Summary: Ohm's Law. Discovery history. Different types of Ohm's law. The history of the discovery of Ohm's law, types of Ohm's law Physics project on the topic of Ohm's laws

slide 1

slide 2

slide 3

slide 4

slide 5

slide 6

Slide 7

Slide 8

Slide 9

slide 10

slide 11

slide 12

slide 13

Research work in physics:

Subject: "Ohm's Law for a Circuit Section".

1. The purpose of the work

2. Ohm's law for a chain section

3. Measurement methods

4. Measurement results

5. Conclusions

6. Literature

Goal of the work:

In carrying out this work, we have set the following goals:

1) Get acquainted with the definition of Ohm's law using the program "Open Physics".

2) Measure Ohm's law on the circuit section.

3) Draw conclusions.

Ohm's law.

The quantitative measure of electric current is current I - scalar physical quantity equal to the charge ratio Δ q , transferred through the cross section of the conductor (Fig. 1.8.1) for the time interval Δ t , to this time interval:

If the strength of the current and its direction do not change with time, then such a current is called permanent .

For the active section of the circuit (section of the circuit containing the current source): the current strength in the section of the circuit containing the current source is equal to the ratio of the sum of the EMF and voltage at the ends of this section to its total resistance, i.e.

(1)

where U = φ1 - φ2, R is the external resistance of the section, and r is the internal resistance of the current source available in this section.

Ohm's law for the active site is otherwise called the generalized Ohm's law.

To derive this law, we take into account that the work done by the electric field to move current carriers along the circuit (current work A), in the absence of any chemical actions in the conductors and the mechanical work done by them, is equal to the amount of heat Q given off by the electric circuit to the environment Wednesday:

A = Q.

But according to the Joule-Lenz law Q = I2 R0 t,

and by definition of current I t = q.

Therefore, the work of the current A \u003d I 2 R 0 t = q I R 0 (2)

Where R0 \u003d R + r is the total resistance of that part of the circuit on which the current work is considered.

On the other hand, this work is made up of the work done by the Coulomb electrical forces, and the work done by external forces acting inside the current source:

A = Shark. + Astor.

By Definition of EMF Astor / q = ξ,

By definition of voltage Akul / q = U,

And according to formula (2) A / q = I R0 .

I R 0 = U + ξ ,

Where does formula (1) come from?

Otherwise, the active section of the circuit is called an inhomogeneous section, and the corresponding law is Ohm's law for an inhomogeneous section of the circuit.

For a passive section of the circuit (a section of the circuit that does not contain a current source): the current strength in the section of the circuit is equal to the ratio of the voltage at its ends to its resistance, i.e.

It was established in 1827 by the German physicist G. Ohm. It can be obtained as a consequence of the generalized Ohm's law by substituting the values ​​ξ = 0 and r = 0 into it.

For a complete (closed) circuit: the current strength in a closed circuit is equal to the ratio of the EMF of the circuit to its total resistance, i.e.

Where R is the external resistance, r is the internal resistance of the current source

It was established in 1826 by the German physicist G. Ohm. It can be obtained as a consequence of the generalized Ohm's law by substituting the value U = 0 into it (when a complete closed circuit is formed from the active section, the ends of the section are connected and the potentials φ1 and φ2 on them become equal).

From Ohm's law for a closed circuit, two important consequences can be obtained:

1. If the external resistance of the circuit is much greater than the internal resistance of the source (R >> r), then the voltage at the source terminals will be approximately equal to the EMF:

An example of such a situation is an open circuit.

1. If the external resistance is small compared to the internal

A similar situation occurs with a short circuit. The current strength becomes large, and therefore the wires can melt or become very hot and cause a fire; the current source may be damaged. To avoid this, fuses are used.

Ohm's law is the basic law of electrodynamics, which establishes a relationship between quantities that characterize the mechanism of electron motion in a conductor.

Due to the impossibility of demonstrating the very mechanism of electron motion, Ohm's law is perceived only quantitatively, which makes it difficult to study the law as a whole.

With the help of computer models, this hidden mechanism can be revealed. Laboratory work"Studying Ohm's Law" contributes to the formation of a correct understanding of the meaning of Ohm's law.

"Studying Ohm's Law for a Circuit Section"

According to Ohm's law, the current through a metal conductor (resistor) is directly proportional to the voltage between its ends. When experimentally studying the dependencies between quantities, it is advisable to use plotting.

In a graphical illustration, the result of joint measurements of two values ​​x and y is represented not just by a dot, but by rectangles, including the measurement error. It is the numerical values ​​xoyo of the measured values ​​that are the coordinates of the center of this rectangle, and the length of its sides is twice the measurement error (Fig. 1).

This implies the rule for constructing a graph by points whose coordinates are obtained as a result of the experiment: the line is drawn so that the same number of points are on opposite sides of it. The figure (Fig. 2) shows an example of such a graph.

Goal of the work: experimental verification Ohm's law for a circuit section.

Devices and materials:"Open Physics 2.5 Part 2", "DC Circuits" model.

Exercise 1.

Plotting a Current Dependence Graph

from voltage

5. Based on the measurement results, build a graph of current versus voltage.

6. Make a conclusion about the nature of this function.

Task 2. Resistor resistance calculation

Using one of the measurement results, calculate the resistance error of the resistor, given that the relative error is equal to the sum of the relative errors of current and voltage:

εR = εI + εU or ∆R = ∆I + ∆U

Send your good work in the knowledge base is simple. Use the form below

Students, graduate students, young scientists who use the knowledge base in their studies and work will be very grateful to you.

Posted on http://www.allbest.ru/

MINISTRY OF EDUCATION OF THE REPUBLIC OF BELARUS

Department of natural sciences

Essay

Ohm's law

Completed:

Ivanov M. A.

Introduction

1. General view of Ohm's law

2. The history of the discovery of Ohm's law, short biography scientist

3. Types of Ohm's laws

4. The first studies of the resistance of conductors

5. Electrical measurements

Conclusion

Literature, other sources of information

Introduction

Phenomena associated with electricity have been seen in ancient China, India and ancient greece several centuries before the beginning of our era. Around 600 BC, as the surviving legends say, the ancient Greek philosopher Thales of Miletus knew the property of amber rubbed on wool to attract light objects. By the way, the word "electron" the ancient Greeks called amber. The word "electricity" also came from him. But the Greeks only observed the phenomena of electricity, but could not explain.

The 19th century was full of discoveries related to electricity. One discovery spawned a whole chain of discoveries over several decades. Electricity from the subject of research began to turn into an object of consumption. It began to be widely introduced into various areas of production. Electric motors, generators, telephone, telegraph, radio were invented and created. The introduction of electricity into medicine begins.

Voltage, current and resistance - physical quantities characterizing the phenomena occurring in electrical circuits. These quantities are related. This connection was first studied by the German physicist 0m. Ohm's law was discovered in 1826.

1. General view of Ohm's law

Ohm's law goes like this: The current strength in a section of the circuit is directly proportional to the voltage in this section (for a given resistance) and inversely proportional to the resistance of the section (for a given voltage): I \u003d U / R, it follows from the formula that U \u003d IChR and R \u003d U / I. Since the resistance of a given conductor does not depend on either voltage or current strength, then the last formula should be read as follows: the resistance of a given conductor is equal to the ratio of the voltage at its ends to the strength of the current flowing through it. In electrical circuits, most often conductors (consumers of electrical energy) are connected in series (for example, bulbs in Christmas tree garlands) and in parallel (for example, household electrical appliances).

When connected in series, the current strength in both conductors (bulbs) is the same: I \u003d I1 \u003d I2, the voltage at the ends of the considered section of the circuit is the sum of the voltage at the first and second bulbs: U \u003d U1 + U2. The total resistance of the section is equal to the sum of the resistances of the bulbs R \u003d R1 + R2.

When the resistors are connected in parallel, the voltage at the circuit section and at the ends of the resistors is the same: U = U1 = U2. the current strength in the unbranched part of the circuit is equal to the sum of the current strengths in the individual resistors: I \u003d I1 + I2. The total resistance of the section is less than the resistance of each resistor.

If the resistances of the resistors are the same (R1 \u003d R2), then the total resistance of the section. If three or more resistors are connected in parallel in the circuit, then the total resistance can be -

found by the formula: 1/R = 1/R1 + 1/R2 + ... + 1/RN. Network consumers are connected in parallel, which are designed for a voltage equal to the mains voltage.

So, Ohm's Law establishes the relationship between the current strength I in the conductor and potential difference (voltage) U between two fixed points (sections) of this conductor:

Proportionality factor R, which depends on the geometric and electrical properties of the conductor and on temperature, is called ohmic resistance or simply the resistance of a given section of the conductor.

2. The history of the discovery of Ohm's law, a brief biography of the scientist

Georg Simon Ohm was born on March 16, 1787 in Erlangen, in the family of a hereditary locksmith. After leaving school, George entered the city gymnasium. The Erlangen Gymnasium was supervised by the university. Classes at the gymnasium were taught by four professors. Georg, after graduating from high school, in the spring of 1805 began to study mathematics, physics and philosophy at the Faculty of Philosophy of the University of Erlangen.

After studying for three semesters, he accepted an invitation to take a position as a mathematics teacher in a private school in the Swiss town of Gottstadt.

In 1811 he returned to Erlangen, graduated from the university and received a Ph.D. Immediately after graduating from the university, he was offered the position of Privatdozent of the Department of Mathematics of the same university.

In 1812 Ohm was appointed teacher of mathematics and physics at the Bamberg school. In 1817, he published his first printed work on teaching methods, "The Best Option for Teaching Geometry in Preparatory Classes." Ohm took up the study of electricity. Ohm based his electrical measuring instrument on the design of Coulomb's torsion balance. The results of his research Om issued in the form of an article entitled "Preliminary report on the law according to which metals conduct contact electricity." The article was published in 1825 in the Journal of Physics and Chemistry, published by Schweigger. However, the expression found and published by Ohm turned out to be incorrect, which was one of the reasons for his long non-recognition. Having taken all precautions, having eliminated in advance all the alleged sources of error, Ohm proceeded to new measurements.

His famous article "The definition of the law according to which metals conduct contact electricity, together with a sketch of the theory of the voltaic apparatus and the Schweigger multiplier", published in 1826 in the Journal of Physics and Chemistry, appears.

In May 1827, "Theoretical Investigations of Electrical Circuits" of 245 pages, which contained Ohm's now theoretical reasoning on electrical circuits. In this work, the scientist proposed to characterize the electrical properties of a conductor by its resistance and introduced this term into scientific use. Ohm found a simpler formula for the law of a section of an electrical circuit that does not contain EMF: "The magnitude of the current in a galvanic circuit is directly proportional to the sum of all voltages and inversely proportional to the sum of the reduced lengths. In this case, the total reduced length is defined as the sum of all individual reduced lengths for homogeneous sections having different conductivity and different cross section".

In 1829, his article "Experimental study of the operation of an electromagnetic multiplier" appeared, in which the foundations of the theory of electrical measuring instruments were laid. Here Ohm proposed a unit of resistance, for which he chose the resistance of a copper wire 1 foot long and with a cross section of 1 square line.

In 1830, Ohm's new study appeared "An Attempt to Create an Approximate Theory of Unipolar Conductivity". It was not until 1841 that Ohm's work was translated into English language, in 1847 - in Italian, in 1860 - in French.

On February 16, 1833, seven years after the publication of the article in which his discovery was published, Ohm was offered a position as a professor of physics at the newly organized Nuremberg Polytechnic School. The scientist begins research in the field of acoustics. Ohm formulated the results of his acoustic research in the form of a law that later became known as Ohm's acoustic law.

Before all of the foreign scientists, Ohm's law was recognized by the Russian physicists Lenz and Jacobi. They also helped its international recognition. With the participation of Russian physicists, on May 5, 1842, the Royal Society of London awarded Om with a gold medal and elected him a member.

In 1845 he was elected a full member of the Bavarian Academy of Sciences. In 1849, the scientist was invited to the University of Munich for the post of extraordinary professor. In the same year, he was appointed curator of the State Collection of Physical and Mathematical Instruments with simultaneous lectures on physics and mathematics. In 1852 Om received the position of ordinary professor. Ohm died on July 6, 1854. In 1881, at an electrical congress in Paris, scientists unanimously approved the name of the unit of resistance - 1 ohm.

3. Types of Ohm's laws

There are several types of Ohm's law.

Ohm's law for a homogeneous section of the chain (not containing a current source): the strength of the current in the conductor is directly proportional to the applied voltage and inversely proportional to the resistance of the conductor:

Ohm's law for a complete circuit - the current strength in the circuit is proportional to the EMF acting in the circuit and inversely proportional to the sum of the circuit resistances and the internal resistance of the source.

where I - current strength

E - electromotive force

R is the external resistance of the circuit (i.e. the resistance of that

part of the circuit that is outside the EMF source)

EMF - the work of external forces (i.e., forces of non-electrical origin) to move the charge in the circuit, referred to the magnitude of this charge.

Units:

EMF - volts

Current - amperes

Resistances (R and r) - ohms

Applying the basic law of an electrical circuit (Ohm's law), many natural phenomena which at first glance seem mysterious and paradoxical. For example, everyone knows that any human contact with live electrical wires is deadly. Just one touch on a broken wire of a high-voltage line can kill a person or an animal with an electric current. But at the same time, we constantly see how birds calmly sit on high-voltage power wires, and nothing threatens the life of these living creatures. So how can we find an explanation for this paradox?

And this phenomenon is explained quite simply, if we imagine that a bird located on an electric wire is one of the sections of the electrical network, the resistance of the second significantly exceeds the resistance of another section of the same circuit (that is, a small gap between the legs of the bird). Consequently, the strength of the electric current acting on the first section of the circuit, that is, on the bird's body, will be completely safe for it. However, complete safety is guaranteed to her only when in contact with a section of high-voltage wire. But as soon as a bird sitting on a power line touches a wire or any object close to the wire (for example, a telegraph pole) with its wing or beak, the bird will inevitably die. After all, the pillar is directly connected to the ground, and the flow of electric charges, passing to the body of a bird, is able to instantly kill it, rapidly moving towards the ground. Unfortunately, for this reason, many birds die in cities.

To protect birds from the harmful effects of electricity, foreign scientists have developed special devices - perches for birds, isolated from electric current. Such devices were placed on high-voltage power lines. Birds, sitting on an isolated perch, can, without any risk to life, touch wires, poles or brackets with their beak, wings or tail. The surface of the upper, so-called stratum corneum of human skin, has the greatest resistance. The resistance of dry and intact skin can reach 40,000 - 100,000 ohms. The horny layer of the skin is very small, only 0.05 - 0.2 mm. and easily breaks through with a voltage of 250 V. In this case, the resistance decreases a hundred times and falls the sooner the longer the current acts on the human body. Dramatically, up to 800 - 1000 Ohm, the resistance of the human body is reduced by increased sweating of the skin, overwork, nervous excitement, intoxication. This explains that sometimes even a small voltage can cause electric shock. If, for example, the resistance of the human body is 700 ohms, then a voltage of only 35 V will be dangerous. That is why, for example, electricians, even when working with a voltage of 36 V, use insulating protective equipment - rubber gloves or tools with insulated handles.

Ohm's law looks so simple that the difficulties that had to be overcome in establishing it are overlooked and forgotten. Ohm's law is not easy to test and cannot be taken as an obvious truth; indeed, for many materials it does not hold.

What are these difficulties anyway? Is it not possible to check what a change in the number of elements of a voltaic column gives by determining the current with a different number of elements?

The point is that when we take different number elements, we are changing the entire chain, because additional elements have additional resistance. Therefore, it is necessary to find a way to change the voltage without changing the battery itself. In addition, a different current heats the wire to a different temperature, and this effect can also affect the current strength. Ohm (1787-1854) overcame these difficulties by taking advantage of the phenomenon of thermoelectricity discovered by Seebeck (1770-1831) in 1822.

So Ohm showed that current is proportional to voltage and inversely proportional to the impedance of the circuit. It was a simple result for a complex experiment. So at least it should seem to us now.

Ohm's contemporaries, especially his compatriots, thought differently: perhaps it was the simplicity of Ohm's law that aroused their suspicion. Om faced difficulties in his service career, experienced a need; Om was especially oppressed by the fact that his works were not recognized. To the credit of Great Britain, and especially of the Royal Society, it must be said that Ohm's work has received the recognition it deserves. Om is one of those great men whose names are often found in lowercase: the name "om" was given to the unit of resistance.

4. The first studies of the resistance of conductors

What is a conductor? It's purely passive. component electric circuit, answered the first researchers. To study it means simply to puzzle over unnecessary riddles, because only the current source is the active element.

This view of things explains to us why scientists, at least until 1840, showed little interest in the few works that were carried out in this direction.

So, at the second congress of Italian scientists, held in Turin in 1840 (the first one met in Pisa in 1839 and even acquired some political significance), speaking in the debate on the report presented by Marianini, De la Rive argued that the conductivity of most liquids is not absolute, "but rather relative and varies with current strength." But Ohm's law was published 15 years before!

Among the few scientists who were the first to deal with the issue of conductivity of conductors after the invention of the galvanometer was Stefano Marianini (1790-1866).

He came to his discovery by accident, studying the voltage of batteries. He noticed that with an increase in the number of elements of the voltaic column electromagnetic influence on the arrow does not increase noticeably. This made Marianini immediately think that each voltaic element was an obstacle to the passage of current. He made experiments with pairs of "active" and "inactive" (i.e., consisting of two copper plates separated by a wet gasket) and empirically found a relationship in which the modern reader will recognize special case Ohm's law, when the resistance of the external circuit is not taken into account, as it was in Marianini's experiment.

Georg Simon Ohm (1789--1854) recognized the merits of Marianini, although his works did not directly help Ohm in his work. Ohm was inspired in his research by the work (“Analytical Theory of Heat”, Paris, 1822) by Jean-Baptiste Fourier (1768-1830) - one of the most significant scientific works of all times, which very quickly gained fame and appreciation among mathematicians and physicists of that time. Ohm came up with the idea that the mechanism of "heat flow" that Fourier speaks of can be likened to an electric current in a conductor. And just as in Fourier theory the heat flow between two bodies or between two points of the same body is explained by the difference in temperature, so Ohm explains the difference in "electroscopic forces" at two points of the conductor, the occurrence of an electric current between them.

Adhering to this analogy, Om began his experimental studies with the determination of the relative values ​​of the conductivity of various conductors. Applying a method that has now become classical, he connected in series between two points of the circuit thin conductors of various materials of the same diameter and changed their length so that a certain amount of current was obtained. The first results that he managed to obtain today seem rather modest. ohm law electric galvanometer

Historians are amazed, for example, by the fact that, according to Ohm's measurements, silver is less conductive than copper and gold, and condescendingly accept the explanation later given by Ohm himself, according to which the experiment was carried out with a silver wire coated with a layer of oil, and this was misleading as to the exact value diameter.

At that time, there were many sources of errors in experiments (insufficient purity of metals, difficulty in calibrating the wire, difficulty in accurate measurements, etc.). The most important source of errors was the polarization of the batteries. Permanent (chemical) elements were not yet known at that time, so that during the time required for measurements, the electromotive force of the element changed significantly. It was these reasons that caused errors that led to the fact that Ohm, on the basis of his experiments, came to the logarithmic law of the dependence of the current strength on the resistance of a conductor connected between two points in the circuit. After the publication of Ohm's first article, Poggendorf advised him to abandon chemical elements and it is better to use the copper-bismuth thermocouple, introduced shortly before by Seebeck.

Ohm heeded this advice and repeated his experiments by assembling an installation with a thermoelectric battery, in the external circuit of which eight copper wires of the same diameter, but of different lengths, were connected in series. He measured the current strength with the help of a kind of torsion balance, formed by a magnetic needle suspended on a metal thread. When the current parallel to the needle deflected it, Om twisted the thread on which it was suspended until the needle was in its usual position;

the current strength was considered proportional to the angle at which the thread was twisted. Ohm concluded that the results of experiments carried out with eight different wires "can be expressed very well by the equation

where X means the intensity of the magnetic action of a conductor whose length is x, and a and b are constants depending, respectively, on the exciting force and on the resistance of the remaining parts of the circuit.

The conditions of the experiment changed: resistances and thermoelectric pairs were replaced, but the results still boiled down to the above formula, which very simply goes into the one we know if X is replaced by the current strength, a by the electromotive force and b + x by the total resistance of the circuit.

Having obtained this formula, Ohm uses it to study the action of the Schweigger multiplier on the deflection of the needle and to study the current that passes in the external circuit of the battery of cells, depending on how they are connected - in series or in parallel. In this way he explains (as is now done in textbooks) what determines the external current of a battery, a subject which was rather obscure to the early investigators. Om hoped that experimental work open the way to the university for him, which he so desired. However, the articles went unnoticed. Then he left his teaching position at the Cologne gymnasium and went to Berlin to theoretically comprehend the results obtained. In 1827, in Berlin, he published his main work Die galvanische Kette, mathe-matisch bearbeitet (Mathematically designed galvanic circuit).

This theory, in the development of which he was inspired, as we have already pointed out, by the analytic theory of Fourier heat, introduces the concepts and precise definitions of the electromotive force, or "electroscopic force", as Ohm calls it, electrical conductivity (Starke der Leitung) and current strength. Having expressed the law he derived in the differential form given by modern authors, Ohm also writes it down in finite values ​​for special cases of specific electrical circuits, of which the thermoelectric circuit is especially important. Based on this, he formulates the known laws of change in electrical voltage along the circuit.

But Ohm's theoretical studies also went unnoticed, and if anyone wrote about them, it was only in order to ridicule "a morbid fantasy, the sole purpose of which is to belittle the dignity of nature." And only ten years later, his brilliant work gradually began to enjoy due recognition: in

In Germany they were appreciated by Poggendorf and Fechner, in Russia by Lenz, in England by Wheatstone, in America by Henry, in Italy by Matteucci.

Simultaneously with Ohm's experiments, A. Becquerel conducted his experiments in France, and Barlow conducted his experiments in England. The experiments of the first are especially remarkable by the introduction of a differential galvanometer with a double winding of the frame and the use of the "zero" method of measurement. Barlow's experiments are worth mentioning because they experimentally confirmed the constancy of the current strength throughout the circuit. This conclusion was verified and extended to the internal current of the battery by Fechner in 1831, generalized in 1851 by Rudolf Kohlrausch

(180E - 1858) on liquid conductors, and then once again confirmed by the thorough experiments of Gustav Nidman (1826 - 1899).

5. Electrical measurements

Becquerel used a differential galvanometer to compare electrical resistances. On the basis of his research, he formulated the well-known law of the dependence of the resistance of a conductor on its length and cross section. These works were continued by Pouillet and described by him in subsequent editions of his famous Elements de

physique experimentale” (“Fundamentals of experimental physics”), the first edition of which appeared in 1827. The resistances were determined by the comparison method.

Already in 1825, Marianini showed that in branching circuits, electric current is distributed over all conductors, regardless of what material they are made of, contrary to the assertion of Volta, who believed that if one branch of the circuit is formed by a metal conductor, and the rest are liquid, then all the current must pass through the metal conductor. Arago and Pouille popularized Marianini's observations in France. Not yet knowing Ohm's law, Pouille in 1837 used these observations and Becquerel's laws to show that the conductivity of a circuit equivalent to two

branched chains is equal to the sum of the conductivities of both chains. With this work, Pouille laid the foundation for the study of branched circuits. Puyet established a number of terms for them,

which are still alive, and some private laws generalized by Kirchhoff in 1845 in his famous "principles" ..

The greatest impetus for electrical measurements, and in particular resistance measurements, was given by the increased needs of technology, and first of all by the problems that arose with the advent of the electric telegraph. For the first time, the idea of ​​using electricity to transmit signals over a distance was born back in the 18th century. Volta described the telegraph project, and Ampère, back in 1820, proposed using electromagnetic phenomena for signal transmission. Ampere's idea was picked up by many scientists and technicians: in 1833, Gauss and Weber built the simplest telegraph line connecting the astronomical observatory and the physical laboratory. But practical use The telegraph was received thanks to the American Samuel Morse (1791-1872), who in 1832 had the good idea to create a telegraph alphabet consisting of only two characters. After many attempts, Morse finally succeeded in privately building the first crude model of a telegraph at New York University in 1835. In 1839, an experimental

line between Washington and Baltimore, and in 1844 the first American company organized by Morse for the commercial exploitation of the new invention arose. It was also the first practical application of the results of scientific research in the field of electricity.

In England, Charles Wheatstone (1802-1875), a former musical instrument maker, took up the study and improvement of the telegraph. Realizing the importance

measurements of resistance, Wheatstone began to look for the simplest and most accurate methods of such measurements. The comparison method then in use, as we have seen, gave unreliable results, mainly due to the lack of stable power supplies. Already in 1840, Wheatstone found a way to measure resistance regardless of the constancy of the electromotive force and showed his device to Jacobi. However, the article in which this device is described and which can be called the first work in the field of electrical engineering, appeared only in 1843. This article describes the famous "bridge", then named after Wheatstone. In fact, such a device has been described -

back in 1833 by Günther Christie and independently in 1840 by Marianini; both of them proposed a method of nullification, but their theoretical explanations, which did not take Ohm's law into account, left much to be desired.

Wheatstone, on the other hand, was an admirer of Ohm and knew his law very well, so that the theory of the “Wheatstone bridge” given by him is no different from that now given in textbooks. In addition, Wheatstone, in order to be able to quickly and conveniently change the resistance of one side of the bridge to obtain zero current strength in a galvanometer included in the diagonal arm of the bridge, designed three types of rheostats (the word itself was proposed by him according to

analogy with the "rheophore" introduced by Ampère, in imitation of which Peclet also introduced the term "rheometer"). The first type of rheostat with a movable bracket, which is still used today, was created by Wheatstone by analogy with a similar device used by Jacobi in 1841. The second type of rheostat was in the form of a wooden cylinder, around which a part of a wire connected to the circuit was wound, which was easily rewound from a wooden cylinder. to bronze. The third type of rheostat was similar to the "resistance box" that Ernst

Werner Siemens (1816-1892), scientist and industrialist, improved and widely disseminated in 1860. The Wheatstone bridge made it possible to measure electromotive forces and resistances.

The creation of an underwater telegraph, perhaps even more than an air telegraph, required the development of methods for electrical measurements. Experiments with an underwater telegraph began as early as 1837, and one of the first problems to be solved was to determine the speed of current propagation. Back in 1834, Wheatstone, with the help of rotating mirrors, as we have already mentioned in Chap. 8, made the first measurements of this speed, but his results contradicted the results of Latimer Clark, and the latter, in turn, did not correspond to later studies by other scientists.

In 1855, William Thomson (who later became Lord Kelvin) explained the reason for all these discrepancies. According to Thomson, the speed of current in a conductor has no definite value. Just as the rate of heat propagation in a rod depends on the material, so the rate of current in a conductor depends on the product of its resistance and electric capacitance. Following this theory of his, which in "" his times

subjected to fierce criticism, Thomson took up the problems associated with the underwater telegraph.

The first transatlantic cable, connecting England and America, functioned for about a month, but then deteriorated. Thomson calculated a new cable, carried out numerous measurements of resistance and capacitance, invented new transmitting devices, of which the astatic reflective galvanometer, replaced by the "siphon recorder" of his own invention, should be mentioned. Finally, in 1866, the new transatlantic cable was successfully put into operation. The creation of this first large electrical structure was accompanied by the development of a system of units of electrical and magnetic measurements.

The basis of the electromagnetic metric was laid by Carl Friedrich Gauss (1777-1855) in his famous article "Intensitas vis magneticae terrestris ad mensuram absolutam revocata" ("The magnitude of the force of terrestrial magnetism in absolute terms"), published in 1832. Gauss noted that various magnetic units are inconsistent between

himself, at least for the most part, and therefore proposed a system of absolute units based on the three basic units of mechanics: the second (a unit of time), the millimeter (a unit of length) and the milligram (a unit of mass). Through them he expressed all the rest physical units and invented a number of measuring instruments, in particular a magnetometer for measuring in absolute units of terrestrial magnetism. Gauss's work was continued by Weber, who built many of his own devices and devices conceived by Gauss. Gradually, especially thanks to the work of Maxwell, carried out in the special commission on measurements created by the British Association, which published annual reports from 1861 to 1867, the idea arose to create unified systems of measures, in particular, a system of electromagnetic and electrostatic measures.

Thoughts on the creation of such absolute systems of units were detailed in the historical report for 1873 of the second commission of the British Association. Convened in Paris in 1881, the International Congress for the first time established international units of measurement, giving each of them a name in honor of some great physicist. Most of these names are still preserved: volt, ohm, ampere, joule, etc. After

many ups and downs in 1935 was introduced international system Georgie, or MKSQ, which takes meter, kilogram-mass, second, and ohm as base units.

The "systems" of units are connected with the "dimension formulas" first used by Fourier in his analytical theory of heat (1822) and extended by Maxwell, who established the designations used in them. Metrology of the last century, based on the desire to explain all phenomena with the help of mechanical models, gave great importance formulas of dimensions in which she wanted to see nothing more and nothing less as a key to the secrets of nature. At the same time, a number of statements of an almost dogmatic nature were put forward. So, almost a mandatory dogma was the requirement that there were certainly three basic quantities. But by the end of the century, they began to realize that the formulas for dimensions are pure convention, as a result of which interest in theories of dimensions began to gradually fall.

Conclusion

Professor of physics at the University of Munich E. Lommel spoke well about the significance of Ohm's research at the opening of the monument to the scientist in 1895:

"Ohm's discovery was a bright torch that lit up that area of ​​​​electricity, which before him was shrouded in darkness. Ohm showed the only correct path through the impenetrable forest of incomprehensible facts. The remarkable successes in the development of electrical engineering, which we have watched with surprise in recent decades, could only be achieved based on the discovery of Om. Only he is able to dominate the forces of nature and control them, who will be able to unravel the laws of nature, Om wrested from nature the secret that she had been hiding for so long and handed it over to the hands of his contemporaries.

List of sources used

Dorfman Ya. G. The World History physics. M., 1979 Om G. Definition of the law according to which metals conduct contact electricity. - In the book: Classics of physical science. M., 1989

Encyclopedia One hundred people. Who changed the world. Ohm.

Prokhorov A. M. Physical Encyclopedic Dictionary, M., 1983

Orir J. Physics, vol. 2. M., 1981

Giancoli D. Physics, vol. 2. M., 1989

http://www.portal-slovo.ru/

http://www.polarcom.ru/~vvtsv/s_doc9c.html)

Hosted on Allbest.ru

Similar Documents

History of Isaac Newton's Discovery of the Law gravity", events preceding this discovery. The essence and limits of the application of the law. The formulation of Kepler's laws and their application to the motion of the planets, their natural and artificial satellites.

presentation, added 07/25/2010


The study of the motion of a body under the action of a constant force. Equation of a harmonic oscillator. Description of the oscillation of a mathematical pendulum. Movement of the planets around the sun. Solution of a differential equation. Application of Kepler's law, Newton's second law.

abstract, added 08/24/2015


The history of the discovery of the law of universal gravitation. Johannes Kepler as one of the discoverers of the law of planetary motion around the sun. Essence and features of the Cavendish experiment. Analysis of the theory of the force of mutual attraction. The main limits of applicability of the law.

presentation, added 03/29/2011


The study of the "Law of Archimedes", conducting experiments to determine the Archimedean force. Derivation of formulas for finding the mass of the displaced fluid and calculating the density. Application of the "Law of Archimedes" for liquids and gases. Methodical development lesson on this topic.

lesson summary, added 09/27/2010


Biographical information about Newton - the great English physics, mathematics and astronomer, his works. Research and discoveries of the scientist, experiments in optics and color theory. Newton's first derivation of the speed of sound in a gas, based on the Boyle-Mariotte law.

presentation, added 08/26/2015


Study of the cause of the magnetic anomaly. Methods for determining the horizontal component of tension magnetic field Earth. Application of the Biot-Savart-Laplace law. Determination of the reason for the turn of the arrow after applying voltage to the coil of the tangent-galvanometer.

control work, added 06/25/2015


Description of Newton's basic laws. Characteristics of the first law on the preservation of the state of rest or uniform motion by the body with compensated actions of other bodies on it. Principles of the law of body acceleration. Features of inertial reference systems.

presentation, added 12/16/2014


Kepler's laws of planetary motion a brief description of. The history of the discovery of the Law of universal gravitation by I. Newton. Attempts to create a model of the universe. The movement of bodies under the influence of gravity. Gravitational forces of attraction. artificial satellites Earth.

abstract, added 07/25/2010


Checking the validity of the ratios for parallel connection of resistors and the first law of Kirchhoff. Features of the resistance of receivers. Method for calculating voltage and current for various connections. The essence of Ohm's law for the site and for the entire circuit.

laboratory work, added 01/12/2010


Fundamental interactions in nature. Interaction of electric charges. Properties of electric charge. The law of conservation of electric charge. Formulation of Coulomb's law. vector shape and physical meaning Coulomb's law. The principle of superposition.

Plan 1) Introduction 2) Electric current 3) DC sources 4) DC electrical circuit 5) Ohm's law for a circuit section 6) Series and parallel connection of conductors. 7) Work and power of electric current. 8) Internal resistance of the current source. 9) Electromotive force. 10) Ohm's law for a complete circuit. 11) Literature

Introduction Ohm's Law (discovered in 1826) is a physical law that determines the relationship between voltage, current and conductor resistance in an electrical circuit. Named in honor of its discoverer Geogra Ohm. Ohm's law states: The current strength in a homogeneous section of the circuit is directly proportional to the voltage applied to the section, and inversely proportional to the electrical resistance of this section. (Current is directly proportional to voltage and inversely proportional to resistance) And is written by the formula: Where: I current (A), U voltage (V), R resistance (Ohm).

Electric current Electric current is the orderly movement of electric charges. Electric charges can move in an orderly manner under the action of an electric field An electric field can be created, for example, by two oppositely charged bodies. By connecting oppositely charged bodies with a conductor, it is possible to obtain an electric current flowing for a short interval of time.

Direct current sources In order for an electric current to exist in a conductor for a long time, it is necessary to maintain unchanged the conditions under which an electric current occurs. If at the initial moment of time the potential of point A of the conductor is higher than the potential of point B (Fig. 148), then the transfer of a positive charge q from point A to point B leads to a decrease in the potential difference between them.

Electric DC circuit In the outer section of the circuit, electric charges move under the influence of electric field forces. The movement of charges inside the conductor does not lead to equalization of the potentials of all points of the conductor, since at each moment in time the current source delivers exactly the same number of charged particles to one end of the electric circuit, which from it passed to the other end of the external electric circuit. Therefore, the voltage between the beginning and the end of the external section of the electrical circuit remains unchanged; the electric field strength inside the conductors in this circuit is non-zero and constant in time.

Series and parallel connection of conductors. Conductors in DC electrical circuits can be connected in series and in parallel. When the conductors are connected in series, the end of the first conductor is connected to the beginning of the second, etc. U = U 1 + U 2 + U 3 According to Ohm's law for the circuit section U 1 = IR 1, U 2 = IR 2, U 3 = IR 3 and U = IR When the conductors are connected in series, their total electrical resistance is equal to the sum of the electrical resistances of all conductors.,

Ohm's law for a circuit section. The German physicist Georg Ohm () in 1826 discovered that the ratio of the voltage U between the ends of a metal conductor, which is a section of an electrical circuit, to the current strength I in the circuit is a constant value: The unit of electrical resistance in SI ohm (Ohm). An electrical resistance of 1 ohm has such a section of the circuit in which, at a current strength of 1 A, the voltage is 1 V:

Ohm's law for a circuit section. Experience shows that the electrical resistance of a conductor is directly proportional to its length l and inversely proportional to the cross-sectional area S: The experimentally established dependence of the current strength I on the voltage U and the electrical resistance R of the circuit section is called Ohm's law for the circuit section:

Work and power of electric current. The work of the forces of the electric field that creates an electric current is called the work of the current. The work A of the forces of the electric field or the work of the electric current in a section of the circuit with electrical resistance R during the time is equal to The power of the electric current is equal to the ratio of the work of the current A to the time for which this work is done:

Work and power of electric current. If mechanical work is not performed on a section of the circuit under the influence of an electric field and chemical transformations of substances do not occur, then the work of the electric field only leads to heating of the conductor. (43.12) The law (43.12) was experimentally established by the English scientist James Joule () and the Russian scientist Emil Khristianovich Lenz (), therefore it is called Joule Lenz's law.

Internal resistance of the current source. In an electrical circuit consisting of a current source and conductors with electrical resistance R, the electric current does work not only on the outer, but also on the inner section of the circuit. The electrical resistance of a current source is called internal resistance. In an electromagnetic generator, the internal resistance is the electrical resistance of the generator winding wire. In the internal section of the electrical circuit, an amount of heat is released equal to: The total amount of heat released when a direct current flows in a closed circuit, the external and internal sections of which have resistances equal to R and r, respectively, is

Electromotive force. The total work of the forces of the electrostatic field during the movement of charges along a closed DC circuit is equal to zero. Consequently, all the work of the electric current in a closed electrical circuit is performed due to the action of external forces that cause the separation of charges inside the source and maintain a constant voltage at the output of the current source.

Ohm's law for a complete circuit. If, as a result of the passage of direct current in a closed electrical circuit, only heating of the conductors occurs, then, according to the law of conservation of energy, the total work of the electric current in a closed circuit, equal to work external forces of the current source, is equal to the amount of heat released in the external and internal sections of the circuit:

Literature 1) Physics textbook for grade 10. Authors: G. Ya. Myakishev 2) Website "Ohm's Law" (om_content&view=article&id=215#q10)