductor, depending on the conditions in which it is located, can pass through itself, without overheating, a current not exceeding a certain permissible value. This quantity is characterized permissible current density , i.e. the magnitude of the current I attributable to 1 mm cross-sectional area s conductor.
- permissible current density I on cross-sectional area.
- winding of electrical machines
- thread light bulb
In case of loose electrical contact and bad connection conductors R together their connections (the so-called contact resistance electrical contact) increases greatly, and increased heat release occurs here. As a result, this can lead to contact burnout and electrical circuit breakage.
Questions for self-control:
1. How it is done serial connection conductors? What laws apply in this chain?
2. How it is done parallel connection conductors? What laws apply in this chain?
3. How is mixed connection of consumers carried out?
4. How to determine the work and power of electric current? In what units are power and work measured?
5. What is efficiency?
6. Formulate the Joule-Lenz law.
7. What is current density and contact resistance?
8. What is an electric field? What is the electric field characterized by?
9.What is called electric potential? Potential difference? In what units is it measured?
10. What is EMF, and in what units is it measured?
11. What is electric current, and in what units is it measured?
12. What is called electrical resistance? What does the resistance of conductors depend on?
13. How is an atom of a substance structured?
14.What are called conductors and dielectrics?
15. How they interact electric charges? Coulomb's law.
16. What is an electric field? What is the electric field characterized by?
17. What is electric current, and in what units is it measured?
18. What is called electrical resistance? What does the resistance of conductors depend on?
19.How can you increase the resistance of a conductor?
20. How is an electrical circuit formed, and what parts does it consist of?
21. Formulate Ohm’s law for an electric circuit and separate area?
22. What is voltage drop and how is it determined?
23. Describe the operating modes of a DC generator?
24. What is called short circuit, what are its consequences?
25.How is Kirchhoff’s first law formulated?
26. How is Kirchhoff’s second law formulated?
Crystal lattice
Electric current. All metals are conductors of electric current. They consist of a spatial crystal lattice, the nodes of which coincide with the centers of positive ions. Free electrons move chaotically around the ions.
In metals, electronic conductivity
Electric current in metals is the ordered movement of free electrons.The direction of current is taken to be the direction of movementpositively charged particles.
Electric charges can move in an orderly manner under the influence of an electric field, so condition for the existence of electricity. current is the presence of an electric field and free charge carriers.
The current strength is numerically equal to the charge flowing through a given cross-section of a conductor per unit time. The current is called constant, e If the current strength and its direction do not change over time.
1 ampere (A) is equal to the strength of direct current at which 1 C of electricity flows through any cross-section of the conductor in 1 s. I = q 0 nvS The current in the circuit is measured. Symbol in a chain
Work and current power. Electric current supplies us with energy. It arises due to the work of the electric field on the movement of free charges in the conductor. Consider the section of the circuit through which current flows I. We denote the voltage in the area U, the section resistance is equal to R. When current flows through a homogeneous section of the circuit, the electric field does work. During time Δtcharge flows through the circuitΔ q = I Δ t . The electric field in a selected area does work.ΔA = U I Δ t — this job is calledwork of electric current . Due to the work in the area under consideration, it can be accomplished mechanical work; chemical reactions may also occur. If this is not the case, then the operation of the electric field only leads to heating of the conductor. The work done by the current is equal to the amount of heat generated by the current-carrying conductor:— Joule-Lenz law
The power of the electric current is equal to the ratio of the work of the current ΔA to the time interval Δ t for which this work was completed on this site: P = IU or . The work done by electric current in SI is expressed in joules (J), power – in watts (W).
Ohm's law for a closed circuit.
The current source has EMF () and resistance ( r
), which is called internal. Electromotive force (EMF) is the ratio of the work done by external forces to move a charge q
along the chain, to the value of this charge ( 1V=1J/1C). Let us now consider a closed (complete) direct current circuit consisting of a source with electromotive force And internal resistance r
and an external homogeneous area with resistance R
. (R+r
) — impedance chains. Ohm's law for a complete circuit is written as 
or
