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 of determining the 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.

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 conversion 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)

In this way, 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).

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. (electric 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 greatest amount of heat will be released in the place. This will melt the contact point of the two conductors and open the 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:

1. Textbook "Physics", grade 8. A.V. Peryshkin.
2. "Collection of problems in physics". IN AND. Lukashik.

Ignorance of the law is not an excuse.
Aphorism

I wonder what laws will be discussed in lesson number three. Is there really a whole mountain or even a bunch of these laws in electrical engineering, and they all need to be remembered? Now we will find out. Hello dear! Probably, many of you are already looking at the next lesson with annoyance in your eyes and thinking to yourself: “What a boring thing!”, Or maybe even going to leave our orderly ranks? Don't rush, everything is just beginning! The initial stage is always boring ... From this lesson, all the most interesting things will go. Today I will tell you who is a friend in electrical engineering and who is an enemy, what will happen if you wake up an electronics student in the middle of the night, and how to understand half of all electrical engineering with one finger. Interesting? Then let's go!

We met our first friend in the last lesson - this is the strength of the current. It characterizes electricity in terms of the rate of charge transfer from one point in space to another under the action of a field. But, as it was noted, the current strength also depends on the properties of the conductor through which this current "runs". The magnitude of the electrical conductivity of the material directly affects the current strength. Now let's imagine a certain conductor (suitable as in Figure 3) with electrons moving in it. The main drawback of the electron, I would call the lack of a steering wheel. Due to this shortcoming, the movement of electrons is determined only by the field acting on them and the structure of the material in which they move.

Since the electrons “can’t” turn, some of them can collide with the nodes of the crystal lattice that fluctuate under the influence of temperature, lose their speed from the collision, and thereby reduce the charge transfer rate, that is, reduce the current strength. Some electrons can lose so much energy that they "stick" to an ion and turn it into a neutral atom. Now, if we increase the length of the conductor, it is obvious that the number of such collisions will also increase, and the electrons will give off even more energy, that is, the current strength will decrease. But with an increase in the cross-sectional area of ​​\u200b\u200bthe conductor, only the number of free electrons increases, and the number of collisions per unit area remains practically unchanged, therefore, with an increase in the area, the current also increases. So, we found out that electrical conductivity (it has already become not specific, since it takes into account the geometric dimensions of a particular conductor) depends immediately on three characteristics of the conductor: length, cross-sectional area and material.

However, the better the material conducts electric current, the less it "resists" its passage. These statements are equivalent. It's time to meet our second friend - electrical resistance. This is the reciprocal of the conductivity and depends on the same characteristics of the conductor.

Figure 3.1 - What determines the resistance of the conductor

In order to take into account the influence of the type of substance on its electrical resistance in numerical calculations, the value of specific electrical resistance is introduced, which characterizes the ability of a substance to conduct electric current. Note that the definitions of electrical conductivity and electrical resistance are identical, as are the statements above. Resistivity is defined as the resistance of a conductor with a length of 1m and a cross-sectional area of ​​1m 2. It is denoted by the Latin letter ρ (“ro”) and has the dimension Ohm m. Ohm is a unit of resistance, which is the reciprocal of Siemens. Also, to determine the resistivity, the dimension of Ohm mm 2 / m can be used, which is a million times less than the main dimension.
Thus, the electrical resistance of a conductor can be described in terms of its geometric and physical properties as follows:

where ρ is the specific electrical resistance of the conductor material;
l is the length of the conductor;
S is the cross-sectional area of ​​the conductor.

It can be seen from the dependence that the resistance of the conductor increases with an increase in the length of the conductor and decreases with an increase in the cross-sectional area, and also directly depends on the value of the resistivity of the material.

And now we recall that the magnitude of the current strength in the conductor is influenced by the strength of the electric field, under the influence of which an electric current arises. Oh, how many millions of thousands of times it has already been mentioned that an electric current arises under the influence of an electric field! This fact must always be kept in mind. There are, of course, other ways to create a current, but for now we will consider only this one. As mentioned above, an increase in the field strength leads to an increase in current, and more recently we found that the more energy an electron retains when moving along a conductor, the higher the value of the electric current. From the course of mechanics it is known that the energy of a body is determined by its kinetic and potential energy. So, a point charge placed in an electric field has at the initial moment of time only potential energy (since its speed is zero). To characterize this potential energy of the field, which the charge possesses, the value of the electrostatic potential was introduced, equal to the ratio of the potential energy to the value of the point charge:

where W p is the potential energy,
q is the value of the point charge.

After the charge falls under the action of an electric field, it will begin to move at a certain speed and part of its potential energy will turn into kinetic energy. Thus, at two points of the field, the charge will have a different value of potential energy, that is, two points of the field can be characterized by different values ​​of the potential. The potential difference is defined as the ratio of the change in potential energy (perfect work of the field) to the value of the point charge:

Moreover, the work of the field does not depend on the path of charge movement and characterizes only the magnitude of the change in potential energy. Potential difference is also called electrical voltage. Voltage is usually denoted by the English letter U (“y”), the unit of voltage is the value volt (V), named after the Italian physicist and physiologist Alessandro Volta, who invented the first electric battery.

Well, we met three inseparable friends in electrical engineering: ampere, volt and ohm or current, voltage and resistance. Any component of an electrical circuit can be unambiguously characterized by these three electrical characteristics. The first who met and became friends with all three at once was Georg Ohm, who discovered that voltage, current and resistance are related to each other by a certain ratio:

which was later called Ohm's law.

The strength of the electric current in a conductor is directly proportional to the voltage at the ends of the conductor and inversely proportional to the resistance of the conductor.

This wording must be known from the capital letter C to the dot at the end. Rumor has it that the first phrase of any electronics student woken up in the middle of the night will be exactly the formulation of Ohm's law. This is one of the basic laws of electrical engineering. This formulation is called integral. In addition to it, there is also a differential formulation that reflects the dependence of the current density on the characteristics of the field and the material of the conductor:

where σ is the conductivity of the conductor,
E is the electric field strength.

This formulation follows from the formula given in the second lesson, and differs from the integral one in that it does not take into account the geometric characteristics of the conductor, taking into account only its physical characteristics. This formulation is interesting only from the point of view of theory and is not applied in practice.
To quickly memorize and use Ohm's law, you can use the diagram shown in the figure below.

Figure 3.2 - Ohm's "triangular" law

The rule for using the diagram is simple: it is enough to close the desired value and two other symbols will give a formula for calculating it. For example.

Figure 3.3 - How to remember Ohm's law

We are done with the triangle. It is worth adding that only one of the above formulas is called Ohm's law - the one that reflects the dependence of current on voltage and resistance. The other two formulas, although they are its consequence, have no physical meaning. So don't get confused!
A good interpretation of Ohm's law is a drawing that most clearly reflects the essence of this law:

Figure 3.4 - Ohm's Law clearly

As we can see, this figure shows just three of our new friends: Ohm, Ampere and Volt. Volt tries to push Ampere through the conductor section (current strength is directly proportional to voltage), and Ohm, on the contrary, interferes with this (and is inversely proportional to resistance). And the more Om "pulls" the conductor, the harder it will be for Ampere to climb. But if Volt kicks harder...

It remains to figure out why the term "many laws" appears in the title of the lesson, because we have one law - Ohm's law. Well, firstly, there are two formulations for it, secondly, we only learned the so-called Ohm's law for a chain section, and there is also Ohm's law for a complete chain, which we will consider in the next lesson, thirdly, we have , at least two consequences from Ohm's law, allowing you to find the resistance value of a circuit section and the voltage in this section. So there is only one law, but it can be used in different ways.

Finally, I will tell you one more interesting fact. 10 years after the appearance of Ohm's law, a French physicist (and Ohm's work was not yet known in France) came to the same conclusions based on experiments. But he was pointed out that the law established by him back in 1827. was discovered by Ohm. It turns out that French schoolchildren are still studying Ohm's law under a different name - for them it is Poulier's law. Here it is. This concludes another lesson. Until we meet again!

• Any section or element of an electrical circuit can be unambiguously characterized using three characteristics: current, voltage and resistance.
• Resistance (R)- a characteristic of a conductor, reflecting the degree of its electrical conductivity and depending on the geometric dimensions of the conductor and the type of material from which it is made.
• Voltage (U)- the same as the potential difference; a value equal to the ratio of the work of the electric field to move a point charge from one point in space to another.
• Current, voltage and resistance are interconnected by the ratio I = U / R, called Ohm's law (the strength of the electric current in the conductor is directly proportional to the voltage at the ends of the conductor and inversely proportional to the resistance of the conductor).

And also puzzles:

• If the length of the wire is doubled by stretching, how will its resistance change?
• Which conductor presents more resistance: a solid copper rod or a copper tube having an outer diameter equal to the diameter of the rod?
• The potential difference at the ends of the aluminum conductor is 10V. Determine the density of the current flowing through the conductor if its length is 3 m.

The basic law of electrical engineering, with which you can study and calculate electrical circuits, is Ohm's law, which establishes the relationship between current, voltage and resistance. It is necessary to clearly understand its essence and be able to use it correctly in solving practical problems. Often mistakes are made in electrical engineering due to the inability to correctly apply Ohm's law.

Ohm's law for a section of a circuit states that current is directly proportional to voltage and inversely proportional to resistance.

If the voltage acting in an electrical circuit is increased several times, then the current in this circuit will increase by the same amount. And if you increase the resistance of the circuit several times, then the current will decrease by the same amount. Likewise, the flow of water in a pipe is greater, the greater the pressure and the less resistance the pipe exerts to the movement of water.

In popular form, this law can be formulated as follows: the higher the voltage for the same resistance, the higher the current strength, and at the same time, the higher the resistance for the same voltage, the lower the current strength.

To express Ohm's law mathematically most simply, consider that the resistance of a conductor in which a current of 1 A flows at a voltage of 1 V is 1 ohm.

The current in amps can always be determined by dividing the voltage in volts by the resistance in ohms. That's why Ohm's law for a circuit section is written by the following formula:

I = U/R.

magic triangle

Any section or element of an electrical circuit can be characterized using three characteristics: current, voltage and resistance.

How to use Ohm's Triangle: close the desired value - the other two characters will give a formula for its calculation. By the way, only one formula from a triangle is called Ohm's law - the one that reflects the dependence of current on voltage and resistance. The other two formulas, although they are its consequence, have no physical meaning.

Ohm's law calculations for a circuit section will be correct when voltage is in volts, resistance is in ohms, and current is in amps. If multiple units of measurement of these quantities are used (for example, milliamps, millivolts, megaohms, etc.), then they should be converted to amperes, volts and ohms, respectively. To emphasize this, sometimes the formula for Ohm's law for a chain section is written like this:

ampere = volt/ohm

You can also calculate the current in milliamps and microamps, while the voltage should be expressed in volts, and the resistance in kiloohms and megaohms, respectively.

Other articles about electricity in a simple and accessible presentation:

Ohm's law is valid for any section of the circuit. If it is required to determine the current in a given section of the circuit, then it is necessary to divide the voltage acting on this section (Fig. 1) by the resistance of this particular section.

Fig 1. Application of Ohm's law for a circuit section

Let's give an example of calculating the current according to Ohm's law. Let it be required to determine the current in a lamp having a resistance of 2.5 ohms, if the voltage applied to the lamp is 5 V. Dividing 5 V by 2.5 ohms, we get the current value equal to 2 A. In the second example, we determine the current, which will be flow under the action of a voltage of 500 V in a circuit whose resistance is 0.5 MΩ. To do this, we express the resistance in ohms. Dividing 500 V by 500,000 ohms, we find the value of the current in the circuit, which is equal to 0.001 A or 1 mA.

Often, knowing the current and resistance, the voltage is determined using Ohm's law. Let's write the formula for determining the voltage

U=IR

From this formula it can be seen that the voltage at the ends of a given section of the circuit is directly proportional to the current and resistance. The meaning of this dependence is not difficult to understand. If you do not change the resistance of the circuit section, then you can increase the current only by increasing the voltage. This means that with constant resistance, more current corresponds to more voltage. If it is necessary to obtain the same current at different resistances, then with a greater resistance there must be a correspondingly greater voltage.

The voltage across a section of a circuit is often referred to as voltage drop. This often leads to misunderstanding. Many people think that a voltage drop is some kind of wasted unnecessary voltage. In fact, the concepts of voltage and voltage drop are equivalent.

The calculation of voltage using Ohm's law can be shown in the following example. Let a current of 5 mA pass through a section of a circuit with a resistance of 10 kΩ, and it is required to determine the voltage in this section.

Multiplying I \u003d 0.005 A at R -10000 ohms, we get a voltage equal to 50 V. We could get the same result by multiplying 5 mA by 10 kOhm: U \u003d 50 V

In electronic devices, current is usually expressed in milliamps and resistance in kiloohms. Therefore, it is convenient to use these units of measurements in calculations according to Ohm's law.

According to Ohm's law, resistance is also calculated if the voltage and current are known. The formula for this case is written as follows: R = U/I.

Resistance is always the ratio of voltage to current. If the voltage is increased or decreased several times, then the current will increase or decrease by the same number of times. The ratio of voltage to current, equal to the resistance, remains unchanged.

The formula for determining resistance should not be understood in the sense that the resistance of a given conductor depends on outflow and voltage. It is known that it depends on the length, cross-sectional area and material of the conductor. In appearance, the formula for determining resistance resembles the formula for calculating current, but there is a fundamental difference between them.

The current in a given section of the circuit really depends on the voltage and resistance and changes when they change. And the resistance of a given section of the circuit is a constant value, independent of changes in voltage and current, but equal to the ratio of these quantities.

When the same current flows in two sections of the circuit, and the voltages applied to them are different, it is clear that the section to which the greater voltage is applied has a correspondingly greater resistance.

And if, under the influence of the same voltage, a different current passes in two different sections of the circuit, then a smaller current will always be in that section that has a greater resistance. All this follows from the basic formulation of Ohm's law for a circuit section, i.e., from the fact that the current is greater, the greater the voltage and the lower the resistance.

We will show the calculation of resistance using Ohm's law for a section of the circuit in the following example. Let it be required to find the resistance of the section through which, at a voltage of 40 V, a current of 50 mA passes. Expressing the current in amperes, we get I \u003d 0.05 A. Divide 40 by 0.05 and find that the resistance is 800 ohms.

Ohm's law can be visualized in the form of the so-called volt-ampere characteristic. As you know, a direct proportional relationship between two quantities is a straight line passing through the origin. Such a dependence is called linear.

On fig. 2 shows, as an example, a graph of Ohm's law for a circuit section with a resistance of 100 ohms. The horizontal axis is voltage in volts and the vertical axis is current in amps. The scale of current and voltage can be chosen as you like. A straight line is drawn so that for any point on it, the ratio of voltage to current is 100 ohms. For example, if U \u003d 50 V, then I \u003d 0.5 A and R \u003d 50: 0.5 \u003d 100 Ohms.

Rice. 2. Ohm's law (voltage characteristic)

The plot of Ohm's law for negative values ​​of current and voltage has the same form. This means that the current in the circuit flows equally in both directions. The greater the resistance, the less current is obtained at a given voltage and the more flat the straight line goes.

Devices in which the current-voltage characteristic is a straight line passing through the origin, i.e., the resistance remains constant when the voltage or current changes, are called linear devices. The terms linear circuits, linear resistances are also used.

There are also devices in which the resistance changes with a change in voltage or current. Then the relationship between current and voltage is expressed not according to Ohm's law, but more complicated. For such devices, the current-voltage characteristic will not be a straight line passing through the origin, but is either a curve or a broken line. These devices are called non-linear.

Mnemonic diagram for Ohm's law